Surface decorated metal carbonyl clusters: bridging organometallic molecular clusters and atomically precise ligated nanoclusters

Abstract

In this Frontier Article, the work carried out within our research group in Bologna in the field of surface decorated metal carbonyl clusters will be outlined and put in a more general context. After a short Introduction, clusters composed of a metal carbonyl core decorated on the surface by metal–ligand fragments will be analyzed. Both metal–ligand fragments behaving as Lewis acids and Lewis bases will be considered. Then, the focus will be moved to clusters composed of a naked metal core decorated and stabilized on the surface by metal-carbonyl fragments. The structure and bonding (where theoretical studies are available) of such surface decorated metal carbonyl clusters will be presented, and compared to atomically precise ligated nanoclusters.

---

Cristiana Cesari

Cristiana Cesari: Cristiana Cesari received the Master Degree in Industrial Chemistry from the University of Bologna in 2012, and in 2016 was awarded a Ph.D. in Chemistry from the University of Bologna. Her doctoral research was focused on the synthesis, characterization and catalytic application of ruthenium N-heterocyclic carbene complexes. After a post-doctoral fellowship in Bologna focused on the study of the transformation process of ethanol to butanol via both homogeneous and heterogeneous catalysis, in 2019, she became a researcher in the field of General and Inorganic Chemistry at the University of Bologna. Her research activityis aimed at the synthesis of molecular organometallic compounds containing low valent transition metals stabilized by ligands, focusing on the study of metal carbonyl cluster as atomically defined nanomaterials for electrochemical applications and as precursors of nanostructured catalytic systems

Cristina Femoni

Cristina Femoni: Cristina Femoni received her Degree cum laude in Industrial Chemistry from the University of Bologna working on an experimental thesis under the supervision of Prof. G. Longoni. After a brief industrial working experience in France, in 1999 she was awarded a PhD in Chemical Sciences from the University of Bologna. Since 2022 she is full professor of General and Inorganic Chemistry. Her main scientific interests and expertise lie in the synthesis and characterization, also by single-crystal X-ray diffraction, of high-nuclearity transition metal clusters stabilized by carbonyl ligands, also referred to as atomically-precise ligand-stabilized nanoclusters, and their application in catalysis and nanoscience. She has been elected President of the European Chemistry Thematic Network (ECTN).

Francesca Forti

Francesca Forti: Francesca Forti received the Master Degree in Industrial Chemistry from the University of Bologna in 2021, and the same year started her Ph.D. in Chemistry at the University of Bologna. Her doctoral research is focused on the synthesis and characterization of new heterometallic carbonyl clusters and their application in catalytic reactions. In 2023 she joined Professor Nordlander's group in University of Lund for a visiting period to conduct synthesis and characterization of Os–Rh carbonyl clusters bearing chiral phosphines, exploited as asymmetric catalysts.

Maria Carmela Iapalucci

Maria Carmela Iapalucci: Maria Carmela Iapalucci received the Degree in Industrial Chemistry from the University of Bologna in 1987 with an experimental thesis under the supervision of Prof. G. Rosini. In 1992, she received her PhD in Chemical Sciences from the University of Bologna, working on the synthesis and characterization of iron and nickel carbonyl clusters containing group XIII and XV heteroatoms, under the supervision of Prof. G. Longoni. Since 2001 she is associate professor of General and Inorganic Chemistry at the University of Bologna.

Giorgia Scorzoni

Giorgia Scorzoni: Giorgia Scorzoni received the Master Degree in Industrial Chemistry from the University of Bologna in 2022. After spending one year as a research fellow in the field of Inorganic Chemistry, she started her Ph.D. in Industrial Chemistry at the University of Bologna. Her research activities focus on synthesis and characterization of new rhodium carbonyl clusters differentially substituted.

Stefano Zacchini

Stefano Zacchini: Stefano Zacchini received the Degree in Industrial Chemistry from the University of Bologna in 1996, working on an experimental Thesis under the supervision of Prof. G. Longoni. In 2001, he received his PhD in Chemistry from the University of Liverpool under the direction of Prof. B. T. Heaton. After a post-doctoral fellowship in Liverpool, he joined the University of Bologna in 2002 as research associate. In 2010, he has been appointed Associate Professor of General and Inorganic Chemistry at the University of Bologna and in 2017 he became full Professor. His research program focuses on the chemistry of metal carbonyl clusters and their applications in nanotechonolgy and nanosciences, molecular electronics and catalysis.

---

1. Introduction

According to F. A. Cotton, metal atom clusters are “…compounds containing a finite group of metal atoms which are held together entirely, mainly, or at least to a significant extent, by bonds directly between the metal atoms even though some non-metal atoms may be associated intimately with the cluster…”.¹ There are nowadays several categories of compounds that fall within Cotton's definition, including clusters composed of many types of transition and main group metals, in low to high oxidation states, and stabilized by a large variety of inorganic and organic ligands.^2–8 Metal carbonyl clusters (MCC) represent one of the most important category of organometallic and low-valent molecular metal clusters.^9–24 The chemistry of MCCs is well developed, and they played a fundamental role for Cotton's definition of metal clusters.

A similar consideration applies to gold-phosphine clusters, another category of low valent metal clusters, that was developed almost contemporarily to MCCs.²⁵ Indeed, following the pioneering work of Malatesta and Naldini in the 1960s,^26–29 several gold-phosphine clusters were characterized between 1970 and 2000. Since the turn of the century the range of gold cluster compounds has been greatly extended by the study of organothiolato-gold clusters. Indeed, molecular gold clusters and nanoclusters protected by thiolate ligands represent a very important field of research in modern inorganic chemistry and nanochemsitry.^30–35 This field has been further expanded, including other organic ligands, as well as other metals, and alloy nanoclusters.^36–38 Molecular clusters find nowadays several applications, including catalysis, electrocatalysis, material chemistry, sensing, medicine, biology.^39–43

As in the early days of metal cluster chemistry, single crystal X-ray diffraction (SC-XRD) represents the most important technique for the complete characterization of the structures of metal clusters. The SC-XRD analysis of [Au₁₀₂(p-MBA)₄₄] (p-MBA = p-mercaptobenzoic acid) represented somehow a milestone and showed some interesting features.⁴⁴ First of all, its metal core does not possess a ccp structure, as found in larger Au nanoparticles and bulk Au, but is based on an icosahedral structure. Moreover, the thiolato ligands are not innocent, but strongly interact with surface Au atoms, forming [Au_n(SR)_n+1]⁻ staple motives which decorate the surface of the cluster. Similar structures have been found for several other thiolate protected Au nanoclusters, and linear RS–Au–SR staple motives are nowadays a common features in cluster chemistry.⁴⁵

Different models have been developed for the interpretation of bonding in molecular metal clusters. In the case of low valent metal clusters, including MCCs, the most noticeable models for their electron counting are the effective atomic number (EAN) rule, Wade's rules, the polyhedral skeletal electron pair theory (PSEPT), and topological electron counting (TEC).^25,46 Regarding thiolate protected gold nanoclusters, and related molecular nanoclusters, their bonding has been usually rationalized based on the superatom model.^47–49 Recently, the superatom model has been successfully applied also to some MCCs, in particular Pt MCCs decorated on the surface by Cd-halide or Au-phosphine fragments, as well as Au, Ag and Cu clusters stabilized on the surface by Fe–CO, Ni–CO, Nb–CO or Ta–CO fragments.^50–53 These are representative examples of the two main categories of surface decorated MCCs, that is: (1) clusters composed of a metal carbonyl core decorated on the surface by ML fragments; (2) clusters composed of a naked metal core decorated and stabilized on the surface by metal-carbonyl fragments. These theoretical studies suggest some analogies between surface decorated MCCs and atomically precise ligated nanoclusters. Further analogies become evident when their structures are analyzed. Indeed, the metal–ligand and metal-CO fragments decorating the surfaces of such MCCs are somehow reminiscent of the staple motives found in atomically precise ligated nanoclusters. One of the aim of this Frontier Article is to stimulate further theoretical studies in order to investigate the possibility of extending the applicability of the superatom model to other MCCs and, at the same time, to compare it with other theoretical models. This might lead to a general unified approach for the interpretation of the different categories of molecular ligated metal clusters.

Our group has worked in the field of surface decorated MCCs over the last 15 years, and the most noticeable results are summarized in this Frontier Article. After this short Introduction, section 2 will be dedicated to clusters composed of a metal carbonyl core decorated on the surface by ML fragments. This will be further divided based on the fact that the surface ML fragments may act as Lewis acids (section 2.1) or Lewis bases (section 2.2). Then, section 3 will be focused on clusters composed of a naked metal core decorated and stabilized on the surface by metal-carbonyl fragments. For each category of surface decorated MCCs, the most representative examples taken from our work will be presented together with the most relevant compounds published in the literature. The structure and bonding (where theoretical studies are available) of such surface decorated MCCs will be presented, and compared to atomically precise ligated nanoclusters. The applications of MCCs in catalysis and electrocatalysis have been recently reviewed.^9,13 A short perspective on the potential and prospects of MCCs in practical applications will be reported in the Conclusions. All the structures reported in the following figures have been determined by SC-XRD.

2. Metal carbonyl core decorated on the surface by metal–ligand fragments

2.1. Metal–ligand fragments as Lewis acids

There are several examples of MCCs decorated on the surface by metal–ligand fragments of general formula {M_xL_y}^z (x, y = 1, 2, 3…; z = 0, positive or negative; M = transition or p-block metal; L = neutral or anionic ligand). These fragments may act as Lewis acids or Lewis bases. The former behaviour will be discussed in this section, the latter in section 2.2.

Cationic [Au(PR₃)]⁺ groups are the archetype of metal–ligand fragments that can coordinate as Lewis acids on the surface of MCCs. Indeed, several examples of MCCs decorated by [Au(PR₃)]⁺ fragments are known, and this topic has been recently reviewed.¹⁵ [Au(PR₃)]⁺ fragments possess 12-electrons, and the d¹⁰ Au(I) centre may accept electron density from the MCC surface on its empty orbitals (s, p or hybridized orbitals). It must be remarked that coordination of a 12 electron fragment does not alter the electron count of a MCC. This concept can be extended to other 12 electron ML fragments, including group 12 metals [ML]²⁺ and [MX]⁺ fragments (M = Zn, Cd, Hg; L = neutral ligand; X = anionic ligand), group 11 metals [ML]⁺ and [MX] fragments (M = Cu, Ag, Au), group 10 metals [ML] fragments (M = Ni, Pd, Pt), and group 9 metals [ML₂]⁺ fragments (M = Co, Rh, Ir).

Earlier interest was due to the isolobal analogy between [Au(PR₃)]⁺ and H⁺.^54–57 As soon as structures of MCCs containing two or more [Au(PR₃)]⁺ fragments were discovered, it was realized that Au(I) centres could reciprocally attract due to aurophilic interactions.^58–63 Indeed, peraurated MCCs revealed to be suitable platforms to study aurophilicity. This generates [{Au(PR₃)}_n]ⁿ⁺ fragments, which are strictly related to the Au-thiolate staple motives found in molecular Au nanoclusters.

Since aurophilic interactions are rather weak, MCCs decorated by [Au(PR₃)]⁺ fragments may show fluxionality and/or isomerism (Fig. 1 and 2).^64–67

Fig. 1 Three isomers of [Co₆C(CO)₁₂(AuPPh₃)₄] are formed upon crystallization, as found in the crystals of [Co₆C(CO)₁₂(AuPPh₃)₄], [Co₆C(CO)₁₂(AuPPh₃)₄]·THF and [Co₆C(CO)₁₂(AuPPh₃)₄]·4THF (blue, Co; yellow, Au; green, P; red, O; grey, C; white, H). Adapted with permission from ref. 64 Copyright 2014 American Chemical Society.

Fig. 2 Reactions of [HM₄(CO)₁₂]³⁻ (M = Fe, Ru) with increasing amounts of Au(PPh₃)Cl result in the formation of [HM₄(CO)₁₂(AuPPh₃)₂]⁻ and [HM₄(CO)₁₂(AuPPh₃)₃]. Anionic Fe and Ru clusters are isostructural, whereas noticeable differences are detected for the neutral clusters. [HFe₄(CO)₁₂(AuPPh₃)₃] displays the unique hydride within the tetrahedral Fe₄ cavity, and its formation is reversed in polar solvents such as acetone. [HRu₄(CO)₁₂(AuPPh₃)₃] exists as two isomers in rapid exchange in solution, both displaying the hydride ligand on the surface of the cluster (orange, Fe or Ru; yellow, Au; purple, P; red, O; grey, C; white, H).

Moreover, coordination of [Au(PR₃)]⁺ fragments may favour the stabilisation and isolation of MCCs, that otherwise cannot be observed as free species.⁶⁸ For instance, perauration allows the isolation of Ni carbide MCCs such as [Ni₆(C)(CO)₉(AuPPh₃)₄]²⁻ and [Ni₆(C)(CO)₈(AuPPh₃)₈]²⁺, containing a carbide atom within an octahedral Ni₆ cages (Fig. 3).^69,70

Fig. 3 Perauration of Ni-carbide MCCs results in the encapsulation of the carbide within an octahedral Ni₆ cage. Usually carbides are present within larger Ni cages, such as capped trigonal prismatic or square-antiprismatic. Molecular structure of (a) [Ni₆(C)(CO)₈(AuPPh₃)₈]²⁺, (b) its Ni₆(C)Au₈ cage, and (c) its [Ni₆(C)(CO)₈]⁶⁻ core (green, Ni; yellow, Au; purple, P; red, O; grey, C; H-atoms have been omitted for clarity). Surface decoration with [Au(PR₃)]⁺ fragments results in the stabilization in the otherwise unknown [Ni₆(C)(CO)₈]⁶⁻ cluster. Adapted from ref. 70 with permission from The Royal Society of Chemistry.

The reaction of [Pt₁₉(CO)₂₂]⁴⁻ with CO affords a purported [Pt₁₉(CO)₂₂]⁴⁻ cluster, which has never been isolated, since the reaction is reversed upon crystallization. Nonetheless, it has been possible to trap this elusive [Pt₁₉(CO)₂₄]⁴⁻ species by addition of 3–4 [Au(PPh₃)]⁺ fragments, affording the surface decorated clusters [Pt₁₉(CO)₂₄(AuPPh₃)₃]⁻ and [Pt₁₉(CO)₂₄(AuPPh₃)₄] (Fig. 4).⁷¹ Interestingly, [Pt₁₉(CO)₂₄(AuPPh₃)₃]⁻ is decorated by three isolated [Au(PPh₃)]⁺ fragments, whereas [Pt₁₉(CO)₂₄(AuPPh₃)₄] contains two [{Au(PPh₃)}₂]²⁺ fragments showing one aurophilic interaction each.

Fig. 4 The elusive [Pt₁₉(CO)₂₄]⁴⁻ species has been trapped upon reaction of [Pt₁₉(CO)₂₂]⁴⁻ with three moles of Au(PPh₃)Cl under CO atmosphere. The resulting [Pt₁₉(CO)₂₄(AuPPh₃)₃]⁻ monoanion further reacts with [Au(PPh₃)(THF)]⁺ affording neutral [Pt₁₉(CO)₂₄(AuPPh₃)₄]. The homoleptic precursor [Pt₁₉(CO)₂₄]⁴⁻ possesses a centered pentagonal prismatic structure, whereas the three- and tetra-aurated clusters display a compact ccp metal core (purple, Pt; yellow, Au; green, P; CO ligands and Ph-groups have been omitted for clarity).

The cluster [Os₁₀C(CO)₂₄Au(AuPCy₃)₃] is rather peculiar, since it may be viewed as a [Os₁₀C(CO)₂₄] tetrahedron of frequency two decorated on one edge by a [Au₄(PCy₃)₃] tetrahedral unit (Fig. 5).⁷² This is a rare case of a MCC decorated by an Au-phosphine cluster.

Fig. 5 A rare example of the MCCs decorated by an Au-phosphine cluster. (a) Molecular structure of [Os₁₀C(CO)₂₄Au(AuPCy₃)₃], and (b) its Os₁₀CAu₄P₃ cage (orange, Os; yellow, Au; purple, P; red, O; grey, C). Hydrogen atoms have been omitted for clarity.

The concepts developed for [Au(PR₃)]⁺ fragments can be applied to related fragments obtained using other monodenate or polydentate ligands, or replacing Au(I) with other coinage metal ions, that is, Cu(I) and Ag(I). Several examples of MCCs decorated by [ML]⁺ (M = Cu, Ag, Au; L = neutral ligand) or MX (M = Cu, Ag, Au; X = anionic ligand) fragments are, indeed, known.

NHC ligands have recently attracted a large interest in catalysis, coordination and organometallic chemistry. Examples of MCCs decorated by [M(NHC)]⁺ (M = Cu, Ag, Au) fragments have been, also, reported, even though their nuclearity is usually rather limited.^73–82 The largest species reported so far are [Pt₆(CO)₁₂(AgIPr)₂] and [Pt₉(CO)₁₈(AgIPr)₂] [IPr = C₃N₂H₂(C₆H₃ⁱPr₂)₂] obtained from the reactions of Chini clusters [Pt₆(CO)₁₂]²⁻ and [Pt₉(CO)₁₈]²⁻, respectively, with Ag(IPr)Cl (Fig. 6).⁸³ In these cases, the two [AgIPr]⁺ fragments cap the external triangular faces of the Chini clusters with retention of their prismatic structures.

Fig. 6 MCCs decorated by [M(NHC)]⁺ (M = Cu, Ag, Au) fragments. The molecular structure of (a) [Pt₆(CO)₁₂(AgIPr)₂] and (b) [Pt₉(CO)₁₈(AgIPr)₂] (purple, Pt; yellow, Ag; blue, N; red, O; grey, C; white, H). Adapted with permission from ref. 83 Copyright 2017 American Chemical Society.

Clusters with higher nuclearities have been obtained upon capping Ni carbide MCCs with CuCl and [Cu(MeCN)]⁺ fragments (Fig. 7), as in the case of [HNi₄₂C₈(CO)₄₄(CuCl)]⁷⁻ and [H₂Ni_29+x(CO)_33+x{Cu(MeCN)}₂]⁴⁻ (x = 0, 1).^84–86 The two [Cu(MeCN)]⁺ fragments in the latter cluster may be formally replaced with isoelectronic [CdX]⁺ (X = Cl, Br, I) or Ni(CO) fragments, affording [H_6−nNi₃₀C₄(CO)₃₄(CdX)₂]ⁿ⁻ (n = 3–6), [H_7−nNi₃₂C₄(CO)₃₆(CdX)]ⁿ⁻ (n = 5–7), and [H_6−nNi_34+xC₄(CO)_38+x]ⁿ⁻ (n = 5, 6; x = 0, 1).^87,88 All these clusters are based on the same tetracarbide Ni₃₀C₄ core. Similarly, replacing the CuCl fragment of [HNi₄₂C₈(CO)₄₄(CuCl)]⁷⁻ with [CdX]⁺ (X = Cl, Br) or Ni(CO) fragments results in related [Ni_42+xC₈(CO)_44+x(CdCl)]⁷⁻ (x = 0, 1), [HNi_42+xC₈(CO)_44+x(CdBr)]⁶⁻ (x = 0, 1), [HNi₄₃C₈(CO)₄₅]⁷⁻, [HNi₄₄C₈(CO)₄₆]⁷⁻, [H₂Ni₄₃C₈(CO)₄₅]⁶⁻, and [H₂Ni₄₄C₈(CO)₄₆]⁶⁻.^84,85,89 All these clusters possess a common Ni₄₂C₈ core differently capped by miscellaneous [CuCl], [CdX]⁺, and [Ni(CO)] fragments and may be viewed as borderline compounds between molecular and quasi-molecular clusters. It must be remarked that from a synthetic point of view, clusters containing Cu and Cd are usually obtained from the reactions of preformed Ni carbide MCCs such as [Ni₉C(CO)₁₇]²⁻, [Ni₁₀(C₂)(CO)₁₆]²⁻, [Ni₁₆(C₂)₂(CO)₂₃]⁴⁻, and [Ni₃₈C₆(CO)₄₂]⁶⁻, with CuCl, [Cu(MeCN)₄][BF₄] and CdX₂. Homometallic Ni species are obtained from the chemical oxidation of the same carbide species. Thus, since Cu(I) and Cd(II) salts may act also as oxidants, sometimes mixtures of species are obtained, that contain a variable amount of [CdX]⁺/Ni(CO), CuCl/Ni(CO) or [Cu(MeCN)]⁺/Ni(CO) fragments.

Fig. 7 Ni-carbide MCCs decorated by Cu and Cd-based fragments: (a) [H₂Ni₃₀C₄(CO)₃₄{Cu(MeCN)}₂]⁴⁻ (green, Ni; orange, Cu; grey, C; red, O; blue, N; white, H); (b) [Ni₃₆C₈(CO)₃₆(Cd₂Cl₃)]⁵⁻, (c) [Ni_42+xC₈(CO)_44+x(CdCl)]⁷⁻ (x = 0.19) (green, Ni; yellow, Cd; orange, Cl; grey, C; red, O); (d) [HNi₄₂C₈(CO)₄₄(CuCl)]⁷⁻ and (e) [HNi_43+xC₈(CO)_45+x]⁷⁻ (two CO ligands with partial occupancy factors have not been located) (green, Ni; purple, Ni with partial occupancy factor; orange Cu; yellow, Cl; grey, C; red, O). Hydride ligands have not been located by SC-XRD. Adapted from ref. 84 and 85 with permission from Springer and Elsevier.

Capping a cluster with Ni(CO) fragments is very common and, indeed, several Ni MCCs displaying the same metal core decorated by a variable amount of Ni(CO) fragments are known (Fig. 8).^{84,85,89–91} Even though apparently different, it must be remarked that both in the cases of Ni MCCs and Au thiolated nanoclusters, capping fragments and metal core are based on the same metals, that is, Ni in the case of Ni MCCs decorated by Ni(CO) fragments, Au in the case of Au nanoclusters with Au–SR staple motives.

Fig. 8 Different Ni(CO) capping modes found in the [HNi₄₂C₈(CO)₄₄(CuCl)]⁷⁻, [HNi_43+xC₈(CO)_45+x]⁷⁻, [H₂Ni_43+xC₈(CO)_45+x]⁶⁻, [Ni_42+xC₈(CO)_44+x(CdCl)]⁷⁻ and [HNi_42+xC₈(CO)_44+x(CdBr)]⁶⁻ clusters. (a) Uncapped Ni₄₂C₈(CuCl) core; (b) mono-capped Ni₄₄C₈ core; (c) mono-capped Ni₄₃C₈(CdX) core (green, Ni; grey, C; blue, Ni not bonded to carbide; purple, Ni with partial occupancy factor; orange, Cu; cyan, Cd; yellow, Cl or Br). Hydride ligands have not been located by SC-XRD. Adapted from ref. 84 with permission from Springer.

As an alternative to Ni(CO), MCCs may also be decorated by M(PR₃) or M(NHC) (M = Ni, Pd, Pt) fragments (Fig. 9).^92–96

Fig. 9 (Top) Molecular structures of the two isomers [Ru₅C(CO)₁₅{μ₄-Pt(P^tBu₃)}] and [Ru₅C(CO)₁₅{μ-Pt(P^tBu₃)}] (orange, Ru; yellow, Pt; green, P; red, O; grey, C; white, H). The two isomers rapidly interconvert in solution, mimicking the shift of a metal atom from a 4-fold to a 2-fold bonding site and back on a metal surface during the hopping process. (Bottom) A schematic representation of the adatom hopping mechanism on a metal surface. Empty circles represent the metal surface; the shaded circle represent the atom that moves. Adapted with permission from ref. 92 Copyright 2003 American Chemical Society.

Some impressive examples of MCCs decorated by Ni(PR₃) and Pd(PR₃) fragments are the homo and heterometallic Pd CO/PR₃-ligated clusters described by Dahl and Mednikov (Fig. 10).^12,97,98

Fig. 10 MCCs decorated by Ni(PR₃) fragments: (a) [Pd₁₆Ni₄(CO)₂₂(PPh₃)₄]²⁻ and (b) [Pd₃₃Ni₉(CO)₄₁(PPh₃)₆]⁴⁻ (orange, Pd; green, Ni; purple, P; grey, C; red, O). H-atoms have been omitted for clarity.

Group 9 metal fragments of the type [ML₂]⁺ (M = Rh, Ir; L = neutral ligand) are isoelectronic to those described above and, thus, may be used in order to decorate MCCs. Nonetheless, very few examples are known.^99,100

As shown above, some MCCs decorated by [CdX]⁺ fragments have been reported, and the literature may be extended to related [ZnX]⁺ and [HgX]⁺ fragments. Cd(II) is rather versatile for this purpose, and other Cd-based fragments may be found, that is, CdX₂, [Cd₂Cl₃]⁺ and [Cd₅(μ-Br)₅Br_5−x(solvent)_x]^x+.^101–105

Thermal decomposition in dfm at 120 °C of Chini clusters [Pt_3n(CO)_6n]²⁻ (n = 2–6) in the presence of CdBr₂·H₂O under different experimental conditions selectively afford the surface decorated Pt–Cd nanoclusters [Pt₁₃(CO)₁₂{Cd₅(μ-Br)₅Br₂(dmf)₃}₂]²⁻, [Pt₁₉(CO)₁₇{Cd₅(μ-Br)₅Br₃(Me₂CO)₂}{Cd₅(μ-Br)₅Br(Me₂CO)₄}]²⁻, and [H₂Pt₂₆(CO)₂₀(CdBr)₁₂]⁸⁻.^106,107

[Pt₁₃(CO)₁₂{Cd₅(μ-Br)₅Br₂(dmf)₃}₂]²⁻ is composed of a Pt₁₃(CO)₁₂ core decorated by two {Cd₅(μ-Br)₅Br₂(dmf)₃} rings (Fig. 11). Its electron count may be justified adopting either an ionic or covalent (radical) model. In the ionic model, the oxidation states of Cd and Br are assumed to be +2 and −1, respectively, and the cluster may be partitioned into two cationic [{Cd₅(μ-Br)₅Br₂(dmf)₃}]³⁺ rings and one [Pt₁₃(CO)₁₂]⁸⁻ anionic core. The two rings act as Lewis acids via the Cd(II) centers, and overall [Pt₁₃(CO)₁₂]⁸⁻ displays 162 cluster valence electrons (CVE) [13(Pt) × 10 + 12(CO) × 2 + 8(negative charge)]. In the covalent model, each {Cd₅(μ-Br)₅Br₂(dmf)₃} ring is assumed as neutral, and binds the inner [Pt₁₃(CO)₁₂]²⁻ cluster as a pseudo π-allyl ligand, contributing three electrons (one per each Cd bonded to dmf). The overall electron count is still 162 CVE [13(Pt) × 10 + 12(CO) × 2 + 2(negative charge) + 2(Cd rings) × 3]. Based on the cluster-borane analogy, an icosahedral transition metal cluster should display 170 CVE rather than 162 CVE.¹⁰⁸ Nonetheless, most gold icosahedral clusters display 162 CVE, as in the case of [Au₁₃Cl₂(PMe₂Ph)₁₀]³⁺ and [Au₉M₄Cl₄(PMe₂Ph)₁₀]⁺ (M = Cu, Ag, Au).¹⁰⁹ Mingos rationalized the low electron count of such clusters based on the predominance of radial over tangential bonding.¹¹⁰ Pyykko further supported the explanation of Mingos, by performing DFT calculations of the hypothetical 162 CVE [WAu₁₂(CO)₁₂] cluster.¹¹¹ The same electron count has been found also in [Pt₁₃{Au₂(PPh₃)₂}₂(CO)₁₀(PPh₃)₄] (Fig. 12), composed of a [Pt₁₃(CO)₁₀(PPh₃)₄]⁴⁻ core [13(Pt) × 10 + 10(CO) × 2 + 4(PPh₃) × 2 + 4(negative charge) = 162 CVE] decorated by two [Au₂(PPh₃)₂]²⁺, acting as Lewis acids.¹¹² Using a similar approach based on Lewis Dot Formulas, staple motives such as [Au(SR)₂]⁻ and [Au₂(SR)₃]⁻, found in thiolated gold clusters, may be viewed essentially as four-electron donors. Within this framework, the [Au₂₅(SCH₂CH₂Ph)₁₈]⁻ nanocluster may be viewed as composed of a [Au₁₃]⁵⁺ core [13(Au) × 11 − 5(positive charge) = 138 CVE] and six [Au₂(SR)₃]⁻ staple motives [contributing 6 × 4 = 24 CVE], with an overall electron count of 162 CVE, as in the above mentioned organometallic carbonyl and phosphine clusters.

Fig. 11 [Pt₁₃(CO)₁₂{Cd₅(μ-Br)₅Br₂(dmf)₃}₂]²⁻ (a) may be described as composed of a [Pt₁₃(CO)₁₂]⁸⁻ anionic core (b) decorated by two cationic [{Cd₅(μ-Br)₅Br₂(dmf)₃}]³⁺ rings (c, tow views). The resulting Pt₁₃ and Pt₁₃Cd₁₀ cores are represented in (d) and (e), respectively (purple, Pt; yellow, Cd; orange, Br; blue, N; red, O; grey, C; white, H). Adapted with permission from ref. 106 Copyright 2011 American Chemical Society.

Fig. 12 [Pt₁₃{Au₂(PPh₃)₂}₂(CO)₁₀(PPh₃)₄] (a) is composed of a [Pt₁₃(CO)₁₀(PPh₃)₄]⁴⁻ core decorated by two [Au₂(PPh₃)₂]²⁺ fragments (b) (purple, Pt; yellow, Au; orange, P; red, O; grey, C). H-atoms have been omitted for clarity in (a). CO ligands and Ph rings have been omitted in (b).

Saillard et al. have analyzed the same clusters within the framework of the superatom model.⁵¹ [Au₁₃Cl₂(PMe₂Ph)₁₀]³⁺ is a superatom with 8 free electrons (fe = free electrons; nfe = number of free electrons) [nfe = 13(Au₁₃) − 2 (Cl₂) − 3(positive charge) = 8] and 1S² 1P⁶ closed-shell configuration. In the case of [Pt₁₃(CO)₁₂{Cd₅(μ-Br)₅Br₂(dmf)₃}₂]²⁻, the 8− charge of the [Pt₁₃(CO)₁₂]⁸⁻ core provides the 8 free electrons for the same superatomic 1S² 1P⁶ configuration. This point has been further corroborated by the Kohn–Sham orbital diagram of [Pt₁₃(CO)₁₂]⁸⁻, where the 1S and 1P orbitals lie at the bottom of the fully occupied 5d block, whereas the empty 1D level is located above the π*(CO) orbitals (Fig. 13). Similar conclusions apply to [Pt₁₃{Au₂(PPh₃)₂}₂(CO)₁₀(PPh₃)₄], which is again a superatom with nfe = 8. These bonding analyses may be extended to other Pt–Cd carbonyl clusters, and the reader may find details in the cited literature.⁵¹

Fig. 13 Kohn–Sham MO diagram of [Pt₁₃(CO)₁₂]⁸⁻, as found in the structure of [Pt₁₃(CO)₁₂{Cd₅(μ-Br)₅Br₂(dmf)₃}₂]²⁻. Adapted from ref. 51 with permission from The Royal Society of Chemistry.

Surface decoration of MCCs with Lewis acids may be extended also to main group metal-based fragments. For instance, reactions of [Ni₆(CO)₁₂]²⁻ with InBr₃ under different experimental conditions lead to the selective formation of [Ni₆(μ₃-InBr₃)(η²-μ₆-In₂Br₅)(CO)₁₁]³⁻, [Ni₆(η²-μ₆-In₂Br₅)₂(CO)₁₀]⁴⁻, and [Ni₁₂(μ₆-In)(η²-μ₆-In₂Br₄OH)(CO)₂₂]⁴⁻ (Fig. 14).¹¹³ These clusters are composed of [Ni₆(CO)₁₁]⁴⁻ or [Ni₆(CO)₁₀]⁶⁻ carbonyl cores decorated by Lewis-acid fragments such as InBr₃, [In₂Br₅]⁺, [In₂Br₄OH]⁺ and [In]³⁺.

Fig. 14 MCCs decorated by main group metal-based fragments acting as Lewis acids. Molecular structures of (a) [Ni₆(μ₃-InBr₃)(η²-μ₆-In₂Br₅)(CO)₁₁]³⁻, and (b) [Ni₁₂(μ₆-In)(η²-μ₆-In₂Br₄OH)(CO)₂₂]⁴⁻ (green, Ni; yellow, In; orange, Br; red, O; grey, C). The H-atom of the OH group has not been located.

2.2. Metal–ligand fragments as Lewis bases

Metal–ligand fragments with a Lewis base character may coordinate to the surface of a MCC as monodentate or polydentate ligands. Such fragments may replace one or more CO ligands in existing MCCs, or stabilize non-existing MCC cores. Therefore, these metal–ligand Lewis base fragments may be viewed as organometallic analogues of organic ligands, such as monodentate or polydentate phosphines.

Among these, Sn(II) based fragments are very effective and versatile. Simple fragments such as [SnCl₃]⁻, SnCl₂ and [SnCl]⁺ may be viewed as two-electron donor ligands, related to alkyl, carbene and carbyne ligands, respectively (Scheme 1). The [Cl₂Sn(μ-OR)SnCl₂]⁻ and [Br₂Sn(μ-Br)SnBr₂]⁻ fragments may be described as bidentate (four electron) ligands, whereas [Cl₂SnOCOSnCl₂]²⁻ is a tridentate ligand, donating four electrons via the two Sn(II)-centers and two electrons via the C-atom. The species [Pt₈(CO)₁₀(SnCl₂)₄]²⁻, [Pt₅(CO)₅{Cl₂Sn(OR)SnCl₂}₃]³⁻ (R = H, Me, Et, ⁱPr), [Pt₆(CO)₆(SnCl₂)₂(SnCl₃)₄]⁴⁻, [Pt₉(CO)₈(SnCl₂)₃(SnCl₃)₂(Cl₂SnOCOSnCl₂)]⁴⁻ and [Pt₁₀(CO)₁₄{Cl₂Sn(OH)SnCl₂}₂]²⁻ can be obtained from the reactions of [Pt_3n(CO)_6n]²⁻ (n = 2–5) Chini clusters with SnCl₂ under different experimental conditions (Fig. 15).^114–116 These may be further transformed into [Pt₆(CO)₈(SnCl₂)(SnCl₃)₄]⁴⁻ and [Pt₆(CO)₈(SnCl₂)(SnCl₃)₂(PPh₃)₂]²⁻ upon reactions with CO and PPh₃. All these species may be viewed as Pt–CO clusters decorated by the above mentioned Sn-based fragments.

Scheme 1 Lewis dot formulas of the [SnCl]⁺, SnCl₂, [SnCl₃]⁻, [Cl₂Sn(μ-OR)SnCl₂]⁻, [Br₂Sn(μ-Br)SnBr₂]⁻ and [Cl₂SnOCOSnCl₂]²⁻ ligands found in Pt carbonyl clusters decorated by Sn-fragments acting as Lewis bases.

Fig. 15 Pt carbonyl clusters decorated by Sn-fragments acting as Lewis bases. Molecular structures of (a) [Pt₆(CO)₆(SnCl₂)₂(SnCl₃)₄]⁴⁻, (b) [Pt₈(CO)₁₀(SnCl₂)₄]²⁻, (c) [Pt₉(CO)₈(SnCl₂)₃(SnCl₃)₂(Cl₂SnOCOSnCl₂)]⁴⁻ and (d) [Pt₁₀(CO)₁₄{Cl₂Sn(OH)SnCl₂}₂]²⁻ (purple, Pt; orange, Sn; green, Cl; red, O; grey, C; white, H). Adapted from ref. 115 with permission from The Royal Society of Chemistry.

Reaction of [Pt₁₅(CO)₃₀]²⁻ with GeCl₄ affords [Pt₈(CO)₁₀(GeCl₂)₄]²⁻, isostructural to [Pt₈(CO)₁₀(SnCl₂)₄]²⁻, that contains four carbene-like GeCl₂ ligands.¹¹⁷ Several examples of Ru, Os and Ir carbonyl clusters decorated by [GeR]⁺ and GeR₂ (R = H, alkyl, aryl) have been reported by Adams and Captain.^118–120 For instance, thermal reaction of Ru₃(CO)₁₂ and ^tBuGeH₃ in heptane affords a mixture of six clusters, that is, Ru₃(CO)₉(μ₃-Ge^tBu)₂, Ru₂(CO)₆(μ-Ge^tBuH)₃, Ru₄(CO)₁₀(μ₄-Ge₂^tBu₂)(μ-Ge^tBuH)₂, Ru₄(CO)₈(μ₄-Ge₂^tBu₂)(μ-Ge^tBuH)₂(μ₃-Ge^tBu)(H), Ru₅(CO)₁₂(μ₃-Ge^tBu)₂(μ₄-Ge^tBu)(H), and Ru₆(CO)₁₂(μ₃-Ge^tBu)₄(H)₂, which can be separated by TLC (Scheme 2). All these Ru–Ge carbonyl clusters are composed of Ru₂, Ru₃, Ru₄, Ru₅ and Ru₆ carbonyl cores decorated by [Ge^tBu]⁺ and Ge^tBuH ligands.

Scheme 2 The products of the thermal reaction of Ru₃(CO)₁₂ and ^tBuGeH₃. The products have been separated by thin layer chromatography (TLC). CO ligands have been omitted for clarity. Adapted from ref. 9 with permission from The Royal Society of Chemistry.

The clusters [Rh₆(CO)₁₅(GaCp*)], [Rh₆(CO)₁₄(GaCp*)₂], [Rh₆(CO)₁₃(GaCp*)₃], and [Rh₆(CO)₁₂(GaCp*)₄] may be viewed as octahedral Rh₆ clusters decorated by GaCp* fragments (Fig. 16).^121,122 This is a two-electron donor fragment and, thus, these species are formally obtained upon replacement of 1–4 CO ligands in [Rh₆(CO)₁₆] by 1–4 GaCp*. A related InCp* species has been also reported, that is, [Rh₆(CO)₁₅(InCp*)]. The cluster [Ni₄(CO)₆(GaCp*)₄] is a tetrahedral 60 CVE species, analogue to the homoleptic [Ni₄(CO)₁₀] cluster, which has never been observed (Fig. 17).¹²³

Fig. 16 Octahedral Rh carbonyl clusters decorated by GaCp* and InCp* fragments. Molecular structures of (a) [Rh₆(CO)₁₂(μ₃-GaCp*)₄] and (b) [Rh₆(CO)₁₅(μ₃-InCp*]) (blue, Rh; yellow, Ga (a) or In (b); red, O; grey, C; white, H). These compounds have been obtained upon reactions of [Rh₆(CO)₁₆] with GaCp* and [Rh₆(CO)₁₅(MeCN)] with InCp*, respectively (Cp* = pentamethylcyclopentadienyl). Adapted from ref. 9 with permission from The Royal Society of Chemistry.

Fig. 17 The tetrahedral 60 CVE [Ni₄(CO)₆(GaCp*)₄] cluster (a), composed of a [Ni₄(CO)₆] core (b) decorated by four GaCp* fragments. The resulting [Ni₄(CO)₆Ga₄] framework is represented in (c) (green, Ni; yellow, Ga; red, O; grey, C; white, H).

3. Metal core decorated on the surface by metal-carbonyl fragments

Metal-carbonyl fragments may behave as ligands and coordinate to single metal ions, dimers or metal clusters. This applies to monometallic M–CO fragments, that is, [M(CO)_n]^z−, or polynuclear MCC fragments, that is, [M_m(CO)_n]^z−. Such M–CO fragments may exist or not exist as free species.

There are several complexes where a single d¹⁰ metal ion (Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺, Hg²⁺) is linearly bonded to two MCC fragments. Some noticeable examples are represented in Fig. 18.^124–126 Such complexes are closely related to MCCs decorated on the surface by ML fragments. In the case of compounds described in section 2.1, the central ion is bonded to one MCC fragment and one L (or X⁻) ligand, whereas in the cases reported in Fig. 18, M is bonded to two MCC fragments.

Fig. 18 Single metal ions stabilised upon coordination to metal carbonyl fragments. Molecular structures of (a) [{Co₅C(CO)₁₂}₂Au]⁻, (b) [{Ni₆(CO)₁₂}₂Au]³⁻, and (c) [{Fe(CO)₅}₂Au]⁺ (blue, Co; green, Ni; orange, Fe; yellow, Au; red, O; grey, C).

In most of the cases so far reported, the single metal ion is coordinated to anionic MCC fragments. An interesting exception is represented by [MFe₂(CO)₁₀]⁺ (M = Cu, Ag, Au), where the M⁺ ions are linearly bonded to two neutral Fe(CO)₅ molecules.¹²⁷ There are also a few examples where a d¹⁰ ion is coordinated to extended cyclic or acyclic MCC fragments, such as [MRu₆(CO)₂₂]⁻ (M = Cu, Ag), [AuRu₅(CO)₁₉]⁻,¹²⁸ and [HgOs₆(μ-PPh₂)₂(CO)₂₀].¹²⁹

In all cases, the MCC fragments act as Lewis bases, and they can also bind to larger M_n fragments. This point is well exemplified by the Hg–Ru clusters [HgRu₆(CO)₂₂]²⁻, [Hg₂Ru₇(CO)₂₆]²⁻, [Hg₃Ru₈(CO)₃₀]²⁻, and [Hg₄Ru₁₀(CO)₃₂]⁴⁻ (Fig. 19).¹³⁰ These contain [Hg]²⁺, [Hg₂]⁴⁺, linear [Hg₃]⁶⁺, and rectangular [Hg₄]⁸⁺ cores, respectively, stabilized by [Ru(CO)₄]²⁻, [Ru₃(CO)₁₁]²⁻, and [Ru₄(CO)₁₂]²⁻ MCC anions. Triangular [Hg₃]⁶⁺ cores have been found in [Hg₃Os₉(CO)₃₃] and [Hg₃Os₁₈(C)₂(CO)₄₂]²⁻ (Fig. 20).^131,132 The former cluster possesses a slightly twisted 2-D structure, since the three [Os₃(CO)₁₁]²⁻ units are side-by-side bonded to the three edges of the [Hg₃]⁶⁺ triangle. Conversely, in the case of [Hg₃Os₁₈(C)₂(CO)₄₂]²⁻, the two [Os₉C(CO)₂₁]⁴⁻ fragments are bonded to the two triangular faces of [Hg₃]⁶⁺, resulting in a sandwich compound. Mercurophilic interactions are present in all these compounds.

Fig. 19 [Hg_n]²ⁿ⁺ (n = 1–4) cores stabilised upon coordination to metal carbonyl fragments. Molecular structures of (a) [HgRu₆(CO)₂₂]²⁻, (b) [Hg₂Ru₇(CO)₂₆]²⁻, (c) [Hg₃Ru₈(CO)₃₀]²⁻, and (d) [Hg₄Ru₁₀(CO)₃₂]⁴⁻ (blue, Hg; orange, Ru; red, O; grey, C). Adapted from ref. 130 with permission from Elsevier.

Fig. 20 (a) The triangular [Hg₃]⁶⁺ unit can be stabilised upon coordination of three [Os₃(CO)₁₁]²⁻ MCCs on its three edges resulting in the 2-D surface decorated MCC [Hg₃Os₉(CO)₃₃]. (b) Alternatively, it can be sandwiched between two [Os₉C(CO)₂₁]⁴⁻ MCCs, resulting in [Hg₃Os₁₈(C)₂(CO)₄₂]²⁻ (blue, Hg; orange, Os; red, O; grey, C).

Square or rectangular [M₄]ⁿ⁺ cores (n = 4, M = Cu, Ag, Au; n = 8, M = Cd, Hg) stabilized by MCC fragments are rather common. Representative examples are [M₄Fe₄(CO)₁₆]⁴⁻ (M = Ag, Au),¹³³ [M₄Co₄(CO)₁₆],⁶³ (M = Cu, Ag), [M₄Mo₄(Cp)₄(CO)₁₂] (M = Ag, Au), [M₄Mo₄(C₅H₄NMe₂)₄(CO)₁₂] (M = Ag, Au),¹³⁴ [Cd₄Fe₄(CO)₁₆], [Hg₄Ru₄(CO)₁₆],¹³⁵ and [Hg₄Mn₄(MeCp)₄(CO)₈] (Fig. 21).¹³⁶ These 2-D clusters contain [M₄]⁴⁺ (M = Cu, Ag, Au) or [M₄]⁸⁺ (M = Cd, Hg) cores stabilized by [Fe(CO)₄]²⁻, [Ru(CO)₄]²⁻, [Co(CO)₄]⁻, [CpMo(CO)₃]⁻, and [(MeCp)Mn(CO)₂]²⁻ fragments. Anionic [M₄Fe₄(CO)₁₆]⁴⁻ (M = Ag, Au) may be further decorated by [Au₂(dppm)]²⁺ and [Au₂(dppe)]²⁺ fragments affording the larger 2-D clusters [Ag₈Fe₄(CO)₁₆(dppm)₂], [Ag₄Au₄Fe₄(CO)₁₆(dppe)₂], [Au₈Fe₄(CO)₁₆(dppe)₂], and, [Au₆Cu₂Fe₄(CO)₁₆(dppe)₂].^137,138 The species [Au₈Mo₄(CO)₂₀(PPh₃)₄] is closely related.¹³⁹

Fig. 21 MCCs based on square [Ag₄]⁴⁺ cores stabilised upon coordination of four [Fe(CO)₄]²⁻ units, which can be further decorated by two [Au₂(dppe)]²⁺ fragments. Molecular structures of (a) [Ag₄Fe₄(CO)₁₆]⁴⁻ and (b) [Ag₄Au₄Fe₄(CO)₁₆(dppe)₂] (blue, Fe; orange, Ag; yellow, Au, purple, P; red, O; grey, C). Hydrogen atoms have been omitted.

Polymerization isomerism has been observed in the case of [M₄Fe₄(CO)₁₆]⁴⁻ (M = Ag, Au). Indeed, it has been possible to synthesize and structurally characterize their triangular isomers [M₃Fe₃(CO)₁₂]³⁻ (M = Ag, Au) (Scheme 3).¹³³ Conversely, only [Cu₃Fe₃(CO)₁₂]³⁻ is known in the case of copper.

Scheme 3 Syntheses of the clusters [M₃Fe₃(CO)₁₂]³⁻ (M = Cu, Ag, Au) and [M₄Fe₄(CO)₁₆]⁴⁻ (M = Ag, Au). In the case of Ag and Au, the trimeric and tetrameric clusters may be viewed as polymerization isomers. Adapted with permission from ref. 133 Copyright 2019 American Chemical Society.

The species [M₅Fe₄(CO)₁₆]³⁻ (M = Cu, Ag, Au) contain a centered rectangular [M₅]⁵⁺ core decorated on the surface by four [Fe(CO)₄]²⁻ fragments, and formally originate upon addition of a M⁺ ion at the center of [M₄Fe₄(CO)₁₆]⁴⁻. Interestingly, complex mixtures of 2-D molecular alloy clusters of the type [M_xM′_5−xFe₄(CO)₁₆]³⁻ (M, M′ = Cu, Ag, Au; M ≠ M′) can be prepared in an almost continuum of compositions.¹⁴⁰

In the case of copper, further addition of a Cu⁺ ion affords the 3-D cluster [Cu₆Fe₄(CO)₁₆]²⁻, composed of an octahedral [Cu₆]⁶⁺ core and four [Fe(CO)₄]²⁻ fragments. Alternatively, this cluster may be viewed as a sp³ four-electron [Cu₆]²⁺ superatom bonded to four [Fe(CO)₄]˙⁻ MCC radical fragments (Fig. 22). The same model has been applied to anionic [Au₆Ni₁₂(CO)₂₄]²⁻ and cationic [Ag₆M₄(CO)₂₄]²⁺ (M = Nb, Ta), which contain a [M₆]²⁺ (M = Ag, Au) superatom tetrahedrally bonded to four [Ni₃(CO)₆]˙⁻ or four [M(CO)₆]˙ (M = Nb, Ta) radical metal carbonyl fragments (Fig. 23 and 24).^52,53 The closely related [Ag₆Nb₅(CO)₃₀]⁺ has been interpreted as composed of a trigonal prismatic [Ag₆]⁺ five electrons core bonded to five [Nb(CO)₆]˙ radicals, which adopt a trigonal bypiramidal arrangement. Octahedral cores have been found also in [Au₆Rh₁₆(CO)₃₆]⁶⁻ and [HPd₆Fe₆(CO)₂₄]³⁻,^141,142 but theoretical studies for their possible rationalization within the superatom model have not been yet carried out.

Fig. 22 (a) The molecular cluster [Cu₆Fe₄(CO)₁₆]²⁻ may be viewed as (b) a sp³ four-electron [Cu₆]²⁺ superatom bonded to four [Fe(CO)₄]˙⁻ MCC radical fragments (blue, Fe; orange, Cu; red, O; grey, C).

Fig. 23 Schematic structural formulae and structures of the metallic cores of the cationic cluster cores of [Ag₆Nb₅(CO)₃₀]⁺ (3⁺) and [Ag₆M₄(CO)₂₄]²⁺ (M = Nb, Ta) (4²⁺). The counterions [Al(OR^F)₄]⁻ and the CO ligands are omitted for clarity in all structures. Ellipsoids were drawn at the 50 % probability level. Sum of the formally available valence electrons (VE) of the metal cores (Ag⁺: 10 VE and M⁻: 6 VE). Adapted from ref. 52 with permission from Wiley.

Fig. 24 The molecular cluster [Au₆Ni₁₂(CO)₂₄]²⁻ (a) may be viewed as a sp³ four-electron [Au₆]²⁺ superatom bonded to four [Ni₃(CO)₆]˙⁻ MCC radical fragments (green, Ni; yellow, Au; red, O; grey, C). The interaction of one of these radical fragments with the octahedral core is schematically represented (b). Adapted from ref. 52 with permission from Wiley.

A further intriguing example where M–CO fragments act as ligands toward a gold cluster is [Au₆Co₂(PPh₃)₄(CO)₈].¹⁴³ Its [Au₆]²⁺ core consists of two tetrahedra with a common edge, bonded to four PPh₃ and two [Co(CO)₄]⁻ units all acting as terminal ligands. Alternatively, it could be interpreted as composed of a neutral [Au₆] core, four PPh₃ ligands and two [Co(CO)₄]˙ radicals.

A few examples of larger nanoclusters containing metal cores decorated by MCC fragments have been reported, that is, [Ag₉Os₁₃(CO)₄₈]⁻, [Ag₉Os₉(μ₃-O)₂(CO)₃₀]⁻,¹⁴⁴ [Ag₁₃Fe₈(CO)₃₂]ⁿ⁻ (n = 3, 4, 5),¹⁴⁰ [Ag₁₆Ni₂₄(CO)₄₀]⁴⁻,¹⁴⁵ [H_3−nPd₉Co₁₅C₃(CO)₃₈]ⁿ⁻ (n = 0–3), [H_6−nPd₁₆Co₂₀C₄(CO)₄₈]ⁿ⁻ (n = 3–6),^146–148 [Au₂₁Fe₁₀(CO)₄₀]⁵⁻, [Au₂₂Fe₁₂(CO)₄₈]⁶⁻, [Au₂₈Fe₁₄(CO)₅₂]⁸⁻, and [Au₃₄Fe₁₄(CO)₅₀]¹⁰⁻.¹⁴⁹

The two Ag–Os clusters are actually composed of three [Ag₃]³⁺ units stabilized upon coordination to four [Os₃(CO)₁₁]²⁻ and one [Os(CO)₄]²⁻ fragments in the case of [Ag₉Os₁₃(CO)₄₈]⁻, and two [Os₃O(CO)₉]²⁻ and three [Os(CO)₄]²⁻ units in the case of [Ag₉Os₉(μ₃-O)₂(CO)₃₀]⁻.¹⁴⁴

The very impressive [Ag₁₆Ni₂₄(CO)₄₀]⁴⁻ cluster reported by Dahl et al.,¹⁴⁵ is composed of a ccp Ag₁₆ kernel whose surface is connected to four tetrahedrally disposed triangular Ni₆(CO)₁₀ fragments. Taking into consideration the negative charge, it may be viewed as a 16 electrons Ag₁₆ core bonded to four [Ni₆(CO)₁₀]˙⁻ radicals (Fig. 25).

Fig. 25 The molecular structure of the cluster [Ag₁₆Ni₂₄(CO)₄₀]⁴⁻ (a), whose Ag₁₆Ni₂₄ metal kernel (b) may be derived from the interaction of a ccp Ag₁₆ core (c) with four [Ni₆(CO)₁₀]˙⁻ radicals (d), adopting a tetrahedral arrangement (green, Ni; orange, Ag; red, O; grey, C).

[H_3−nPd₉Co₁₅C₃(CO)₃₈]ⁿ⁻ (n = 0–3) and [H_6−nPd₁₆Co₂₀C₄(CO)₄₈]ⁿ⁻ (n = 3–6) display some interesting features (Fig. 26).^146–148 First of all, they are composed of compact Pd₉ and Pd₁₆ cores decorated by three or four [Co₅C(CO)₁₂] MCC fragments arranged in a trigonal planar and tetrahedral geometry, respectively (Fig. 27). Moreover, they are poly-hydrides and each single hydride species display some reversible redox processes, as assessed by electrochemical studies. SQUID measurements indicate that [HPd₉Co₁₅C₃(CO)₃₈]²⁻ is paramagnetic with two unpaired electrons. The structure of the Pd₉ core of [H_3−nPd₉Co₁₅C₃(CO)₃₈]ⁿ⁻ (n = 0–3) reversibly changes from trigonal prismatic to octahedral upon protonation/deprotonation reactions. Core isomerism has been observed in the case of [HPd₉Co₁₅C₃(CO)₃₈]²⁻ that may adopt both trigonal prismatic and octahedral structures of the Pd₉ core, depending on packing forces.

Fig. 26 Synthesis of [H_6−nCo₂₀Pd₁₆C₄(CO)₄₈]ⁿ⁻ (n = 3–6) and its transformation into [HPd₉Co₁₅C₃(CO)₃₈]²⁻ upon reaction with HBF₄ in CH₂Cl₂. Protonation/deprotonation reactions of [H_3−nCo₁₅Pd₉C₃(CO)₃₈]ⁿ⁻ (n = 0–3) are reported. [H₃Co₁₅Pd₉C₃(CO)₃₈] and [H₂Co₁₅Pd₉C₃(CO)₃₈]⁻ display an Oh-Pd₉ core, whereas [Co₁₅Pd₉C₃(CO)₃₈]³⁻ adopt a TP-Pd₉ structure (Oh = octahedron; TP = trigonal prism). Both isomers have been found in the case of [HCo₁₅Pd₉C₃(CO)₃₈]²⁻ (orange, Pd; blue, Co; red, O; grey, C). Hydride ligands have not been located by SC-XRD. Adapted from ref. 147 with permission from The Royal Society of Chemistry.

Fig. 27 Schematic representation of (a) one of the [Co₅C(CO)₁₂] fragment decorating the [H_3−nPd₉Co₁₅C₃(CO)₃₈]ⁿ⁻ (n = 0–3) and [H_6−nPd₁₆Co₂₀C₄(CO)₄₈]ⁿ⁻ (n = 3–6) clusters, and their (b) Pd₁₆, (c) octahedral Pd₉ and (d) trigonal prismatic Pd₉ cores. The tetrahedral and trigonal planar arrangements of the [Co₅C(CO)₁₂] fragments around these Pd_n cores is represented in (e) and (f).

The [Au₂₁Fe₁₀(CO)₄₀]⁵⁻, [Au₂₂Fe₁₂(CO)₄₈]⁶⁻, [Au₂₈Fe₁₄(CO)₅₂]⁸⁻, and [Au₃₄Fe₁₄(CO)₅₀]¹⁰⁻ nanoclusters may be viewed as organometallic counterparts of thiolate-protected gold nanoclusters.¹⁴⁹ Indeed, these Au–Fe–CO molecular nanoclusters are stabilized on the surface by linear Fe–Au–Fe fragments, reminiscent of the S–Au–S staple motives present in ligand protected Au nanoclusters. Because of this, the real nuclearity of the metal cores of such Au–Fe–CO and Au–SR nanoclusters are reduced compared to the nominal ones. The linear fragments on the surface may generate larger motives, such as Au(SR)₂ and Au₂(SR)₃ in the case of Au–SR nanoclusters, and Au{Fe(CO)₄}₂ and Au₂{Fe(CO)₄}₃ for Au–Fe–CO MCCs. Both an ionic and neutral model may be used in order to interpret their bonding. Using the ionic model, [Au₂₁Fe₁₀(CO)₄₀]⁵⁻ may be partitioned into a [Au₁₁]⁵⁺ core and two [Au₅{μ-Fe(CO)₄}₅]⁵⁻ rings containing linear Fe–Au–Fe motives (Fig. 28).^106,149 Similarly, [Au₂₅(SCH₂CH₂Ph)₁₈]⁻ may be viewed as composed by a [Au₁₃]⁵⁺ core and six [Au₂(SR)₃]⁻ staple motives.

Fig. 28 Molecular structure (a) and metal core (b) of [Au₂₁Fe₁₀(CO)₄₀]⁵⁻ (yellow, Au; blue, Fe; grey, C; red, O). The central [Au₁₁]⁵⁺ unit is bonded to two [Au₅{μ-Fe(CO)₄}₅]⁵⁻ rings containing linear Fe–Au–Fe motives.

Häkkinen and Femoni have further analyzed these iron-carbonyl-protected gold clusters by near-infrared (NIR) and Raman spectroscopy in conjunction with linear-response time-dependent density functional theory (LR-TDDFT).⁵⁰ In particular, the analyses of [Au₂₁Fe₁₀(CO)₄₀]⁵⁻ and [Au₂₂Fe₁₂(CO)₄₈]⁶⁻ (Fig. 29) indicate that their bonding and electronic structures display some analogies to thiolate-monolayer-protected Au clusters, and the frontier orbitals responsible for their NIR absorption may be rationalized in the framework of the gold superatom model.

Fig. 29 NIR absorption spectrum recorded in CH₃CN, molecular structure and Au₂₂Fe₁₂ frame of [Au₂₂Fe₁₂(CO)₄₈]⁶⁻ (yellow, Au; green, Fe; grey, C; red, O). Adapted with permission from ref. 50 Copyright 2009 American Chemical Society.

The species [Au₁₆S{Fe(CO)₄}₄(IPr)₄]²⁺ (IPr = C₃N₂H₂(C₆H₃ⁱPr₂)₂) was obtained upon thermal decomposition of [Fe(CO)₄(AuIPr)₂] in DMSO.¹⁵⁰ It consists of a μ₁₂-S centered Au₁₂-cubeoctahedron, decorated on the surface by four μ₃-Fe(CO)₄ and four μ₃-AuIPr fragments, with pseudo-T_d symmetry (Fig. 30 and 31). Icosahedral Au₁₃ and Au₁₂M cages have been found in several ligand-protected molecular gold nanoclusters,^{30,31,35,44,151,152} whereas the cubeoctahedral core is less common.¹⁵³ The electron count of [Au₁₆S{Fe(CO)₄}₄(IPr)₄]²⁺ can be derived assuming that the μ₃-AuIPr fragments, isolobal to μ₃-H, contribute one electron each, the μ₃-Fe(CO)₄ groups are four electron donors, and the interstitial μ₆-S atom donates six electrons.¹⁵⁰ Overall, [Au₁₆S{Fe(CO)₄}₄(IPr)₄]²⁺ possesses 156 CVE [11 × 12 (Au) + 6 × 1 (μ₆-S) + 4 × 1 (μ₃-AuIPr) + 4 × 4 (μ₃-Fe(CO)₄) −2 (charge +2)]. According to the EAN (Effective Atomic Number) rule, a cubeoctahedron should have 168 CVE. PSEPT (Polyhedral Skeletal Electron Pair Theory) predicted 170 CVE by interpreting a cubeoctahedron as a four-connected polyhedron. Conversely, assuming that radial bonding predominates, on the basis of Mingos Rules a cubeoctahedron should have 162 CVE. In all cases, [Au₁₆S{Fe(CO)₄}₄(IPr)₄]²⁺ results electron poor, as often found for gold clusters.

Fig. 30 Two different views of the molecular structure of [Au₁₆S{Fe(CO)₄}₄(IPr)₄]²⁺ (IPr = C₃N₂H₂(C₆H₃ⁱPr₂)₂). Au–C(O) contacts [2.636(4)–2.723(4) Å] were represented as fragmented lines. Hydrogen atoms have been omitted (green Fe; yellow Au; orange S; blue N; red O; grey C). Adapted with permission from ref. 150 Copyright 2020 American Chemical Society.

Fig. 31 Schematic representation of (a) the Au₁₂S cubeoctahedral core of [Au₁₆S{Fe(CO)₄}₄(IPr)₄]²⁺ (IPr = C₃N₂H₂(C₆H₃ⁱPr₂)₂), and the tetrahedral arrangements of (b) the four μ₃-Fe(CO)₄ groups and (c) the four μ₃-AuIPr fragments (green Fe; yellow Au; orange S; blue N; red O; grey C). The center of the cluster is represented as a white sphere in (b) and (c).

4. Conclusions

Two general types of molecular clusters may be classified as surface decorated MCCs. These include clusters composed of a metal carbonyl core decorated on the surface by ML fragments, as well as clusters composed of a naked metal core decorated and stabilized on the surface by metal-carbonyl fragments. All of these possess a metal core of various geometries stabilized upon coordination to its surfaces of different metal-ligands or metal-CO fragments. Most of the work appeared to date concerns the syntheses of surface decorated MCCs and their structural characterization by SC-XRD. Some of the most noticeable examples appeared in the literature have been reported in this Frontier Article, mainly outlining the contribution of our research group in Bologna to this field.

The examples herein reported show the rich structural and chemical diversity of surface decorated MCCs, as well as the possibility of obtained higher nuclearity clusters. Beside the aesthetical pleasure generated by these species, they allow to access unusual structural motives and geometries. At this regard, some structural analogies can be drawn among some of these surface decorated MCCs and other categories of ligated clusters, such as ligand protected coinage metal nanoclusters. In particular, the M-ligand and M–CO fragments decorating some of these surface decorated MCCs display some resemblances to the staple motives reported for ligand protected Au nanoclusters and related species. Moreover, the superatom model, usually applied to coinage metal nanoclusters, has been recently applied also to some representative examples of surface decorated MCCs.^50–53 Further structural and theoretical work will be required in order to fully explore the potentialities of these analogies, hopefully leading to a general unified approach for the interpretation of the different categories of molecular ligated metal clusters.

At this regard, it is noteworthy that, adopting the electron counting rules of low valent metal clusters (whose classically employed for MCCs), the surface decorated MCCs [Pt₁₃(CO)₁₂{Cd₅(μ-Br)₅Br₂(dmf)₃}₂]²⁻ and [Pt₁₃{Au₂(PPh₃)₂}₂(CO)₁₀(PPh₃)₄], the Au-phosphine cluster [Au₁₃Cl₂(PMe₂Ph)₁₀]³⁺, and the Au-thiolate cluster [Au₂₅(SCH₂CH₂Ph)₁₈]⁻, all possess 162 CVE (see section 2.1).^106–109 As predicted by Mingos, this electron counting is consistent with an icosahedral M₁₃ core where radial bonds predominate over tangential ones.^110,111 Applying the superatom model to the same four icosahedral clusters, they result to be superatom with 8 free electrons.⁵¹ This apparent “abnormal” difference in the electron counting (162 vs. 8) is due to how and which electrons are counted in the two schemes. CVE rules add all the valence electrons of the cluster, without differentiating between core electrons and those used for binding the ligands. Moreover, also the d electrons of the transition metals are included, even if the question might be debated in the case of Au. Of course, the metal framework (M–M bonding) is due to a reduced number of electrons (as a function of geometry). Most electrons are used to bind ligands (σ and π interactions) or are inert low lying core electrons. Conversely, in the superatom model only the electrons responsible for M–M bonds are counted, and d electrons are excluded. After considering these two points, the two models perfectly agree for the above mentioned four icosahedral clusters. Somehow, they say the same thing, but in a different manner. It would be important to extend this theoretical comparison among different electron counting models to other molecular metal clusters, in order to better appreciate such analogies.

From a practical point of view, MCCs already found several applications in catalysis and electrocatalysis.^{9,13,128,154–158} Enhanced molecular electrocatalysts might be obtained based on the electron sponge behavior of larger MCCs.^19,159 Paramagnetic MCCs could be exploited as single molecular magnets.¹⁶⁰ MCCs were employed as models in surface science, particularly for Fischer–Tropsch chemistry, and gave a fundamental contribution to the development of the cluster-surface analogy.^161,162 More recently, iron carbide MCCs garnered attention in bioinorganic chemistry, as spectroscopic, structural and functional models of the nitrogenase active site cluster.¹⁶³ Such studies require the partial replacement of carbonyls with other ligands, and the introduction of organic and inorganic sulfur based ligands is rather challenging.¹⁶⁴ CO-substitution may lead also to water-soluble MCCs, that might have potential biological and pharmaceutical applications.¹⁶⁵ Isomerism and chirality are two other fields of increasing interest in molecular cluster chemistry,^166,167 whose investigation in the case of MCCs is at its beginning.^168,169 New molecular materials integrated with MCCs can be obtained by self-assembly phenomena exploiting the formation of homometallic or heterometallic bonds, or employing suitable bidentate ligands.^11,18,101 Indeed, self-assembly can be exploited for the construction of cluster-based polymers for optoelectronics, magnetism, catalysis and nanotechnology.^11,170,171 A further strategy for the preparation of MCC based materials is represented by their encapsulation within metal organic framework (MOF),¹⁷² affording nanostructures materials for potential applications in catalysis and electrocatalysis.¹⁷³ Finally, MCCs may be used as molecular precursors in controlled thermal processes for the synthesis of supported metal catalysts, metal nanocrystals embedded in porous matrices, magnetic nanoalloys, and conductive sub-micrometric metal wires.^18,174–178

Data availability

No primary research results, software, or code have been included and no new data were generated or analyzed as part of this Frontier Article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financed by the European Union – NextGenerationEU through the Italian Ministry of University and Research under PNRR – Mission 4 Component 1, Investment 4.1 (DM 118/2023) – CUP J33C23002200002. We thank the referees for useful suggestions in revising the manuscript.

References

1. F. A. Cotton, Transition-metal compounds containing clusters of metal atoms, Q. Rev., Chem. Soc., 1966, 20, 389–401.
2. A. V. Virovets, E. Peresypkina and M. Scheer, Structural Chemistry of Giant Metal Based Supramolecules, Chem. Rev., 2021, 121, 14885–14554.
3. J. E. McGrady, F. Weigend and S. Dehnen, Electronic structure and bonding in endohedral Zintl clusters, Chem. Soc. Rev., 2002, 51, 628–649.
4. J. Heine, B. Peerless, S. Dehnen and C. Lichtenberg, Charge Makes a Difference: Molecular Ionic Bismuth Compounds, Angew. Chem., Int. Ed., 2023, 62, e202218771.
5. M. Schütz, C. Gemel, W. Klein, R. A. Fischer and T. F. Fässler, Intermetallic phases meet intermetalloid clusters, Chem. Soc. Rev., 2021, 50, 8496–8510.
6. J. Qian, Z. Yang, J. Lyu, Q. Yao and J. Xie, Molecular Interactions in Atomically Precise Metal Nanoclusters, Precis. Chem., 2024, 2, 495–517.
7. K. Wang, T. Iwano and S. Uchida, Keplerate polyoxometalate compounds: a multifunctional nano-platform for advanced materials, Dalton Trans., 2024, 53, 16797–16806.
8. E. L. Albright, T. I. Levchenko, V. K. Kulkarni, A. I. Sullivan, J. F. DeJesus, S. Malola, S. Takano, M. Nambo, K. Stamplecoskie, H. Häkkinen, T. Tsukuda and C. M. Crudden, N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters, J. Am. Chem. Soc., 2024, 146, 5759–5780.
9. C. Cesari, J.-H. Shon, S. Zacchini and L. A. Berben, Metal carbonyl clusters of groups of 8–10: synthesis and catalysis, Chem. Soc. Rev., 2021, 50, 9503–9539.
10. P. R. Raithby, The growth of higher nuclearity carbonyl clusters of ruthenium and osmium, J. Organomet. Chem., 2024, 1005, 122979.
11. M. Shieh, Y.-H. Liu, Y.-H. Li and R. Y. Lin, Metal carbonyl cluster-based coordination polymers: diverse syntheses, versatile network structures, and special properties, CrystEngComm, 2019, 21, 7341–7364.
12. E. G. Mednikov and L. F. Dahl, Syntheses, structures and properties of primarily nanosized homo/heterometallic palladium CO/PR₃-ligated clusters, Philos. Trans. R. Soc., A, 2010, 368, 1301–1332.
13. C. Cesari, C. Femoni, F. Forti, M. C. Iapalucci, G. Scorzoni and S. Zacchini, Molecular hydride carbonyl clusters and nanoclusters, Inorg. Chim. Acta, 2025, 574, 122394.
14. C. Cesari, C. Femoni, M. C. Iapalucci and S. Zacchini, Molecular Fe, Co and Ni carbide carbonyl clusters and nanoclusters, Inorg. Chim. Acta, 2023, 544, 121235.
15. I. Ciabatti, C. Femoni, M. C. Iapalucci, S. Ruggieri and S. Zacchini, The role of gold in transition metal carbonyl clusters, Coord. Chem. Rev., 2018, 355, 27–38.
16. B. Berti, C. Femoni, M. C. Iapalucci, S. Ruggieri and S. Zacchini, Functionalization, Modification, and Transformation of Platinum Chini Clusters, Eur. J. Inorg. Chem., 2018, 3285–3296.
17. I. Ciabatti, C. Femoni, M. C. Iapalucci, G. Longoni and S. Zacchini, Platinum Carbonyl Clusters Chemistry: Four Decades of Challenging Nanoscience, J. Cluster Sci., 2014, 25, 115–146.
18. S. Zacchini, Using Metal Carbonyl Clusters To Develop a Molecular Approach towards Metal Nanoparticles, Eur. J. Inorg. Chem., 2011, 4125–4145.
19. C. Femoni, M. C. Iapalucci, F. Kaswalder, G. Longoni and S. Zacchini, The possible role of metal carbonyl clusters in nanoscience and nanotechnologies, Coord. Chem. Rev., 2006, 250, 1580–1604.
20. B. F. G. Johnson and S. McIndoe, Spectroscopic and mass spectrometric methods for the characterisation of metal clusters, Coord. Chem. Rev., 2000, 200–202, 901–932.
21. K. H. Whitmire, Transition metal complexes of the naked pnictide elements, Coord. Chem. Rev., 2018, 376, 114–195.
22. J. A. Cabeza and P. G. Álvarez, The N-heterocyclic carbene chemistry of transition-metal carbonyl clusters, Chem. Soc. Rev., 2011, 40, 5389–5405.
23. G. Hogarth, S. E. Kabir and E. Nordlander, Cluster chemistry in the Noughties: new developments and their relationship to nanoparticles, Dalton Trans., 2010, 39, 6153–6174.
24. M. Sellin and I. Krossing, Homoleptic Transition Metal Carbonyl Cations: Synthetic Approaches, Characterization and Follow-Up Chemistry, Acc. Chem. Res., 2023, 56, 2776–2787.
25. D. M. P. Mingos, Structural and bonding patterns in gold clusters, Dalton Trans., 2015, 44, 6680–6695.
26. M. L. Ganadu, F. Demartin, A. Panzanelli, E. Zangrando, M. Peana, S. Medici and M. A. Zoroddu, Gold Clusters: From the Dispute on a Gold Chair to the Golden Future of Nanostructures, Molecules, 2021, 26, 5014.
27. L. Malatesta, Cluster compounds of gold, Gold Bull., 1975, 8, 48–52.
28. L. Naldini, F. Cariati, G. Simonetta and L. Malatesta, Gold-tertiary phosphine derivatives with intermetallic bonds, Chem. Commun., 1966, 647–648.
29. L. Malatesta, L. Naldini, G. Simonetta and F. Cariati, Triphenylphosphine-gold(0)-gold(I) compounds, Coord. Chem. Rev., 1966, 1, 255–262.
30. R. Jin, C. Zeng, M. Zhou and Y. Chen, Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities, Chem. Rev., 2016, 116, 10346–10413.
31. I. Chakraborty and T. Pradeep, Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles, Chem. Rev., 2017, 117, 8208–8271.
32. S. Li, N.-N. Li, X.-Y. Dong, S.-Q. Zang and T. C. W. Mak, Chemical Flexibility of Atomically Precise Metal Clusters, Chem. Rev., 2024, 124, 7262–7378.
33. X. Zou, X. Kang and M. Zhu, Recent developments in the investigation of driving forces for transforming coinage metal nanoclusters, Chem. Soc. Rev., 2023, 52, 5892–5967.
34. M. F. Matus and H. Häkkinen, Understanding ligand-protected noble metal nanoclusters at work, Nat. Rev. Mater., 2023, 8, 372–389.
35. W. W. Xu, X. C. Zeng and Y. Gao, The structural isomerism in gold nanoclusters, Nanoscale, 2018, 10, 9476–9483.
36. C. Sun, B. K. Teo, C. Deng, J. Lin, G.-G. Luo, C.-H. Tung and D. Sun, Hydrido-coinage-metal-clusters: Rational design, synthetic protocols and structural characteristics, Coord. Chem. Rev., 2021, 427, 213576.
37. T.-H. Chiu, J.-H. Liao, R. P. B. Silalahi, M. N. Pillay and C. W. Liu, Hydride-doped coinage metal superatom and their catalytic applications, Nanoscale Horiz., 2024, 9, 675–692.
38. X. Kang, Y. Li, M. Zhu and R. Jin, Atomically precise alloy nanoclusters: syntheses, structures, and properties, Chem. Soc. Rev., 2020, 49, 6443–6514.
39. T. Kawawaki, T. Okada, D. Hirayama and Y. Negishi, Atomically precise metal nanoclusters as catalysts for electrocatalytic CO₂ reduction, Green Chem., 2024, 26, 122–163.
40. R. Jin, G. Li, S. Sharma, Y. Li and X. Du, Toward Active-Site Tailoring in Heterogeneous Catalysis by Atomically Precise Metal Nanoclusters with Crystallographic Structures, Chem. Rev., 2021, 121, 567–648.
41. N. D. Loewen, T. V. Neelakantan and L. A. Berben, Renewable Formate from C-H Bond Formation with CO₂: Using Iron Carbonyl Clusters as Electrocatalysts, Acc. Chem. Res., 2017, 50, 2362–2370.
42. S. Mai, J. Sun, Z. Fang, G.-B. Xiao and J. Cao, Metal Clusters Based Multifunctional Materials for Solar Cells, Chem. – Eur. J., 2024, 30, e202303973.
43. F. Zaera, Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysis?, Chem. Rev., 2022, 122, 8594–8757.
44. P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and R. D. Kornberg, Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution, Science, 2007, 318, 430–433.
45. H. Häkkinen, The gold-sulfur interface at the nanoscale, Nat. Chem., 2012, 4, 443–455.
46. S. M. Owen, Electron counting in clusters: a view of the concepts, Polyhedron, 1988, 7, 253–283.
47. W. W. Xu, X. C. Zeng and Y. Gao, Application of Electronic Counting Rules for Ligand-Protected Gold Nanoclusters, Acc. Chem. Res., 2018, 51, 2739–2747.
48. H. Häkkinen, Atomic and electronic structure of gold clusters: understanding flakes, cages and superatoms from simple concepts, Chem. Soc. Rev., 2008, 37, 1847–1859.
49. C. M. Aikens, R. Jin, X. Roy and T. Tsukuda, From atom-precise nanoclusters to superatom materials, J. Chem. Phys., 2022, 156, 170401.
50. O. Lopez-Acevedo, J. Rintala, S. Virtanen, C. Femoni, C. Tiozzo, H. Grönbeck, M. Pettersson and H. Häkkinen, Characterization of Iron-Carbonyl Protected Gold Clusters, J. Am. Chem. Soc., 2009, 131, 12573–12575.
51. J. Wei, R. Marchal, D. Astruc, S. Kahlal, J.-F. Halet and J.-Y. Saillard, Looking at platinum carbonyl nanoclusters as superatoms, Nanoscale, 2022, 14, 3946–3957.
52. W. Unkrig, K. Kloiber, B. Butschke, D. Kratzert and I. Krossing, Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters, Chem. – Eur. J., 2020, 26, 12373–12381.
53. A. Muñoz-Castro, sp³-hybridization in superatomic clusters. Analogues to simple molecules involving the Au₆ core, Chem. Sci., 2014, 5, 4749–4754.
54. J. W. Lauher and K. Wald, Synthesis and Structure of [FeCo₃(CO)₁₂AuPPh₃]: A Trimetallic Trigonal-Bipyramidal Cluster. Gold Derivatives as Structural Analogues of Hydrides, J. Am. Chem. Soc., 1981, 103, 7648–7650.
55. P. Braunstein, J. Rosé, Y. Dusausoy and J.-P. Mangeot, Complexes à liaisons métal-métal. XXI. Synthèse et structure du “cluster” (Ph₃P)AuRuCo₃(CO)₁₂: une bipyramide trigonale à trois métaux différents, C. R. Chim., 1982, 294, 967–970.
56. P. Braunstein and J. Rosé, Gold in Bimetallic Molecular Clusters. Their Synthesis, Bonding and Catalytic Reactivities, Gold Bull., 1985, 18, 17–30.
57. P. Braunstein, H. Lehner, D. Matt, A. Tiripicchio and M. Tiripicchio-Camellini, Synthesis of the first Pt-Au cluster by an unexpected H⁺-substitution at trans-PtH(Cl)L₂, Angew. Chem., Int. Ed. Engl., 1984, 23, 304–304.
58. S. Sculfort and P. Braunstein, Intramolecular d¹⁰-d¹⁰ interactions in heterometallic clusters of the transition metals, Chem. Soc. Rev., 2011, 40, 2741–2760.
59. H. Schmidbaur and A. Schier, Aurophilic interactions as a subject of current research: an up-date, Chem. Soc. Rev., 2012, 41, 370–412.
60. P. Pyykkö, Strong closed-shell interactions in inorganic chemistry, Chem. Rev., 1997, 97, 597–636.
61. A. Das, U. Das and A. K. Das, Relativistic effects on the chemical bonding properties of the heavier elements and their compounds, Coord. Chem. Rev., 2023, 479, 215000.
62. N. Mirzadeh, H. Privér, A. J. Blake and H. Schmidbaur, Innovative Molecular Design Strategies in Materials Science Following the Aurophilicity Concept, Chem. Rev., 2020, 120, 7551–7591.
63. C. Cesari, B. Berti, F. Calcagno, C. Femoni, M. Garavelli, M. C. Iapalucci, I. Rivalta and S. Zacchini, Polymerization isomerism in Co-M (M = Cu, Ag, Au) carbonyl clusters: synthesis, structures and computational investigation, Molecules, 2021, 26, 1529.
64. I. Ciabatti, C. Femoni, M. Hayatifar, M. C. Iapalucci, A. Ienco, G. Longoni, G. Manca and S. Zacchini, Octahedral Co-Carbide Carbonyl Clusters Decorated by [AuPPh₃]⁺ Fragments: Synthesis, Structural Isomerism, and Aurophilic Interactions of Co₆C(CO)₁₂(AuPPh₃)₄, Inorg. Chem., 2014, 53, 9761–9770.
65. C. Cesari, M. Bortoluzzi, C. Femoni, F. Forti, M. C. Iapalucci and S. Zacchini, Peraurated ruthenium hydride carbonyl clusters: aurophilicity, isolobal analogy, structural isomerism, and fluxionality, Dalton Trans., 2024, 53, 3865–3879.
66. M. Bortoluzzi, I. Ciabatti, C. Femoni, M. Hayatifar, M. C. Iapalucci, G. Longoni and S. Zacchini, Hydride Migration from a Triangular Face to a Tetrahedral Cavity in Tetranuclear Iron Carbonyl Clusters upon Coordination of [AuPPh₃]⁺ Fragments, Angew. Chem., Int. Ed., 2014, 53, 7233–7237.
67. M. Bortoluzzi, I. Ciabatti, C. Cesari, C. Femoni, M. C. Iapalucci and S. Zacchini, Synthesis of the highly reduced [Fe₆C(CO)₁₅]⁴⁻ carbonyl carbide cluster and its reactions with H⁺ and [Au(PPh₃)]⁺, Eur. J. Inorg. Chem., 2017, 3135–3143.
68. I. Ciabatti, C. Femoni, M. Hayatifar, M. C. Iapalucci and S. Zacchini, Co₅C and Co₄C carbido carbonyl clusters stabilized by [AuPPh₃]⁺ fragments, Inorg. Chim. Acta, 2015, 428, 203–211.
69. I. Ciabatti, C. Femoni, M. C. Iapalucci, A. Ienco, G. Longoni, G. Manca and S. Zacchini, Intramolecular d¹⁰-d¹⁰ Interactions in a Ni₆C(CO)₉(AuPPh₃)₄ Bimetallic Nickel-Gold carbide Carbonyl Cluster, Inorg. Chem., 2013, 52, 10559–10565.
70. M. Bortoluzzi, I. Ciabatti, C. Femoni, M. Hayatifar, M. C. Iapalucci, G. Longoni and S. Zacchini, Peraurated nickel carbide carbonyl clusters: the cationic [Ni₆(C)(CO)₈(AuPPh₃)₈]²⁺ monocarbide and the [Ni₁₂(CO)(C₂)(CO)₁₇(AuPPh₃)₃]⁻ anion containing one carbide and one acetylide unit, Dalton Trans., 2014, 43, 13471–13475.
71. A. Ceriotti, P. Macchi, A. Sironi, S. El Afefey, M. Daghetta, S. Fedi, F. Fabrizi de Biani and R. Della Pergola, Cooperative Effects of Electron Donors and Acceptors for the Stabilization of Elusive Metal Cluster Frameworks: Synthesis and Solid-State Structures of [Pt₁₉(CO)₂₄(μ₄-AuPPh₃)₃]⁻ and [Pt₁₉(CO)₂₄{μ₄-Au₂(PPh₃)₂}₂], Inorg. Chem., 2013, 52, 1960–1964.
72. V. Dearing, S. R. Drake, B. F. G. Johnson, J. Lewis, M. McPartlin and H. R. Powell, The synthesis of the first tetra- and penta-gold decaosmium clusters, X-ray structure analysis of [Os₁₀Au₄C(CO)₂₄{P(C₆H₁₁)₃}₃], J. Chem. Soc., Chem. Commun., 1988, 1331–1333.
73. M. Bortoluzzi, C. Cesari, I. Ciabatti, C. Femoni, M. Hayatifar, M. C. Iapalucci, R. Mazzoni and S. Zacchini, Bimetallic Fe-Au Carbonyl Clusters Derived from Collman's Reagent: Synthesis, Structure and DFT Analysis of Fe(CO)₄(AuNHC))₂ and [Au₃Fe₂(CO)₈(NHC)₂]⁻, J. Cluster Sci., 2017, 28, 703–723.
74. R. Della Pergola, A. Sironi, A. Rosehr, V. Colombo and A. Sironi, N-heterocyclic carbene copper complexes tethered to iron carbidocarbonyl clusters, Inorg. Chem. Commun., 2014, 49, 27–29.
75. R. D. Adams, J. Tedder and Y. O. Wong, Phenyl-gold complexes of Ru₆ and Ru₅ carbonyl clusters, J. Organomet. Chem., 2015, 795, 2–10.
76. C.-N. Lin, C.-Y. Huang, C.-C. Yu, Y.-M. Chen, W.-M.- Ke, G.-J. Wang, G.-A. Lee and M. Shieh, Iron carbonyl cluster-incorporated Cu(I) NHC complexes in homocoupling of arylboronic acids: an effective [TeFe₃(CO)₉]²⁻ ligand, Dalton Trans., 2015, 44, 16675–16679.
77. M. K. Karunananda, S. R. Parmelee, G. W. Waldhart and N. P. Mankad, Experimental and Computational Characterization of the Transition State for C-X Bimetallic Oxidative Addition at a Cu-Fe Reaction Center, Organometallics, 2015, 34, 3857–3864.
78. R. D. Adams and G. Elpitiya, The addition of Gold and Tin to Bismuth-Triiridium Carbonyl Complexes, Inorg. Chem., 2015, 54, 8042–8048.
79. R. D. Adams, P. Dhull, V. Rassolov and Y. O. Wong, Synthesis and Reactivity of Electronically Unsaturated Dirhenium Carbonyl Compounds Containing Bridging Gold-Carbene Groups, Inorg. Chem., 2016, 55, 10475–10483.
80. B. Berti, M. Bortoluzzi, C. Cesari, C. Femoni, M. C. Iapalucci, R. Mazzoni, F. Vacca and S. Zacchini, Synthesis and Characterization of Heterobimetallic Carbonyl Clusters with Direct Au-Fe and Au⋯Au Interactions Supported by N-Heterocyclic Carbene and Phosphine Ligands, Eur. J. Inorg. Chem., 2019, 3084–3093.
81. B. Berti, M. Bortoluzzi, C. Cesari, C. Femoni, M. C. Iapalucci, R. Mazzoni and S. Zacchini, A Comparative Experimental and Computational Study of Heterometallic Fe-M (M = Cu, Ag, Au) Carbonyl Clusters Containing N-Hetericyclic Carbene Ligands, Eur. J. Inorg. Chem., 2020, 2191–2202.
82. C. Cesari, B. Berti, F. Calcagno, C. Lucarelli, M. Garavelli, R. Mazzoni, I. Rivalta and S. Zacchini, Bimetallic Co-M (M = Cu, Ag, and Au) Carbonyl Complexes Supported by N-Heterocyclic Carbene Ligands: Synthesis, Structures, Computational Investigation, and Catalysis for Ammonia Borane Dehydrogenation, Organometallics, 2021, 40, 2724–2735.
83. M. Bortoluzzi, C. Cesari, I. Ciabatti, C. Femoni, M. C. Iapalucci and S. Zacchini, Reactions of Platinum Carbonyl Chini Clusters with Ag(NHC)Cl Complexes: Formation of Acid-Base Lewis Adducts and Heteroleptic Clusters, Inorg. Chem., 2017, 56, 6532–6544.
84. C. Cesari, I. Ciabatti, C. Femoni, M. C. Iapalucci and S. Zacchini, Capping [H_8−nNi₄₂C₈(CO)₄₄]ⁿ⁻ (n = 6, 7, 8) Octa-carbide Carbonyl Nanoclusters with [Ni(CO)] and [CuCl] Fragments, J. Cluster Sci., 2017, 28, 1963–1979.
85. A. Bernardi, I. Ciabatti, C. Femoni, M. C. Iapalucci, G. Longoni and S. Zacchini, Molecular nickel poly-carbide carbonyl nanoclusters: The octa-carbide [HNi₄₂C₈(CO)₄₄(CuCl)]⁷⁻ and the deca-carbide [Ni₄₅C₁₀(CO)₄₆]⁶⁻, J. Organomet. Chem., 2016, 812, 229–239.
86. A. Bernardi, I. Ciabatti, C. Femoni, M. C. Iapalucci, G. Longoni and S. Zacchini, Ni-Cu tetracarbide carbonyls with vacant Ni(CO) fragments as borderline compounds between molecular and quasi-molecular clusters, Dalton Trans., 2013, 42, 407–421.
87. A. Bernardi, C. Femoni, M. C. Iapalucci, G. Longoni, F. Ranuzzi, S. Zacchini, P. Zanello and S. Fedi, Synthesis, Molecular Structure and Properties of the [H_6−nNi₃₀C₄(CO)₃₄(CdCl)₂]ⁿ⁻ (n = 3–6) Bimetallic Carbide Carbonyl Cluster: A Model for the Growth of Noncompact Interstitial Metal Carbides, Chem. – Eur. J., 2008, 14, 1924–1934.
88. A. Bernardi, C. Femoni, M. C. Iapalucci, G. Longoni and S. Zacchini, Cadmium-substitution promoted by nucleophilic attack of [Ni₃₀C₄(CO)₃₄(CdX)₂]⁶⁻ (X = Cl, Br, I) carbide carbonyl clusters: Synthesis and characterization of the new [H_7−nNi₃₂C₄(CO)₃₆(CdX)]ⁿ⁻ (X = Cl, Br, I; n = 5, 6, 7), Inorg. Chim. Acta, 2009, 362, 1239–1246.
89. A. Bernardi, C. Femoni, M. C. Iapalucci, G. Longoni, S. Zacchini, S. Fedi and P. Zanello, Synthesis, Structures and Electrochemistry of New Carbonylnickel Octacarbide Clusters: The Distorting Action of Carbide Atoms in the Growth of Ni Cages and the First Example of the Inclusion of a Carbon Atom within a (Distorted) Ni Octahedral Cage, Eur. J. Inorg. Chem., 2010, 4831–4842.
90. C. Capacci, C. Cesari, C. Femoni, M. C. Iapalucci, F. Mancini, S. Ruggieri and S. Zacchini, Structural Diversity in Molecular Nickel Phosphide Carbonyl Nanoclusters, Inorg. Chem., 2020, 59, 16016–16026.
91. C. Femoni, M. C. Iapalucci, G. Longoni, S. Zacchini, S. Fedi and F. Fabrizi de Biani, Nickel poly-acetylide carbonyl clusters: structural features, bonding and electrochemical behaviour, Dalton Trans., 2012, 41, 4649–4663.
92. R. D. Adams, B. Captain, W. Fu, P. J. Pellechia and M. D. Smith, Remarkable Dynamical Opening and Closing of Platinum and Palladium Pentaruthenium Carbido Carbonyl Cluster Complexes, Inorg. Chem., 2003, 42, 2094–2101.
93. R. D. Adams, B. Captain, W. Fu and M. D. Smith, Lewis Acid-Base Interactions between Metal Atoms and Their Applications for the Synthesis of Bimetallic Cluster Complexes, J. Am. Chem. Soc., 2002, 124, 5628–5629.
94. S. Saha and B. Captain, Synthesis and Structural Characterization of Ruthenium Carbonyl Cluster Complexes Containing Platinum with a Bulky N-Heterocyclic Carbene Ligand, Inorg. Chem., 2014, 21, 1210–1216.
95. R. D. Adams and B. Captain, Unusual Structures and Reactivity of Mixed Metal Cluster Complexes Containing the Palladium/Platinum Tri-t-butylphosphine Grouping, Acc. Chem. Res., 2009, 42, 409–418.
96. A. Koppaka, V. Zollo Jr., S. Etezadi, D. C. Lever and B. Captain, Addition of Pt(IPr) Groupings to Ru₅ Carbide Gives New Mixed-Metal Pt-Ru Cluster Complexes, J. Cluster Sci., 2016, 27, 1671–1681.
97. M. Kawano, J. W. Bacon, C. F. Campana, B. E. Winger, J. D. Dudek, S. A. Sirchio, S. L. Scruggs, U. Geiser and L. F. Dahl, High-Nuclearity Close-Packed Palladium-Nickel Carbonyl Phosphine Clusters: Heteropalladium [Pd₁₆Ni₄(CO)₂₂(PPh₃)₄]²⁻ and [Pd₃₃Ni₉(CO)₄₁(PPh₃)₆]⁴⁻ Containing Pseudo-T_d ccp Pd₁₆Ni₄ and Pseudo-D_3h hcp Pd₃₃Ni₉ Cages, Inorg. Chem., 2001, 40, 2554–2569.
98. J. D. Erickson, E. G. Mednikov, S. A. Ivanov and L. F. Dahl, Isolation and Structural Characterization of a Makay 55-Metal-Atom Two-Shell Icosahedron of Pseudo-I_h Symmetry, Pd₅₅L₁₂(μ₃-CO)₂₀ (L = PR₃, R = isopropyl): Comparative Analysis with Interior Two-Shell Icosahedral Geometries in Capped Three-Shell Pd₁₄₅, Pt-Centered Four-Shell Pd-Pt M₁₆₅, and Four-Shell Au₁₃₃ Nanoclusters, J. Am. Chem. Soc., 2016, 138, 1502–1505.
99. M. Bortoluzzi, I. Ciabatti, C. Femoni, M. Hayatifar, M. C. Iapalucci and S. Zacchini, [H_3−nFe₄(CO)₁₂(IrCOD)]ⁿ⁻ (n = 1, 2) and [H₂Fe₃(CO)₁₀(IrCOD)]⁻ Bimetallic Fe-Ir Hydride Carbonyl Clusters, Organometallics, 2015, 34, 189–197.
100. F. Forti, C. Cesari, M. Bortoluzzi, C. Femoni, T. Funaioli, M. C. Iapalucci and S. Zacchini, Heterometallic Ru-Ir hydride carbonyl clusters, J. Cluster Sci., 2025, 36, 6.
101. C. Femoni, F. Kaswalder, M. C. Iapalucci, G. Longoni and S. Zacchini, Copolymerization of Pt-carbonyl clusters with Lewis acids: synthesis and crystal structure of the molecular {Cd₂Cl₄[Pt₉(CO)₁₈]²⁻}₍ 1-D polymer, Chem. Commun., 2006, 2135–2137.
102. A. Bernardi, C. Femoni, M. C. Iapalucci, G. Longoni and S. Zacchini, The problems of detecting hydrides in metal carbonyl clusters by ¹H NMR: the case study of [H_4−nNi₂₂(C₂)₄(CO)₂₈(CdBr)₂]ⁿ⁻ (n = 2–4), Dalton Trans., 2009, 4245–4251.
103. C.-Y. Miu, H.-H. Chi, S.-W. Chen, J.-J. Cherng, M.-H. Hsu, Y.-X. Huang and M. Shieh, Reactions of the μ₃-sulfido triiron cluster [SFe₃(CO)₉]²⁻ with functionalized organic halides and mercury salts: selective reactivity, electrochemistry, and theoretical calculations, New J. Chem., 2011, 35, 2442–2455.
104. U. Brand and J. R. Shapley, Carbidohexarhenate Cluster Cores Bicapped by Mercury with Acetate or Thiolate Ligands, Inorg. Chem., 2000, 39, 32–36.
105. C. A. Wright and J. R. Shapley, Carbidoheptarhenate Cluster Complexes of Cadmium and Zinc Units: The Structure of [PPh₄]₂[Re₇C(CO)₂₁(μ₃-ZnCl)], Inorg. Chem., 2001, 40, 6338–6340.
106. C. Femoni, M. C. Iapalucci, G. Longoni, S. Zacchini and S. Zarra, Icosahedral Pt-Centered Pt₁₃ and Pt₁₉ Carbonyl Clusters Decorated by [Cd₅(μ-Br)₅Br_5−x(solvent)_x]^x+ Rings Reminiscent of the Decoration of Au-Fe-CO and Au-Thiolate Nanoclusters: A Unifying Approach to Their Electron Counts, J. Am. Chem. Soc., 2011, 133, 2406–2409.
107. I. Ciabatti, C. Femoni, M. C. Iapalucci, G. Longoni, S. Zacchini and S. Zarra, Surface decorated platinum carbonyl clusters, Nanoscale, 2012, 4, 4166–4177.
108. J.-F. Halet, D. G. Evans and D. M. P. Mingos, Effect of Cavity Size on the Charge Distribution in Carbido-Metal Carbonyl Clusters and Its Possible Catalytic Implications, J. Am. Chem. Soc., 1988, 110, 87–90.
109. C. E. Briant, B. R. C. Theobald, J. W. White, L. K. Bell, D. M. P. Mingos and A. J. Welch, Synthesis and X-ray structural characterization of the centred icosahedral gold cluster compound [Au₁₃(PMe₂Ph)₁₀Cl₂](PF₆)₃; the realization of a theoretical prediction, J. Chem. Soc., Chem. Commun., 1981, 201–202.
110. D. M. P. Mingos, Theoretical analyses and electron counting rules for high nuclearity clusters, J. Chem. Soc., Chem. Commun., 1985, 1352–1354.
111. M. P. Johansson and P. Pyykkö, WAu₁₂(CO)₁₂?, Chem. Commun., 2010, 46, 3762–3764.
112. N. de Silva and L. F. Dahl, Synthesis and Structural Analysis of the First nanosized Platinum-Gold Carbonyl/Phosphine Cluster, Pt₁₃[Au₂(PPh₃)₂]₂(CO)₁₀(PPh₃)₄, Containing a Pt-Centered [Ph₃Pau-AuPPh₃]-Capped Icosahedral Pt₁₂ Cage, Inorg. Chem., 2005, 44, 9604–9606.
113. F. Demartin, M. C. Iapalucci and G. Longoni, Synthesis and Characterization of Novel Types of Adducts of Nickel Carbonyl Clusters with Indium Halides: X-ray Structures of [NEt₄]₃[Ni₆(μ₃-InBr₃)(η²-μ₆-In₂Br₅)(CO)₁₁]·Me₂CO, [NEt₄]₄[Ni₆(η²-μ₆-In₂Br₅)₂(CO)₁₀]·Me₂CO, and [NEt₄]₄[Ni₁₂(μ₆-In)(η²-μ₆-In₂Br₄OH)(CO)₂₂], Inorg. Chem., 1993, 32, 5536–5543.
114. B. Berti, M. Bortoluzzi, C. Cesari, C. Femoni, M. C. Iapalucci and S. Zacchini, Reactions of [Pt₆(CO)₆(SnX₂)₂(SnX₃)₄]⁴⁻ (X = Cl, Br) with Acids: Syntheses and molecular structures of [Pt₁₂(CO)₁₀(SnCl)₂(SnCl₂)₄{Cl₂Sn(μ-OH)SnCl₂}₂]²⁻ And [Pt₇(CO)₆(SnBr₂)₄{Br₂Sn(μ-OH)SnBr₂}{Br₂Sn(μ-Br)SnBr₂}]²⁻ Platinum carbonyl clusters decorated by Sn(II)-Fragments, Inorg. Chim. Acta, 2020, 503, 119432.
115. M. Bortoluzzi, A. Ceriotti, I. Ciabatti, R. Della Pergola, C. Femoni, M. C. Iapalucci, A. Storione and S. Zacchini, Platinum carbonyl clusters stabilized by Sn(II)-based fragments: syntheses and structures of [Pt₆(CO)₆(SnCl₂)₂(SnCl₃)₄]⁴⁻, [Pt₉(CO)₈(SnCl₂)₃(SnCl₃)₂(Cl₂SnOCOSnCl₂)]⁴⁻ and [Pt₁₀(CO)₁₄{Cl₂Sn(OH)SnCl₂}₂]²⁻, Dalton Trans., 2016, 45, 5001–5013.
116. M. Bortoluzzi, A. Ceriotti, C. Cesari, I. Ciabatti, R. Della Pergola, C. Femoni, M. C. Iapalucci, A. Storione and S. Zacchini, Syntheses of [Pt₆(CO)₈(SnCl₂)(SnCl₃)₄]⁴⁻ and [Pt₆(CO)₈(SnCl₂)(SnCl₃)₂(PPh₃)₂]²⁻ Platinum-Carbonyl Clusters Decorated by SnII Fragments, Eur. J. Inorg. Chem., 2016, 3929–3949.
117. E. Brivio, A. Ceriotti, L. Garlaschelli, M. Manassero and M. Sansoni, Coordination of SnCl₂ and GeCl₂ onto a one dimensional platinum network: synthesis and structural characterization of the [PPh₄]₂[Pt₈(ECl₂)₄(CO)₁₀] (E = Sn, Ge) complexes, J. Chem. Soc., Chem. Commun., 1995, 2055–2056.
118. S. Saha, D. Isrow and B. Captain, Build-up of a Ru₆ octahedral cluster core stabilized by tert-butyl germyl ligands, J. Organomet. Chem., 2014, 751, 815–820.
119. A. B. Hungria, R. Raja, R. D. Adams, B. Captain, J. M. Thomas, P. A. Midgley, V. Golovko and B. F. G. Johnson, Single-Step Conversion of Dimethyl Terephthalate into Cyclohexanedimethanol with Ru₅PtSn, a Trimetallic Nanoparticle Catalyst, Angew. Chem., Int. Ed., 2006, 45, 4782–4785.
120. R. D. Adams, B. Captain and W. Fu, Facile introduction of bridging MPh₂ groups (M = Ge, Sn, Pb) into platinum-pentaruthenium and hexaruthenium carbide carbonyl cluster complexes, J. Organomet. Chem., 2003, 671, 158–165.
121. E. V. Grachova, P. Jutzi, B. Neumann, L. O. Schebaum, H.-G. Stammler and S. P. Tunik, Unusual selective substitution of triply bridging carbonyl ligands for GaCp* in Rh₆(CO)₁₆. Synthesis and structural characterization of the Rh₆(μ₃-CO)_4−x(μ₃-GaCp*)_x(CO)₁₂ clusters, x = 1–4, J. Chem. Soc., Dalton Trans., 2002, 302–304.
122. E. V. Grachova and G. Linti, Reactivity of InCp* Towards Transition Metal Carbonyl Clusters: Synthesis and Structural Characterization of the Rh₆(CO)_16−x(InCp*)_x Mixed-Metal Cluster Compounds, x = 1–2, Eur. J. Inorg. Chem., 2007, 3561–3564.
123. P. Jutzi, B. Neumann, G. Reunmann and H.-G. Stammler, Organometallics, 1998, 17, 1305–1314.
124. M. Bortoluzzi, I. Ciabatti, C. Femoni, T. Funaioli, M. Hayatifar, M. C. Iapalucci, G. Longoni and S. Zacchini, Homoleptic and heteroleptic Au(I) complexes containing the new [Co₅C(CO)₁₂]⁻ cluster as ligand, Dalton Trans., 2014, 43, 9633–9646.
125. I. Ciabatti, C. Femoni, M. C. Iapalucci, G. Longoni, S. Zacchini, S. Fedi and F. Fabrizi de Biani, Syntehsis, Structure, and Electrochemsitry of the Ni-Au Carbonyl Cluster [Ni₁₂Au(CO)₂₄]³⁻ and Its Relation to [Ni₃₂Au₆(CO)₄₄]⁶⁻, Inorg. Chem., 2012, 51, 11753–11761.
126. G. Manca, F. Fabrizi de Biani, M. Corsini, C. Cesari, C. Femoni, M. C. Iapalucci, S. Zacchini and A. Ienco, Inverted Ligand Field in a Pentanuclear Bow Tie Au/Fe Carbonyl Cluster, Inorg. Chem., 2022, 61, 3484–3492.
127. S. Pan, S. M. N. V. T. Gorantla, D. Parasar, H. V. R. Dias and G. Frenking, Chemical Bonding in Homoleptic Carbonyl Cations [M{Fe(CO)₅}₂]⁺ (M = Cu, Ag, Au), Chem. – Eur. J., 2021, 27, 6936–6944.
128. C. Cesari, M. Bortoluzzi, F. Forti, L. Gubbels, C. Femoni, M. C. Iapalucci and S. Zacchini, 2-D Molecular Alloy Ru-M (M = Cu, Ag, and Au) Carbonyl Clusters: Synthesis, Molecular Structure, Catalysis, and Computational Studies, Inorg. Chem., 2022, 61, 14726–14741.
129. H. Egold, M. Schraa, U. Flörke and J. Partyka, Synthesis of Novel Mixed Metal Cluster Complexes of Osmium and Mercury: Formation of the Wheel-Shaped Cluster Complexes Os₆(μ₆-Hg)(μ-PR₂)₂(CO)₂₀ (R = Ph, i-Bu), Organometallics, 2002, 21, 1925–1932.
130. C. Cesari, M. Bortoluzzi, C. Femoni, M. C. Iapalucci and S. Zacchini, Mercurophilic interactions in heterometallic Ru-Hg carbonyl clusters, Inorg. Chim. Acta, 2023, 545, 121281.
131. M. Fajardo, H. D. Holden, B. F. G. Johnson, J. Lewis and P. R. Raithby, Syntehsis and structural characterisation of the heteronuclear raft complex [Os₃(CO)₁₁Hg]₃, J. Chem. Soc., Chem. Commun., 1984, 24–25.
132. L. H. Gade, B. F. G. Johnson, J. Lewis, M. McPartlin, T. Kotch and A. J. Lee, Photochemical Core Manipulation in High-Nuclearity Os-Hg Clusters, J. Am. Chem. Soc., 1991, 113, 8698–8704.
133. B. Berti, M. Bortoluzzi, C. Cesari, C. Femoni, M. C. Iapalucci, R. Mazzoni, F. Vacca and S. Zacchini, Polymerization Isomerism in [{MFe(CO)₄}_n]ⁿ⁻ (M = Cu, Ag, Au; N = 3, 4) Molecular Clusters Supported by Metallophilic Interactions, Inorg. Chem., 2019, 58, 2911–2915.
134. P. Croizat, S. Sculfort, R. Welter and P. Braunstein, Hexa- and Octanuclear Heterometallic Clusters with Copper-, Silver-, or Gold-Molybdenum Bonds and d¹⁰-d¹⁰ Interactions, Organometallics, 2016, 35, 3949–3958.
135. N. Masciocchi, P. Cairati, F. Ragaini and A. Sironi, Ab initio XPRD structure determination of metal carbonyl clusters: the case of [HgRu(CO)₄]₄, Organometallics, 1993, 12, 4499–4502.
136. W. Gäde and E. Weiss, [(η⁵-CH₃C₅H₄)Mn(CO)₂Hg]₄, a Compound with an Mn₄Hg₄ Eight-Membered Ring and Additional Hg-Hg Bonds, Angew. Chem., Int. Ed. Engl., 1981, 20, 803–804.
137. V. G. Albano, C. Castellari, C. Femoni, M. C. Iapalucci, G. Longoni, M. Monari and S. Zacchini, Synthesis, Chemical Characterization, and Molecular Structure of Au₈{Fe(CO)₄}₄(dppe)₂ and Au₆Cu₂{Fe(CO)₄}₄(dppe)₂, J. Cluster Sci., 2001, 12, 75–87.
138. V. G. Albano, M. C. Iapalucci, G. Longoni, M. Monari, A. Paselli and S. Zacchini, Synthesis, Chemical Characterization, and Molecular Structures of Ag₈Fe₄(CO)₁₆(dppm)₂ and Ag₄Au₄Fe₄(CO)₁₆(dppe)₂, Organometallcis, 1998, 17, 4438–4443.
139. R. Della Pergola, L. Garlaschelli, M. C. Malatesta, C. Manassero and M. Manassero, A Traditional Synthetic Method, and a New Structural Motif, for Molybdenum-Gold Clusters: Synthesis and Solid-State Structure of Au₈{Mo(CO)₅}₄(PPh₃)₄, Inorg. Chem., 2006, 45, 8465–8467.
140. B. Berti, M. Bortoluzzi, C. Cesari, C. Femoni, M. C. Iapalucci, L. Soleri and S. Zacchini, Synthesis, Structural Characterization, and DFT Investigations of [M_xM’_5−xFe₄(CO)₁₆]³⁻ (M, M′ = Cu, Ag, Au; M ≠ M′) 2-D Molecular Alloy Clusters, Inorg. Chem., 2020, 59, 15936–15952.
141. C. Femoni, M. C. Iapalucci, S. Ruggieri and S. Zacchini, From Mononuclear Complexes to Molecular Nanoparticles: The Buildup of Atomically Precise Heterometallic Rhodium Carbonyl Nanoclusters, Acc. Chem. Res., 2018, 51, 2748–2755.
142. G. Longoni, M. Manassero and M. Sansoni, Synthesis and characterization of new iron-palladium and iron-platinum carbonyl anionic clusters, J. Am. Chem. Soc., 1980, 102, 3242–3244.
143. J. W. A. van der Velden, J. J. Bour, W. P. Bosman and J. H. Noordik, Reactions of Cationic Gold Clusters with Lewis Bases. Preparation and X-ray Structure Investigation of [Au₈(PPh₃)₇](NO₃)₂·2CH₂Cl₂ and Au₆(PPh₃)₄[Co(CO)₄]₂, Inorg. Chem., 1983, 22, 1913–1918.
144. Y.-B. Lee and W.-T. Wong, Synthesis and characterization of high-nuclearity osmium-silver mixed-metal clusters, Chem. Commun., 2007, 3924–3926.
145. J. Zhang and L. F. Dahl, First-known high-nuclearity silver-nickel carbonyl cluster: nanosized [Ag₁₆Ni₂₄(CO)₄₀]⁴⁻ possessing a new 40-atom cubic T_d closed-packed metal-core geometry, J. Chem. Soc., Dalton Trans., 2002, 1269–1274.
146. B. Berti, I. Ciabatti, C. Femoni, M. C. Iapalucci and S. Zacchini, Cluster Core Isomerism Induced by Crystal Packing Effects in the [HCo₁₅Pd₉C₃(CO)₃₈]²⁻ Molecular Nanocluster, ACS Omega, 2018, 3, 13239–13250.
147. I. Ciabatti, C. Femoni, M. Gaboardi, M. C. Iapalucci, G. Longoni, D. Pontiroli, M. Riccò and S. Zacchini, Structural rearrangements induced by acid-base reactions in metal carbonyl clusters: the case of [H_3−nCo₁₅Pd₉C₃(CO)₃₈]^n– (n = 0-3), Dalton Trans., 2014, 43, 4388–4399.
148. I. Ciabatti, F. Fabrizi de Biani, C. Femoni, M. C. Iapalucci, G. Longoni and S. Zacchini, Metal Segregation in Bimetallic Co-Pd Carbide Carbonyl Clusters: Synthesis, Structure, Reactivity and Electrochemistry of [H_6−nCo₂₀Pd₁₆C₄(CO)₄₈]^n– (n = 3-6), ChemPlusChem, 2013, 78, 1456–1465.
149. C. Femoni, M. C. Iapalucci, G. Longoni, C. Tiozzo and S. Zacchini, An Organometallic Approach to Gold Nanoparticles: Synthesis and X-Ray Structure of CO-Protected Au₂₁Fe₁₀, Au₂₂Fe₁₂, Au₂₈Fe₁₄, and Au₃₄Fe₁₄ Clusters, Angew. Chem., Int. Ed., 2008, 47, 6666–6669.
150. B. Berti, M. Bortoluzzi, C. Cesari, C. Femoni, M. C. Iapalucci, R. Mazzoni, F. Vacca and S. Zacchini, Thermal Growth of Au-Fe Heterometallic Carbonyl Clusters Containing N-Heterocyclic Carbene and Phosphine Ligands, Inorg. Chem., 2020, 59, 2228–2240.
151. K. Konishi, M. Iwasaki and Y. Shichibu, Phosphine-Ligated Gold Clusters with Core + exo Geometries: Unique Properties and Interactions at the Ligand-Cluster Interface, Acc. Chem. Res., 2018, 51, 3125–3133.
152. M. R. Narouz, K. M. Osten, P. J. Unsworth, R. W. Y. Man, K. Salorinne, S. Takano, R. Tomihara, S. Kaappa, S. Malola, C.-T. Dinh, J. D. Padmos, K. Ayoo, P. J. Garrett, M. Nambo, J. H. Horton, E. H. Sargent, H. Häkkinen, T. Tsukuda and C. M. Crudden, N-heterocyclic carbine-functionalized magic-number gold nanoclusters, Nat. Commun., 2019, 11, 419–425.
153. S. Zhou, W. Pei, Q. Du and J. Zhao, Foreign atom encapsulated Au₁₂ golden cages for catalysis of CO oxidation, Phys. Chem. Chem. Phys., 2019, 21, 10587–10593.
154. M. R. Radzhabov and N. P. Mankad, Activation of robust bonds by carbonyl complexes of Mn, Fe and Co, Chem. Commun., 2023, 59, 11932–11946.
155. T. Kawawaki, Y. Mitomi, N. Nishi, R. Kurosaki, K. Oiwa, T. Tanaka, H. Hirase, S. Miyajima, Y. Niihori, D. J. Osborn, T. Koitaya, G. F. Metha, T. Yokoyama, K. Iida and Y. Negishi, Pt₁₇ nanocluster electrocatalysts: preparation and origin of high oxygen reduction reaction activity, Nanoscale, 2023, 15, 7272–7279.
156. C. Schmitt, N. Da Roit, M. Neumaier, C. B. Maliakkal, D. Wang, T. Henrich, C. Kübel, M. Kappes and S. Behrens, Continuous flow synthesis of atom-precise platinum clusters, Nanoscale Adv., 2024, 6, 2459–2468.
157. Y. Negishi, Metal-nanocluster science and technology: my personal history and outlook, Phys. Chem. Chem. Phys., 2022, 24, 7569–7594.
158. F. Forti, C. Cesari, M. Bortoluzzi, C. Femoni, M. C. Iapalucci and S. Zacchini, Heterometallic Ru-Ir clusters as catalyst precursors for hydrogenation and hydrogen transfer reactions, New J. Chem., 2023, 47, 19289–19303.
159. G. Bussoli, A. Boccalini, M. Bortoluzzi, C. Cesari, M. C. Iapalucci, T. Funaioli, G. Scorzoni, S. Zacchini, S. Ruggieri and C. Femoni, Atomically precise rhodium-indium carbonyl nanoclusters: synthesis, characterization, crystal structure and electron-sponge features, Nanoscale, 2024, 16, 17852–17867.
160. C. Femoni, M. C. Iapalucci, G. Longoni, J. Wolowska, S. Zacchini, P. Zanello, S. Fedi, M. Riccò, D. Pontiroli and M. Mazzani, Magnetic Behavior of Odd- and Even-Electron Metal Carbonyl Clusters: The Case Study of [Co₈Pt₄C₂(CO)₂₄]ⁿ⁻ (n = 1, 2) Carbide Cluster, J. Am. Chem. Soc., 2010, 132, 2919–2927.
161. E. I. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker and W. R. Pretzer, Clusters and Surfaces, Chem. Rev., 1979, 79, 91–137.
162. E. I. Muetterties and J. Stein, Mechanistic Features of Catalytic Carbon Monoxide Hydrogenation Reactions, Chem. Rev., 1979, 79, 479–490.
163. C. R. Cobb, R. K. Ngo, E. J. Dick, V. M. Lynch and M. J. Rose, Multi-phosphine-chelated iron-carbide clusters via redox-promoted ligand exchange on an inert hexa-iron-carbide carbonyl cluster, [Fe₆(μ₆C)(μ₂-CO)₄(CO)₁₂]²⁻, Chem. Sci., 2024, 15, 11455–11471.
164. C. Joseph, C. R. Cobb and M. J. Rose, Single-Step Sulfur Insertion into Iron Carbide Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco Analogues, Angew. Chem., Int. Ed., 2021, 60, 3433–3437.
165. L. K. Batchelor, B. Berti, C. Cesari, I. Ciabatti, P. J. Dyson, C. Femoni, M. C. Iapalucci, M. Mor, S. Ruggieri and S. Zacchini, Water soluble derivatives of platinum carbonyl Chini clusters: synthesis, molecular structures and cytotoxicity of [Pt₁₂(CO)₂₀(PTA)₄]²⁻ and [Pt₁₅(CO)₂₅(PTA)₅]²⁻, Dalton Trans., 2018, 47, 4467–4477.
166. X. Wang and M. Zhu, Structural Isomerism in Atomically Precise Nanoclusters, Chem. Mater., 2021, 33, 39–62.
167. J.-H. Huang, X.-Y. Dong, Y.-J. Wang and S.-Q. Zang, Generation and manipulation of chiroptical activities in coinage-metal clusters, Coord. Chem. Rev., 2022, 470, 214729.
168. C. Cesari, C. Femoni, F. Forti, M. C. Iapalucci, G. Scorzoni and S. Zacchini, Isomerism in Molecular Metal Carbonyl Clusters, Eur. J. Inorg. Chem., 2024, 27, e202400220.
169. C. Cesari, I. Ciabatti, C. Femoni, M. C. Iapalucci, F. Mancini and S. Zacchini, Heteroleptic Chini-Type Platinum Clusters: Synthesis and Characterization of Bis-Phosphine Derivatives of [Pt_3n(CO)_6n]²⁻ (n = 2–4), Inorg. Chem., 2017, 56, 1655–1668.
170. M.-C. Hsu, R. Y. Lin, T.-Y. Sun, Y.-X. Huang, M.-S. Li, Y.-H. Li, H.-L. Chen and M. Shieh, Inorganic-organic hybrid Cu-dipyridyl semiconducting polymers based on the redox-active cluster [SFe₃(CO)₉]²⁻: filling the gap in iron carbonyl chalcogenide polymers, Dalton Trans., 2024, 53m, 7303–7314.
171. X. Zhao, W.-J. Chen, Q.-M. Liang, S.-K. Chen, J. Xun, B.-J. Geng, H.-F. Su and Y. Yang, Ag⁺-Induced Assembly of Pt Clusters for Photocatalytic Hydrogen Production, Inorg. Chem., 2024, 63, 17672–17680.
172. K. L. Kollmannsberger, Poonam, C. Cesari, R. Khare, T. Kratky, M. Boniface, O. Tomanec, J. Michalička, E. Mosconi, A. Gagliardi, S. Günther, W. Kaiser, T. Lunkenbein, S. Zacchini, J. Warnan and R. A. Fischer, Mechanistic Insights into ZIF-8 Encapsulation of Atom-Precise Pt(M) Carbonyl Clusters, Chem. Mater., 2023, 35, 5475–5486.
173. P. M. Schneider, K. L. Kollmannsberger, C. Cesari, R. Khare, M. Boniface, B. Roldán Cuenya, T. Lunkenbein, M. Elsner, S. Zacchini, A. S. Bandarenka, J. Warnan and R. A. Fischer, Engineering ORR Electrocatalysts from Co₈Pt₄ Carbonyl Clusters via ZIF-8 Templating, ChemElectroChem, 2024, 11, e202300476.
174. D. Bonincontro, A. Lolli, A. Storione, A. Gasparotto, B. Berti, S. Zacchini, N. Dimitratos and S. Albonetti, Pt and Pt/Sn carbonyl clusters as precursors for the synthesis of supported metal catalysts for the base-free oxidation of HMF, Appl. Catal., A, 2019, 588, 117279.
175. F. Bertolotti, D. Moscheni, A. Migliori, S. Zacchini, A. Cervellino, A. Guagliardi and N. Masciocchi, A total scattering Debye function analysis study of faulted Pt nanocrystals embedded in a porous matrix, Acta Crystallogr., 2016, A72, 632–644.
176. I. Robinson, S. Zacchini, L. D. Tung, S. Maenosono and N. T. K. Thanh, Synthesis and Characterization of Magnetic Nanoalloys from Bimetallic Carbonyl Clusters, Chem. Mater., 2009, 21, 3021–3026.
177. D. A. Serban, P. Greco, S. Melinte, A. Vlad, C. A. Dutu, S. Zacchini, M. C. Iapalucci, F. Biscarini and M. Cavallini, Towards All-Organic Field-Effect Transistors by Additive Soft Lithography, Small, 2009, 10, 1117–1122.
178. P. Greco, M. Cavallini, P. Stoliar, S. D. Quiroga, S. Dutta, S. Zacchini, M. C. Iapalucci, V. Morandi, S. Milita, P. G. Merli and F. Biscarini, Conductive Sub-micrometric Wires of Platinum-Carbonyl Clusters Fabricated by Soft-Lithography, J. Am. Chem. Soc., 2008, 130, 1177–1182.

---