f-Element silicon and heavy tetrel chemistry

The last three decades have seen a significant increase in the number of reports of f-element carbon chemistry, whilst the f-element chemistry of silicon, germanium, tin, and lead remain underdeveloped in comparison. Here, in this perspective we review complexes that contain chemical bonds between f-elements and silicon or the heavier tetrels since the birth of this field in 1985 to present day, with the intention of inspiring researchers to contribute to its development and explore the opportunities that it presents. For the purposes of this perspective, f-elements include lanthanides, actinides and group 3 metals. We focus on complexes that have been structurally authenticated by single-crystal X-ray diffraction, and horizon-scan for future opportunities and targets in the area.


Introduction
Recent decades have seen a rapid growth in the development and study of f-elements for a wide variety of applications including catalysis, 1,2 imaging, 3,4 data storage 5,6 and nuclear fuel. 7,8 The chemistry of the f-elements, the lanthanides (Ln; dened here to include the group 3 metals scandium, yttrium and lanthanum as well as cerium to lutetium inclusive) and actinides (An), is dened by predominantly ionic bonding regimes, high coordination numbers, and a preference for hard f-element cations to bond with hard bases (by the HSAB hard-so-acid-base denition). As such, the aqueous solution chemistry of the f-elements is dominated by compounds containing essentially electrostatic bonds between the f-element ion and the most electronegative p-block elements (e.g. halides, O, N). 9 Non-aqueous techniques have facilitated the recent expansion of f-element heavy chalcogen and pnictogen chemistry; [10][11][12][13] f-element carbon chemistry is now mature, with a wide variety of ligand types extensively reviewed including alkyls, 14,15 carbenes, 16-18 cyclopentadienyls 19,20 and arenes. [21][22][23] However, the f-element solution chemistry of heavier tetrels has remained in the shadow of the lightest member carbon; this is exemplied by analysis of the number of crystallographically characterised examples of f-element carbon and silicon s-bonds that have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as of August 2020 (Fig. 1). The contrast is even greater upon descent of Group 14, with only a handful of examples of structurally authenticated complexes of f-elements bonded to germanium (4 examples), tin (11 examples) or lead (2 examples) in total. 24 The paucity of f-block-heavier tetrel linkages is representative of the chemistry of these elements being less developed than the d-transition metals (TM); 9,25 this is also exemplied by a comparative dearth of f-element-metal bonds. 26 Although interest in f-element silicon chemistry has started to grow in recent years, the current number of f-element-silicon bonds reported (63 examples) is comparable to the total number of f-element-carbon bonds that were reported by 1986. 24 The most investigated application of complexes containing M-Si bonds to date is the (hydro)-silylation of unsaturated hydrocarbons; TM complexes have been shown to promote this chemistry for decades, and attention has turned to the f-block for comparative studies. [27][28][29][30] Also of signicance is the potential application of uranium silicides (e.g. U 3 Si 2 , USi 2 ) as alternatives to conventional UO 2 fuel due to an increase in uranium density and a larger thermal conductivity in the former materials, potentially allowing prolonged generation of energy from nuclear fuels. [31][32][33] Given historical experimental and computational limitations, early reports of f-element silicon chemistry lack in-depth analysis of the bonding and properties of the M-Si linkage compared to what can be done now; a higher level of analysis has only started to be practicable recently and these fundamental studies are the necessary rst step for this eld to develop further. The increasing rate of development in felement silicon chemistry in the last two decades has provided the motivation for this perspective; the heavier tetrels are discussed herein to inspire the future development of the felement chemistry of these elements also, but silicon remains the focus.
A silicon donor atom is commonly introduced to an felement metal centre in one of two ways: (1) salt elimination/ metathesis of a group 1 or 2 metal silanide anion (SiR 3 À ) transfer agent (Group 11 and 12 metal silanides are also known but are less commonly used) with an f-element halide-precursor to produce a polarised-covalent f-element tetrel linkage; or, (2) dative coordination of a neutral Si(II) silylene reagent to form an adduct with an f-element complex that has a vacant coordination site, or, where these sites can be generated in situ by the displacement of weakly bound donor solvent molecules. The chemistry of both of these silicon reagent families have previously been reviewed. [34][35][36] Examples of f-element silylene adducts are relatively scarce, and f-element silanide complexes are dominated by the tris-(trimethylsilyl)silanide anion, ({Si(SiMe 3 ) 3 }) À , frequently referred to as hypersilanide, and its derivatives.
This perspective highlights work reported in the eld of felement silicon and heavier tetrel chemistry to August 2020, focusing on structurally characterised examples Table 1. Our aims are to show the state-of-the-art in the eld, the limitations of our current understanding, and to inspire researchers to develop and progress f-element silicon chemistry more rapidly in future. This perspective is split into four sections, with separate components on Ln(II)-, Ln(III)-, and An-Si chemistry, and a nal section on f-element heavy tetrel chemistry, with subsections for ease of reference.

(A) Ln(II)-Si chemistry
Ln(II) silicon complexes to date are only known with the classical divalent ions Sm, Eu and Yb, as divalent precursors are most readily available for these elements. 37 However, since 1999 synthetic routes have been developed for solvated LnI 2 precursors for Tm, Dy and Nd, 38 and the 2+ oxidation state is now known for all Ln (with the exception of radioactive Pm), 39 hence an extension of Ln(II) silicon chemistry to other elements in this series is feasible. Considering that the 2+ oxidation state is considerably underdeveloped for the Ln series compared to Ln(III) chemistry, complexes containing Ln(II)-Si bonds are relatively abundant as soer Ln(II) centres are more amenable to bonding with silicon. 9 Ln(II) silicon complexes are organised into three categories herein: (1) Ln(II) silanide complexes that have their coordination spheres completed by THF; (2) Ln(II) silanide complexes that are stabilised by supporting ancillary ligands; (3) Ln(II) silylene complexes.

A1. Ln(II) silanide complexes with their coordination spheres completed by THF
THF is the one of the most commonly utilised coordinating solvents in non-aqueous f-element chemistry, particularly as Ln Fig. 1 Graph depicting the number of structures deposited into the CCDC per year (as of August 2020) for Ln/An-C/Si s-bonds. 24 Total David P. Mills is a Reader at the University of Manchester Department of Chemistry, where he started his independent academic career in 2012. His current research interests are centred around the synthesis, stabilisation and properties of complexes with unusual geometries, oxidation states and bonding motifs, predominantly with the f-block elements.  36  In addition, this paper also included the rst examples of using DFT calculations to probe the nature of the Ln-Si bonds; these computed results supported NMR spectroscopic data in the assignment of a highly shielded anionic silanide fragment.
In 2017 Sadow and co-workers reacted two equivalents of a smaller derivative of potassium hypersilanide, [K{Si(SiMe 2 -H) 3 }], with solvated YbI 2 to afford [Yb{Si(SiMe 2 H) 3 } 2 (THF) 3 ] (4, Fig. 2). 42 Complex 4 features b-Si-H groups, which have frequently been employed in silylalkyl and silylamide chemistry, to stabilise the complex with electrostatic interactions between the metal centre and the electron density associated with the b-Si-H bond.
Complexes 2-Ln and 4 exhibit approximate trigonal bipyramidal geometries, with the two silanide ligands and one THF molecule in the trigonal plane, and the remaining two THF molecules in axial positions. The Si-Ln-Si angles of complexes 2-Ln do not vary greatly with a change in Ln (mean 123.45(5) ), though the Si-Ln-Si angle for 2-Yb (124.51 ) is signicantly smaller than that of 4 (129.69(6) ) due to differences in the size of the silanide ligands. Complexes 3-Ln exhibit mean Si-Ln-Si angles of 90.727(10) as a result of the two hypersilanide moieties being tethered together; this results in pseudo-cisoctahedral geometries with the remainder of the Ln coordination spheres completed by four THF molecules. Ln-Si bond lengths vary with the decreasing size of Ln radii across the series as expected; 43 this is particularly apparent for the 2-Ln and 3-Ln families, with the Yb-Si bond lengths approximately 0.1 A shorter than the respective Sm/Eu-Si distances for the same ligand set. The Ln-O distances corresponding to the THF molecules in 1-4 appear to be essentially independent of the identity of the silanide ligand.
In the same publication, Baumgartner, Marschner and coworkers also explored functionalised silanide ligands featuring oxygen donor atoms to suppress THF coordination to a greater extent. 44 Although THF molecules remained coordinated in the products isolated from initial work towards this goal, these complexes exhibited distinctive geometrical features. The bis-silanide ligands in [Ln{[Si(SiMe 3 ) 2 SiMe 2 ] 2 -O}(THF) 3 ] (7-Ln; Ln ¼ Sm, Yb, Fig. 3) contain Si-O-Si linkages that additionally coordinate the Ln(II) ions on the same hemisphere as the two silanide groups, providing distorted facoctahedral geometries. The two chelating silyl-ether ligands in [Yb{[Si(SiMe 3 ) 2 SiMe 2 ]OMe} 2 (THF)] (8, Fig. 3) are derived from the addition of a methoxy group at the terminus of the oligosilane. The Yb(II) centre in 8 exhibits an approximate squarebased pyramidal geometry, with a THF molecule occupying the axial position and the Si-and O-donors of the two ligands coordinated in a mutually trans-arrangement. The authors concluded in this work that THF could not be readily avoided as a reaction solvent, and that increased steric bulk and coordinating heteroatoms in the ligand scaffold would be required to furnish Ln(II) silanide complexes that are free of coordinated donor-solvents. 44 Further work targeting donor solvent-poor or -free complexes in Ln(II) silanide chemistry saw the development of a multidentate silocanylsilanide ligand, {Si(SiMe 3 ) 2 Si(Me)[{OCH 2 -CH 2 } 2 NMe]} À , which contains four heteroatoms that can potentially donate electron density to metal centres; three of these sites are coordinated to Ln(II)  This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 10871-10886 | 10875 Perspective Chemical Science À182.0 ppm) indicate predominantly silanide character. 44 Although 9-Ln were found to be sensitive to visible light, the lack of donor solvent led to a drastic improvement in their stability compared to the solvated complexes reported previously. Around the same time, Baumgartner and co-workers explored differing degrees of electron density on a silatranesubstituted silicon atom and the effect on the Si-N interaction detected by 29 Si NMR spectroscopy and single crystal XRD analysis. 45 In this study the authors synthesised a variety of metal silanide complexes, including the Yb Fig. 4). One of the silatrane oxygen atoms was found to coordinate to the Yb centre in the solid state structure of 10, which was in contrast to other complexes presented in this study i.e. an analogous reaction of the potassium silyl-silatrane with ZnBr 2 lead to the formation of a completely linear Si-Zn-Si (180 ) Fig. 5), with elimination of LiCp*. 46 This study was the rst example of Yb-Si coupling observed by both 29 Si and 171 Yb NMR spectroscopy, with J Yb-Si ¼ 829 Hz. The amount of s-character in the Si sp 3 -hybridised orbital directed at the Yb metal centre primarily determines the strength of coupling; a larger coupling constant is indicative of a stronger, more covalent Yb-Si bond. 46 In 2003, Hou and co-workers also used Cp* as a supporting ancillary ligand when targeting the synthesis of the phenylsilanide complexes [Ln(Cp*) 2 Fig. 5). 48 Unlike in the previous example of the synthesis of 11, there was no elimination of the expected by-product [K {N(SiMe 3 ) 2 }] in this reaction. Complex 13 exhibits an approximate trigonal planar geometry about Yb with respect to the silanide and amide donor atoms, and is additionally stabilized by Sib-Cg agostic-type interactions from the framework of the silylamide ligand. 48 The K-N bond distances observed in 13 (2.909(3) A) are longer than those found in the structurally similar dimer [K{N(SiMe 3 ) 2 }] 2 (2.787(3) A); from these data Niemeyer suggested the interactions of the silylamide with ytterbium and potassium are competing with a stronger Yb-N interaction observed due to preferential binding to the harder Lewis acid, resulting in longer K-N bonds. The Yb-Si bond length in 13 (3.0387(10) A) is not signicantly different to other reported Yb-Si bond lengths [range: 3.017(4)-3.0644 (7) A], 24 suggesting the potassium cation plays a spectator role in the formation of 13.

A3. Ln(II) complexes containing silylene ligands
Silylenes (R 2 Si) are silicon analogues of carbenes and are two electron s-donor ligands that can form adducts with metals through dative bonding. 49 Analogous to N-heterocyclic carbene (NHC) chemistry, N-heterocyclic silylenes (NHSi) are relatively well-developed due to the push-pull mesomeric-inductive stabilisation mechanism provided by the two N-substituents. 50 The [Sm(Cp*) 2 {C(NMeCMe) 2 }], the authors suggested that NHSi ligands bind to Sm(II) centres with a similar bond strength to NHCs, 52 though there is not always a direct correlation of bond strength and bond length in "long bond" organometallics. 51 In 2015 Baumgartner investigated Sm(II) silylene complexes, this time employing an amidinate substituent and a supporting alkoxide or aryloxide substituent to generate the 3-coordinate Fig. 6). 53 These complexes exhibit signicantly lower room temperature magnetic moments (15: 2.7 m B ; 16: 2.6 m B ) than the starting material [Sm(Cp*) 2 (OEt 2 )] (3.6 m B ). Although there is a considerable difference in the magnetic moments of 15, 16, and the starting material [Sm(Cp*) 2 (OEt 2 )], DFT analyses indicated a predominantly electrostatic donor-acceptor type interaction. 53 Roesky and co-workers have recently reported the synthesis and reactivity of the bis(silylene)-coordinated Eu(II) and Yb(II) Fig. 6), 54 56 In later work, Tilley and Sadow investigated why only bulky primary and secondary silanes could be used in alkane elimination reactions to produce Sc-Si bonds, discussed possible mechanisms for the formation of such bonds, and explored the catalytic potential of 19-Sc and 21 in the alkylation of silanes by s-bond metathesis. 57 Subsequently, Tilley and co-workers investigated the reactivity of the rst Sc-Si linkage in [Sc(Cp) 2 {Si(SiMe 3 ) 3 }(THF)] (22, Fig. 7), which was prepared from the reaction of the Sc(III) dimer [{Sc(Cp) 2 (m-Cl)} 2 ] with two equivalents of [Li{Si(SiMe 3 ) 3 }]. 58 Analogous Sc(III) complexes containing other silanide ligands ({Si(SiMe 3 ) 2 Ph} À , {Si t BuPh 2 } À and {SiPh 3 } À ) were characterised by elemental analysis, and IR and NMR spectroscopy, but no single crystal XRD data was reported for these analogues.   Fig. 7), which contains a rare example of a Lu-Si bond. 59 Although the authors did not explicitly explore the reactivity of the Lu-Si linkage in 23, they did investigate the utility of Lu complexes in the hydrogenolysis of organosilanes, including the conversion of phenylsilane into benzene and polysilanes under an atmosphere of dihydrogen; this work concluded that these catalytic processes proceed via a Lu/Si transition state. 59 60 Unusually for f-element silicon chemistry, 24-Ln and 25-Ln feature halides as ancillary ligands; these distorted octahedral complexes exhibit mer-congurations and were found to rapidly decompose when exposed to vacuum, signifying the facile loss of THF and the importance of the saturation of the metal coordination spheres to their stability. Migratory insertion reactions of isocyanide and carbodiimides into the Ln-Si bonds of 24-Ln and 25-Ln were performed, exemplifying the reactivity of these linkages and mirroring that which is known for f-element alkyl complexes. 15 Analysis of the multiplicity of the 29 Si NMR spectra in diamagnetic 24-Y and 25-Y was shown to be a useful tool for monitoring these insertion reactions.
In 1996, Tilley and Rheingold reported that reaction of the Sm(III) alkyl complex [Sm(Cp*) 2 {CH(SiMe 3 ) 2 }] with the secondary phenyl silane Ph 2 SiH 2 produces the trinuclear cluster [Sm 3 Cp* 6 (m-Si 2 H 4 )(m-SiH 3 )] (26A, Fig. 8) by alkane elimination and silane redistribution, with triphenylsilane (Ph 3 SiH) as a byproduct. 61 On the assumption that all the Sm centres in 26A remain in the 3+ oxidation state and guided by the Si-Si bond length of 2.458(7) A, the depiction of 26A in Fig. 8 is an accurate representation of the product. In contrast, the reaction of [Sm(Cp*) 2 {CH(SiMe 3 ) 2 }] with the primary silane PhSiH 3 furnished a distribution of phenylsilanes as well as three trinuclear clusters: 26A, [Sm 3 Cp* 6 (m-SiH 3 ) 3 ] (26B) and [Sm 3 Cp* 6 (m-Si 3 H 6 )(m-SiH 3 )] (26C); in all of these aggregates the SiH 3 ligands each bridge two Sm(III) centres (Fig. 8). Analysis of disorder in the crystal structure determined that the product distribution ratio was 1 : 5 : 4 for 26A : 26B : 26C. 61 The authors reasoned that the reaction with a less substituted and more hydride-rich silane is the reason for 26B and 26C being present in a higher abundance than 26A in these mixtures. In later work, Tilley and Castillo showed that the addition of hard Lewis bases such as Ph 3 PO and (Me 2 N) 3 PO to reaction mixtures led to the trinuclear aggregates being broken down into mononuclear complexes of the general formula [Sm(Cp*) 2 (SiH 3 )(L)] (L ¼ Lewis base), although no solid state structures were reported; these species presumably contain terminal Sm-SiH 3 linkages, which were found to be more amenable to reactivity studies, including the 1,2-migratory insertion of benzophenone into the Sm-Si bond. 62 Gambarotta and Korobkov reported the rst structurally  Fig. 8) was found to form from a Tm-pyrrolide/ aluminate complex. 63 The Tm centre in 27 was assigned a 3+ oxidation state based on the pale colour of the complex. The SiH 3 unit of the aluminate-based ligand in 27 was disordered over two positions in a ratio of 64 : 36 in the single crystal XRD data, with Tm-Si distances of 3.087(6) A and 2.573(6) A in the respective components, indicating that there is some delocalisation of charge in this ligand that engenders a high degree of exibility in how it binds to the metal.
In the same paper where the synthesis of 4 was disclosed, Sadow and co-workers reported the reaction of YCl 3 with three equivalents of [K{Si(SiMe 2 H) 3 }] in diethyl ether at À78 C for 8 hours to afford a polymeric yttrium silanide 'ate' complex, [Y {Si(SiMe 2 H) 3 } 2 (OEt 2 )(m 2 -Cl) 2 (m 3 -Cl)K 2 (OEt 2 ) 2 ] N (28, Fig. 8). 42 The authors found the characterisation of 28 problematic due to its rapid decomposition at room temperature in both solution and the solid state into unidentied silyl-containing species. The solid state structure of 28 features monomeric units with approximately octahedral Y(III) centres, with trans-disposed silanide groups with mean Y-Si bond lengths of 3.035(1) A, which are signicantly longer than the corresponding distances in 24-Y (2.9613(18) A) and 25-Y (2.979(3) A). 60 The equatorial plane about the Y(III) centres in 28 is composed of one molecule of diethyl ether and three chlorides, with the halides bridging to two potassium cations to form a six-membered YCl 3 K 2 ring; each potassium vertex is capped with a molecule of diethyl ether. The 1D polymeric chain in the solid state structure of 28 is formed by the potassium centres completing their coordination spheres by each bridging to a chloride and one of the H-Si groups of the SiMe 2 H substituents. A low temperature (200 K) 29 Si NMR spectrum of 28 exhibited a resonance at À141.6 ppm for the Y-Si atoms and a signal at À9.1 ppm for the SiMe 2 H moieties. 42

B2. Charged Ln(III) silanide complexes
The rst report of structurally authenticated Ln-Si bonds was in 1985 by Schumann and co-workers, where they synthesised and   43 DFT calculations indicated that the amount of covalency in the Ln-Si bond also decreases across the Ln series; although the bonding is predominantly ionic in all cases, less orbital extension of the silicon sp 3 -hybridized lone pair towards the metal was suggested for the smaller Ln. 53 In a later report, Baumgartner and co-workers reported a samarium analogue 30-Sm to add to the 30-Ln series. 67 In 2018, Evans and co-workers investigated the reduction of a Y(III) complex [Y(C 5 H 4 Me) 3 Fig. 10). 68 Evans and co-workers attempted to characterise 32 by 1 H NMR spectroscopy, but they could not identify the -SiH   Chemical Science complexes discussed in Section A3, the silylene ligands in 35-Ln dissociate in solution; this was monitored for the Y analogue by variable temperature 29 Si NMR spectroscopy studies, which showed complete dissociation at 338 K. A J Y-Si coupling constant of 59 Hz was found for 35-Y, which is of a comparable magnitude to J Y-C coupling constants for yttrium alkyl complexes, thus the authors suggested that there is some orbital overlap in the Y-Si bond. Lappert and co-workers noted that as [Y(Cp) 3 ] and [La(Cp) 3 ] are polymeric chains in the solid state, the breaking of these chains is required to form the monomeric complexes 35-Ln, thus they postulated in the case of La the energy required to rupture the polymeric chain is too high for the silylene to overcome. The authors concluded that Ln-Si bond formation in this case is not dependent on the size of the metal but is instead based upon the ability of the silylene binding to overcome the depolymerisation term associated with the [Ln(Cp) 3 ] starting material.

(C) An-Si chemistry
In comparison to the Ln series, the silicon chemistry of Ans has developed very slowly. Practical issues related to radiological hazards limits the number of researchers working with naturally occurring uranium and thorium, and the hazard and scarcity of synthetic transuranic elements (Np, Pu, Am, etc.) requires specialist facilities, hindering the development of their chemistry further. 25 There are only ve crystallographically authenticated examples of complexes exhibiting An-Si bonds to date, however, there have been several other reports of complexes containing An-Si bonds that have been characterised by other techniques. Therefore, this section will be arranged into the following two categories; (1) noncrystallographically authenticated examples of An-Si complexes; (2) crystallographically authenticated An-Si complexes.

C1. Non-crystallographically authenticated examples of An-Si complexes
The rst report of a complex exhibiting an An-Si bond was in 1989 by Porchia and co-workers, which followed previous reports of germanium and tin analogues (see Section D). [70][71][72] In the silanide work the authors reacted [U(Cp) 3 Cl] with a freshly prepared sample of [LiSiPh 3 ] at low temperature in THF. Aer 30 minutes the reaction was worked up to yield a brown-green powder, which was identied as the U(IV) complex [U(Cp) 3 (-SiPh 3 )] (36, Fig. 12) by consideration of elemental analysis and mass spectrometry data. 72 The authors noted that if the synthesis of 36 was attempted with the potassium salt [KSiPh 3 ] then a mixture of products was obtained, and if the reaction mixture was warmed to room temperature in ethereal solvents the main product formed was [U(Cp) 3 (OSiPh 3 )]. The IR spectrum of 36 exhibits vibrational band positions and intensities that are comparable to previously reported germanium and tin homologues, 70,71 indicating that the uranium centre has a similar coordination environment and further supporting the formulation of 36. An interesting reactivity prole of 36 is the sbond metathesis reaction with HSnPh 3 to yield [U(Cp) 3 (SnPh 3 )], with the thermodynamic driving force being the greater pK a of the Sn-H bond versus the Si-H bond formed. In a follow up report Nolan et al. investigated the thermochemistry of 36 through 1 H NMR spectroscopy titrations and iodinolytic calorimetry experiments, through which they determined a bond dissociation energy of 35(4) kcal mol À1 for the U-Si bond, indicating that this linkage is relatively unstable and is prone to displacement or insertion of oxygen, as had already been evidenced in the initial work. 73 In continued efforts to explore the energetics of metalsilicon bonds, in 1995 Marks and King synthesised a variety of group 4 and f-element silicon complexes and collected thermochemical data to provide insights on the bonding and reactivity of these complexes. 74 Within this report they synthesised [U(Cp) 3 {Si(SiMe 3 ) 3 }] (37, Fig. 12) from the salt metathesis reaction of [U(Cp) 3 Cl] with [(THF) 3 Li{Si(SiMe 3 ) 3 }] in diethyl ether. Elemental analysis and 1 H NMR spectroscopy both supported the formulation of 37; the bond dissociation energy calculated for 37 was determined to be 37(3) kcal mol À1 , indicating that the U-Si linkage in this complex is stronger than the corresponding bond in 36.
Also in 1995, Tilley and Rheingold reported the double insertion reaction of carbon monoxide into Th-Si bonds. 75 The authors rst reacted [Th(Cp*) 2 (Cl) 2 ] with [(THF) 3 Li{Si(SiMe 3 ) 3 }] to give [Th(Cp*) 2 (Cl){Si(SiMe 3 ) 3 }] (38, Fig. 12), however, this yellow complex readily decomposed into [Th(Cp*) 2 (Cl) 2 ], HSi(SiMe 3 ) 3 and other products. As a result of this instability 38 was only characterised by 1 H NMR spectroscopy, but when it was formed in a pressurised CO atmosphere carbonylation occurred to yield the silylated ketene product [Th(Cp*) 2 Fig. 13). 77 Complexes 41 and 42 have U-Si bond lengths of 3.1637(7) and 3.1750(6) A, respectively. As a result of containing a bulkier silylene, one of the amidinates bound to silicon in 42 has switched to a monodentate binding mode upon coordinating to uranium to minimise steric strain (see Fig. 13). No signal was observed in the 29 Si NMR spectra of 41 and 42 for the silicon atoms bonded to uranium, but these spectra revealed that the Cp 0 silicon signal was largely unchanged from the starting material. These data, coupled with relatively strong f-d transitions in the visible region of UV-Vis-NIR spectra indicate a U(III) oxidation state in 41 and 42. Moreover, for 42 the UV-Vis-NIR spectrum is largely the summation of the individual starting materials, whereas FTIR spectroscopy indicates that the U-Si bond is intact in the solid state, suggesting that in solution 42 is in a dynamic equilibrium. These data imply that the strength of the U-Si bonding interactions in 41 and 42 is strongly dependent upon the steric effects of the silylene used. DFT calculations support the experimental evidence of weak U-Si interactions in 41 and 42, with respective bond dissociation energies of 11.9 and 6.9 kcal mol À1 determined. Complex 41 was found to have a low-lying bonding molecular orbital for the U-Si bond, whereas this is more accessible for 42 and is therefore more easily perturbed. The U-Si interactions in both complexes are best described as polarised s-bonding, with the second-order NBO level revealing a signicant p-back bonding component in 41, which is not as prevalent in 42.
We . Quantum chemical calculations revealed strongly polarised single An-Si s-bonds, with largely similar 7s/6d/5f An contributions to the An-Si bonds for Th and U; these were quantied by the QCT interatomic exchange-correlation energy, V XC , to provide a covalency metric for the An-Si interaction of À0.092 and À0.096 for 43-Th and 43-U respectively, which agree with both NBO-based metrics and delocalisation indices. The An-Si interactions in 43-An were found to be kinetically stable in the solid state and in solution for a range of non-aqueous solvent systems, which was attributed to the strong polarised covalent An-Si bonds between the actinide ion and the hypersilanide ligand. 80

(D) Germanium, tin & lead
There have only been a handful of reports of f-element complexes containing bonds with heavier tetrels, thus their chemistry is far less developed than silicon. To date no structurally authenticated examples of actinide germanium complexes have been reported, though [U(Cp) 3 (GePh 3 )] has been synthesised and characterised by elemental analysis and mass spectrometry. 71 All four examples of lanthanide germanium complexes to date are exclusively limited to phenyl substituted ligands; additionally, only three different lanthanides been employed in these studies. For tin there are multiple examples which span a number of ligand types including stannanide and stannylene across both lanthanides and actinides, including a number of Zintl clusters reported in recent years. 81,82 Finally, there are only two reports of molecular f-block plumbylene complexes, however again there are a number of Chemical Science reports of lead Zintl clusters. 83,84 This section will summarise molecular examples of f-element complexes of the heavier tetrels in separate sections.

D1. Germanium
The rst structurally authenticated example of an f-block germanium complex was disclosed in 1994 by Bochkarev and co-workers in the same report as the rst lanthanide(II)-silanide complex (1). 40 The authors reported the synthesis of [Yb(GePh 3 ) 2 (THF) 4 ] (44, Fig. 14), in the same manner as 1, . 85 The cisarrangement of the ytterbium-bound germanium atoms in 45 rival the trans-arrangement found in 44, with a shorter Yb-Ge bond length seen in the metallacycle (3.104(2) A). 85 In 1999, Schumann and co-workers reported the synthesis and structure of [Eu(GePh 3 ) 2 (DME) 3 ] (46, Fig. 14) from the reaction of [Eu(C 10 H 8 )(THF) 2 ] with two equivalents of Ph 3 GeH liberating dihydrogen as a by-product to yield 46. 86 Additionally, the synthesis of the mono-germanide complex [Eu(GePh 3 )(-I)(DME) 2 ] was reported from the reaction of C 10 H 8 [EuI(DME) 2 ] 2 with two equivalents of Ph 3 GeH, although the product was found to be unstable in solution and rapidly disproportionated to 46 and [EuI 2 (DME) 2 ], precluding structural elucidation. 86 In 2019, Gao and co-workers reported the synthesis of [Dy(C 5 H 4i Pr) 2 (GePh 3 )(THF)] (47, Fig. 14 Fig. 15). 90 The authors postulated that 51 formed via the association of in situ-generated "[Yb(SnPh 3 )(Ph)(THF) n ]" with "[Yb(Ph) 2 (THF) 3 ]" to alleviate steric unsaturation. The Yb-Sn length in 51 is 3.379(1) A, which is longer than the mean Yb-Sn distance previously reported for 50 (3.305(1) A). 89 The mechanism of formation of 51 was proposed by the authors to proceed by a 2 electron transfer from [Yb(C 10 H 8 )(THF) 2 ], generating neutral C 10 H 8 and reductively cleaving one C Ph -Sn bond, formally resulting in {Ph 3 Sn} and {Ph} anions and a solvated Yb(II) cation. Various ligand scrambling processes can occur by Schlenk-type equilibria, with one eventuality leading to complex 50 and another outcome yielding 51. 90 In 1993 Bochkarev and co-workers reported the reaction of Me 3 SnCl with either  Fig. 16) alongside the disclosure of 47; in contrast to the salt metathesis reaction used to synthesise 47, complex 56 was prepared by the acid-base reaction of [Dy(Cp*) 2 (CH 2 Ph)(THF)] with HSnPh 3 , eliminating toluene as a by-product. 87 Similar to the analysis of 47, DFT calculations on a model of 56 were interpreted by the authors to propose a polarised covalent Dy-Sn single bond, and 56 was also found to show single-molecule magnet behaviour with an effective barrier to magnetic reversal of 620 K and a hysteresis temperature of 6 K. 87

D3. Lead
Besides the two reports of f-block lead Zintl clusters, 83,84 there is only one report on molecular lead complexes in the f-block by Zeckert and co-workers in 2013, specically the reactions of a lithium tris(organo)plumbylene [LiPb(2-py 6-OtBu ) 3

Outlook & conclusions
Although the eld of f-element silicon chemistry has developed at a relatively steady rate over the last 35 years, reports have started to increase in frequency over the last decade. This can be attributed to the recent renaissance in f-element solution chemistry, 97-99 with new oxidation states and starting materials  This journal is © The Royal Society of Chemistry 2020 Chem. Sci., 2020, 11, 10871-10886 | 10883 Perspective Chemical Science becoming more readily available, and the emergence of new motivations for developing the fundamental chemistry of these elements. 9,25 Throughout the history of f-element silicon chemistry researchers have supported continued progress towards realising the potential of Ln/An-Si bonds through synthetic, spectroscopic and theoretical studies to probe the nature of this linkage; comparative reactivity studies performed with carbon-based analogues has allowed differences and similarities in these bonding regimes to be probed. 51,56,57,59,60,75 It is clear from this perspective that the tris-(trimethylsilyl)silyl ligand {Si(SiMe 3 ) 3 } À and its derivatives constitute the dominant examples of complexes containing Ln/An-Si bonds reported thus far, so an obvious direction for researchers to explore in future is to expand the scope of Si-containing ligands that are utilised in order to provide data on a larger variety of complexes and to allow further benchmarking calculations by computational chemists to be performed. 41,53 Following the same trend of progress in carbon-based organometallic chemistry, the synthesis and study of Ln/An ¼ Si double bonds is a major target in this area to open up new vistas for future exploration, and to deepen our understanding of the degree of covalency in the predominantly ionic bonding regimes of the f-elements. The f-element chemistry of the heavier tetrels germanium, tin and lead is even less developed and more poorly understood than that of silicon. 26,98 Although silicon chemistry presents its own unique set of challenges the heavier tetrels will undoubtedly provide new synthetic problems to overcome. We envisage that the parallel development of f-element silicon chemistry with that of the heavier tetrels will provide new and transferable insights to allow more rapid developments in future.

Conflicts of interest
There are no conicts to declare.