f-Element heavy pnictogen chemistry

The coordination and organometallic chemistry of the f-elements, that is group 3, lanthanide, and actinide ions, supported by nitrogen ligands, e.g. amides, imides, and nitrides, has become well developed over many decades. In contrast, the corresponding f-element chemisty with the heavier pnictogen analogues phosphorus, arsenic, antimony, and bismuth has remained significantly underdeveloped, due largely to a lack of suitable synthetic methodologies and also the inherent hard(f-element)–soft(heavier pnictogen) acid–base mismatch, but has begun to flourish in recent years. Here, we review complexes containing chemical bonds between the f-elements and heavy pnictogens from phosphorus to bismuth that spans five decades of endeavour. We focus on complexes whose identity has been unambiguously established by structural authentication by single-crystal X-ray diffraction with respect to their synthesis, characterisation, bonding, and reactivity, in order to provide a representative overview of this burgeoning area. By highlighting that much has been achieved but that there is still much to do this review aims to inspire, focus and guide future efforts in this area.


Introduction
Due to their widespread and important applications in magnetic materials, 1,2 electronic devices, 3,4 bioimaging, 5,6 synthesis, 7,8 catalysis, 9,10 materials science 11,12 and nuclear technologies, 13,14 there has been burgeoning interest in the fundamental chemistry of the f-elements over the past few decades.Since f-element metal ions, that is group 3, lanthanide, and actinide ions, are hard Lewis acids with typically large radii and high coordination numbers, they preferentially bind with hard bases (by the hard-so-acid-base denition); the chemical bonds of these ions are understood to be predominantly ionic, thus their solution chemistry is dominated by N-, O-, and halide-donor ligands. 15With ever-developing synthetic methods   transformations, and materials precursors.Reecting the growing nature of this eld, there have been a number of excellent but very general or ligand-specic review articles and book chapters covering the historical developments of some of the subtopics, 20,[24][25][26][27][28][29] but recent developments justify a broad but detailed review specically focussed on this topic.This review highlights the most notable achievements in the eld of f-element heavy pnictogen chemistry from phosphorus to the heaviest abundant main group element bismuth up to August 2023.In line with the criteria for reviews, a representative selection, rather than a complete literature survey, is presented, and discussions concentrate on structurally characterised molecules.5][26][27][28][29] Here we present current challenges to inspire researchers and focus and guide future efforts of the eld to develop f-element heavy pnictogen chemistry more rapidly in the future.In this review, we include the group 3 elements scandium, yttrium, and lanthanum under the heading of lanthanide sections for convenience. 31P NMR chemical shis for P-bound complexes covered in this review are compiled in Table 1.

Nomenclature
Metal heavy pnictogen nomenclature depends upon the pnictogen identity, charge and binding mode.The prex is determined by the pnictogen identity; the general prex is 'pnict-', whilst bonds involving phosphorus, arsenic, antimony and bismuth begin with 'phosph-', 'ars-', 'stib-', and 'bism-', respectively.The suffix denotes the charge of the pnictogen and binding mode; the suffix 'ide' is used for a terminally bound pnictogen bearing a formal −1 charge, whereas a terminal pnictogen with a −2 charge ends with '-inidene'.A bridging pnictogen with a −2 charge has the suffix '-inidiide', and lastly a pnictogen bearing a −3 charge ends with '-ido', independent of the binding mode.This gives the four bonding types: pnictide (I), pnictinidiide (II), pnictinidene (III) and pnictido (IV), Fig. 2. Exceptions to these rules are seen for (As) 3− and (R 2 As) − ligands, which are given the prex 'arsen-' to give the respective terms arsenido and arsenide when bound to metal centres.An additional exception is made for the parent phosphide (H 2 P) − , which is given the unique moniker 'phosphanide'.However, within some f-element pnictinidiide and pnictido examples, the ligands can be bridged by more than two metal centres to form more complex bonding modes which are not presented in Fig. 2, but will be discussed with specic examples in the following sections.Fig. 2 General nomenclature for pnictogen metal bonding.

Synthetic methodologies for generating f-element pnictogen bonds
Precise synthetic strategies can vary depending on the type of pnictogen reagents and f-element precursors, but a pnictogen donor ligand is commonly installed on an f-element metal centre in one of the following general ways: (1) Dative coordination of a neutral phosphorus or arsenic ligand to form an adduct with an f-element complex that has an available vacant coordination site; this tends to not be the case for antimony or bismuth, which need to be negatively charged to coordinate to an f-element metal centre.
(2) Salt elimination/metathesis of alkali metal pnictogen anions with an f-element halide (or halide equivalent) precursor to produce a polarised-covalent f-element pnictogen linkage.
(3) Alkane elimination between a primary or secondary pnictogen precursor and f-element alkyl (or cyclometallate) complex exploiting the acidic nature of the proton on the pnictogen atom.
(5) Combining salt and alkane elimination approaches using a primary pnictide alkali metal salt to react with an f-element alkyl and halide starting material (mainly used to produce metal-ligand multiple bonds).

Review
Chemical Science bond distance is 2.770(1) Å. Complex 5 is diamagnetic and the 31 P NMR spectrum exhibits a doublet of doublets resonance at −181.1 ppm with 1 J YP and   6), Fig. 3, which contains two (PH 2 ) − groups that bridge to two lithium cations. 45It was found that 6 is unstable in both solid and solution states, and decomposes under argon atmosphere at room temperature to produce PH 3 as well as a small amount of H 2 PSiMe 3 , reecting the synthetic challenge of stabilising terminal lanthanide-PH 2 species.

Lanthanide phosphorin and biphosphinine complexes
In 1997, Cloke and co-workers reported the synthesis and molecular structure of the bis(2,4,6-tri-tert-butyl-phosphorin) holmium(0) complex [Ho(h 6 -Ttp) 2 ] (7), Fig. 4, prepared by cocondensation of holmium vapor with an excess of 2,4,6-tritert-butyl-phosphorin at −196 °C followed by further work-up and recrystallisation. 46The remarkable thermostability of 7 (T sublimation = 160 °C, 10 −5 mbar, 90% recovery) arises from the better p-acceptor capability for phosphorin over arenes, which was conrmed by optical and magnetic data for 7.The structure of 7 was found to exhibit extensive disorder, with the P-atoms equally disordered over the three possible positions of each phosphorin ligand, so no preference for syn or anti conformations could be inferred.
During 2014 to 2016 Nocton, Clavaguéra, and co-workers reported biphosphinine complexes of the general formula [Ln(L) 2 {(PCHCMeCMeCHC) 2 }] (Ln = Tm, L = {P(CBu t CMe) 2 }, 8a; Ln = Sm or Yb, L = Cp*, C 5 Me 5 , 8b, 8c), 47,48 Fig. 4. In these complexes the biphosphinine ligands are formally radical anions but the extent of electron transfer for the Yb complex was ambiguous, with characterisation data intermediate to closed or fully open shell formulations.The Tm-P, Sm-P, and Yb-P distances were found to be 2.825/2.862(2),2.909(2)/ 2.927(2), and 2.872(2)/2.938(2)Å, consistent with the radii of the lanthanide ions.We note that this work followed on from prior work on P-methylated phosphinine ligands with chelating side arms from Arliguie, Mézailles, and co-workers; however, in those complexes the anion charge partially delocalises into the C 5 P rings, resulting in rather long M-P bonds (∼3 Å for Ce, Nd, and U) that are between dative phosphines and covalent phosphides, 49 thus we do not discuss them further.

Lanthanide phosphinidiide complexes
Unlike their d-transition metal counterparts, lanthanide-pnictogen multiple bonds are relatively rare as a result of the valence orbital spatial and energy mismatch of 4f metal ions and pnictogen ligands.Most lanthanide pnictinidenes form bridging dimeric pnictinidiide complexes where the "Ln]Pn" moiety is stabilised through additional interactions with adjacent rare earth metal centres or electropositive alkali metal cations. 26This section describes the progresses made in isolation of a handful of lanthanide pnictinidiide complexes before the terminal phosphinidene species was nally secured very recently.
Soon aerwards, Chen and co-workers reported the synthesis of the rst early lanthanide phosphinidiide complex, [{Nd(I)(m-PDipp)(THF) 3 } 2 ] (10, Dipp = 2,6-i Pr 2 C 6 H 3 ), via the concomitant salt elimination and silyl redistribution reaction of [NdI 3 (THF) 3.5 ] with two equivalents of KP(SiMe 3 )(Dipp) to eliminate one equivalent of P(SiMe 3 ) 2 (Dipp) and two equivalents of KI, Scheme 2. 51 The Nd(III) ions in 10 exhibits pseudooctahedral geometries, with the two bridging phosphinidiides forming an asymmetric Nd 2 P 2 core that is analogous to the Ln 2 P 2 core of 9, with Nd-P bond distances of 2.7314(15) and 2.7769(16) Å.In common with 9, the phosphorus atoms in 10 are trigonal planar, with the sum of bond angles equalling 359.1°.The authors carried out preliminary investigations into the reactivity of 10, establishing that it reacts in a similar fashion to carbenes with substrates such as benzophenone to give a phosphaalkene.
In 2010 Chen and co-workers later expanded the range of neodymium phosphinidiide complexes, utilising 10 in salt metathesis reactions with two equivalents of either KCp* or KTp Ph (KHB(3-Ph-N 2 C 3 H 2 ) 3 ) to yield the phosphinidiide complexes [{Nd(Cp*)(m-PDipp)(THF)} 2 ] (11) and [{Nd(Tp Ph )(m-PDipp)(THF)} 2 ] (12), respectively, Scheme 2. 52 Complex 11 was isolated as the major product in a yield of 52%.Complex 12, however, was initially isolated as a crystalline mixture with the cyclometallated complex [Nd(Tp Ph -cyclo)(TpPh)], with purication of the two complexes performed via the manual separation of crystals.The solid-state structure of 11 revealed that there is a loss of the trigonal planar geometry of the bridging phosphinidiides, indicated by the decrease of the sum of bond angles of the phosphorus atom to 349.2(3)°when compared to complexes 12 [358.9(4)°]and 10 [359.1°], which could be a result of the coordination of the sterically demanding Cp* ligand.Complex 11 exhibits an analogous Nd 2 P 2 core to that of 10, with inequivalent Nd-P distances of 2.7456(11) and 2.7827(10) Å, whilst 12 demonstrates a more regular core geometry with Nd-P distances of 2.7808(16) and 2.7911(15) Å.The molecular structure of 12 exhibits one inverted pyrazolyl group on each Tp Ph ligand, which is a result of isomerisation of the ligand via a 1,2-shi to relieve steric buttressing between the Tp Ph ligand and the phosphinidiide Dipp group.
Mindiola and co-workers subsequently disclosed the rst mononuclear rare earth phosphinidiide complexes.The Licapped complexes [Sc(PNP iPr )(m-PDmp)(m-Br)Li(DME) n ] (Dmp = 2,6-Mes 2 C 6 H 3 ; n = 0, 14; n = 1, 15) were prepared from the concomitant salt metathesis and protonolysis reactions of the bulky primary phosphide lithium salt, LiPHDmp with [(PNP iPr ) Sc(Me)(Br)], eliminating methane as the driving force, to give 14 (55%) and 15 (47%), Scheme 4. 53 The exact product depends on the reaction solvent; complex 15 was also synthesised by the addition of a stoichiometric amount of DME to 14 (21%).Single crystal X-ray diffraction studies revealed that both 14 and 15 have short Sc-P bond lengths of 2.338(2) and 2.3732(18) Å, respectively.Calculations performed on 15 indicated that the Sc]P bond has signicant multiple bond character with a Mayer bond order of 1.46.The authors varied the reaction conditions for the synthesis of 14 and 15 to investigate if the elimination of LiBr was possible.However, heating reaction mixtures up to 100 °C did not liberate the occluded LiBr.
In the same publication Maron, Chen and co-workers conducted a reactivity study of 21; this revealed that, in contrast with the previously reported complex 16, the phosphinidiide ligand in 21 is relatively unreactive, with all small molecules reacting at the adjacent methylidene centre, Scheme 8.The reaction of 21 with either CO 2 , PhCN, t BuNC or CS 2 resulted in Scheme 7 Synthesis of the Sc-phosphinidiide complex 21.12) Å] and two short [2.7142(11) and 2.7317( 12) Å] Y-P bond lengths.The longer distance is within the range of previously reported dative R 2 P:/Ln(III) interactions.Once the change in metal radii is accounted for, this rationale can be applied to 26Lu, which exhibits similar asymmetric bonding between the phosphinidiide and three bonded Ln(III) ions, with Lu-P distances of 2.902(2), 2.684(2) and 2.639(2) Å.Both complexes contain P-C bonds that are bent out of the Ln 3 plane (Lu: 49.3°; Y: 59.1°); this differs from analogous transition metal bridging phosphinidiide complexes where the P-C bond is approximately perpendicular to the M 3 plane.In common with other rare earth phosphinidiide complexes, the authors reported that complexes 26Ln exhibited reactivity towards unsaturated molecules such as ketones, thiones, or isothiocyanates, where the complex undergoes phospha-Wittig chemistry, with the exchange of the phosphinidene for an oxo or a suldo group.Upon heating complexes 26Ln in toluene, an additional molecule of methane was eliminated to yield the m 2bridging phosphinidiide complexes [{Ln[PhC(NDipp) 2 ]} 3 (m 2 -Me) 2 (m 3 -Me)(m 2 ,h 2 :h 3 -PC 6 H 4 )], 27Ln, in high yields (Ln = Lu, 94%; Y, 91%).Complex 27Lu exhibits Lu-P distances of 2.649(4)/2.644(4)Å, which is a similar range to that of the two shorter Lu-P bonds in 26Lu, whilst 27Y displays a range of Y-P distances [2.698(2)/2.692(2)Å] that are shorter than those seen for 26Y.The authors have additionally reported the crystal structure of the yttrium bridging phosphinidiide complex 57 which exhibits Y-P bond distances of 2.701(3) and 2.700(3) Å; these are statistically indistinguishable from the corresponding distances seen in 27Y.However, the synthetic route and additional characterisation data for this complex have not been reported to date.
In Complex 29 displays interesting reactivity towards unsaturated small molecules such as alkynes when compared to 30. 59he reaction of 29 with one equivalent of PhC^CR (R]H or Me) gave [Sc(NCCN Dipp ){h 2 -P]P(EDA Dipp )CR = CPh}], Fig. 5, (R = H, 31; R = Me, 32), which was surprising as reactions with unsaturated molecules typically occur at the more nucleophilic centre, i.e. the phosphinidene (P a -phosphorus).Indeed, the authors reported that 30 reacts as expected at the phosphinidene centre with the same alkynes PhC^CR to yield [(NCCN Me ) Sc{P(H)P(EDA Dipp )}(C^CPh)] (33) and [(NCCN Me )Sc{h 2 -PP(E-DA Dipp )MeC = CPh}] (34).The calculated reaction pathways for 29 and 30 with PhC^CH illustrated that the interesting reactivity at the P b -phosphorus in the case of 29 was a consequence of the coordinated THF blocking the access of reactants to the P a -phosphinidene centre.

Lanthanide phosphinidene complexes
Whilst there were many lanthanide metal phosphinidiide complexes reported in the past two decades, a bona de terminal phosphinidene complex for any lanthanide metal remained elusive for decades.A breakthrough in this eld was achieved very recently; in 2021, Maron, Chen and co-workers reported the synthesis and molecular structure of the rst terminal scandium phosphinidene complex, Scheme 12. 60 Building on their Sc-phosphinophosphinidene work, Maron, Chen and co-workers reacted the boronylphosphine H 2 PB{N(Dipp)CHCHN-(Dipp)} with KCH 2 Ph to give in situgenerated K[HPB{N(Dipp)CHCHN(Dipp)}], which was treated with [Sc(NCCN Me )(Me)(Cl)] in a THF/toluene mixture to afford a K/Sc heterometallic phosphinidiide complex 35 aer heating at 50 °C for 24 h.Dimeric 35 could be converted to a terminal monomeric boronylphosphinidene complex 36 as a dark purple solid in 83% yield by reacting with dibenzo-18-crown-6 in toluene.The solid-state structure conrmed the boronylphosphinidene ligand of 36 adopts an end-on coordination even though the chloride ligand is still bridged between the Sc and K metal centres.The Sc-P bond length in 36 (2.381(1) Å) is close to that in 35 (2.397(2) Å) but shorter than that in the scandium phosphinophosphinidene complex 30 (2.484(1) Å).The 31 P{ 1 H} NMR spectrum for 36 at 25 °C shows a very broad signal, but data recorded at −30 °C gave a sharper resonance at 19.6 ppm.DFT studies on 36 revealed a three-centre twoelectron (3c-2e) Sc-P-B s bond with a strong Sc-P p-interaction.In line with the nucleophilic nature of the phosphinidene ligand, a preliminary reactivity study showed that 36 reacted with N,N ′ -diisopropylcarbodiimide at room temperature via a [2 + 2]-addition fashion to give a four-membered scandium metallaheterocycle complex, [Sc(NCCN Me ){N( i Pr)C(PB{N(Dipp) CHCHN(Dipp)})N( i Pr)}] (37) in 82% yield.
Very recently, Sirsch, Anwander and co-workers reported the synthesis and molecular structure of the rst terminal yttrium phosphinidene complex, Scheme 13.

Lanthanide phosphido complexes
In common with their pnictinidene derivatives, there are relatively few examples of f-element, and hence lanthanide, heavy pnictido complexes.To date the only structurally authenticated complexes to feature f-element pnictido bonding are with phosphorus or arsenic, with the pnictogen bridging between two or more metal centres. 20There have been no structurally authenticated f-element stibido or bismuthido derivatives to date, therefore they are highly sought-aer synthetic targets.For lanthanide metal complexes, only a few clusters were reported containing phosphido ligand.
In 2011, Chen and co-workers reported the rst structurally authenticated rare earth phosphido complex, [{Y(I)}{Y[

Review
Chemical Science are present as well.In the cases of 41, 42 and 45 an additional interaction with a potassium ion is present, rendering the phosphido bridging modes as either (m 5 -P) 3− or (m 6 -P)  examples of rare earth metal Zintl P 7 compounds prepared from P 4 activation directly.The solid-state structures of 47Sc and 47Y revealed that the P 7 3− units in both cases are similar to the molecular structure of the inorganic salt Li 3 P 7 .The ionic interactions between the rare earth metals and the Zintl P 8 4− or P 7 3− units above was conrmed by DFT calculations.
Although actinide-P 5 and P 6 complexes are known, 20 and indeed cyclo-P 5 complexes are well-known for transition metals, such species remain rare for the lanthanides. 26xamples of lanthanide-P 5 /P 6 complexes have been stabilised by transition metals, resulting in 3d/4d-4f clusters.For instance, the rst lanthanide complex containing cyclo-P by alkyl migration of an organosubstituted cyclo-P 4 R 2 precursor followed by encapsulation of K + cation with 18crown-6 reagent, Fig. 6. 68 Very recently, using a redox synthetic strategy, Roesky and co-workers reported another two cyclo-P 3 and -P 4 inverse sandwich complexes for lanthanides supported by a xanthene-diamide ligand. 69Lastly, diphosphorus (P 2 ) complexes, which are heavy N 2 analogues, remain elusive for lanthanides because of the synthetic challenges of making and stabilising P 2 . 69

Actinide phosphorus complexes
Although actinide-nitrogen chemistry is well-developed over several decades, this is not the case for actinide-phosphorus chemistry. 16,20Nevertheless, actinide-phosphorus chemistry is the most developed compared with the analogous lanthanide chemistry, likely because actinides can deploy 5f and 6d orbitals in bonding to form more covalent chemical bonds than lanthanides.[74][75] Fig. 6 Rare earth metal polyphosphorus complexes 46-49.
Chemical Science Review

Actinide phosphide complexes
In terms of actinide complexes containing metal-phosphorus single bond interactions, monoanionic charged phosphide ligands can have stronger interactions with actinide metal centres than neutral phosphine ligands, which only form dative bonds to actinide ions. 20Furthermore, some actinide phosphide complexes are useful precursors to novel linkages such as actinide-metal and -ligand multiple bonds (see below), lowvalent U-P bonds, 76 and hydrophosphination catalysts. 77,78he rst examples of actinide phosphide complexes were reported in 1985 by Ryan and co-workers.The mononuclear complex [Th(Cp*) 2 (PPh 2 ) 2 ] (50) was prepared by a salt metathesis reaction between [Th(Cp*) 2 (Cl) 2 ] and potassium diphenylphosphide, Scheme 15. 79 Importantly, the phosphide ligands are able to coordinate to transition metals as well, supporting actinide and transition metal interactions, making it possible to investigate actinide-metal bonding.Treatment of 50 with [Ni(COD) 2 ] (COD = 1,5-cyclooctadiene) under a CO atmosphere, or [Pt(COD) 2 ] in the presence of PMe 3 , led to the formation of the heterobimetallic compounds [Th(Cp*) 2 (m-PPh 2 ) 2 Ni(CO) 2 ] (51) 79 and [Th(Cp*) 2 (m-PPh 2 ) 2 Pt(PMe 3 )] (52) 80 in moderate yields, respectively, Scheme 15.The 31 P{ 1 H} NMR spectrum for 51 exhibits a signal at 177 ppm, which is shied downeld from the resonance for 50 found at 143 ppm.In contrast, the 31 P NMR spectrum for 52 shows a doublet at 149.3 ppm, and a triplet at −3.3 ppm.These were attributed to the [PPh 2 ] − and PMe 3 ligands, respectively.Both resonances contain coupling to Pt, suggesting a direct interaction between the phosphorus atoms of both ligands and the transition metal.The solid-state structure of 51 revealed the Th-P bond lengths to be 2.869(4) and 2.900(4) Å, which are close to the Th-P distance of 2.866(7) Å found in the mononuclear starting material 50.The Th-P bond lengths in 52 are unexceptional and are similar to those in 50.The distance between the thorium and nickel atoms in 51 was found to be 3.206(2) Å, which is longer than the sum of the covalent single bond radii of Th and Ni (2.85 Å).In contrast, in 52 the distance between the thorium and platinum centres was found to be 2.984(1) Å, which is similar to the sum of the covalent single bond radii of Th and Pt (2.98 Å).The metalmetal bonding interactions in 51 and 52 were interpreted as a weak, donor-acceptor dative bonds from the low-valent electron-rich Ni(0) and Pt(0) ions to the electron-poor thorium(IV) ions.
As well as actinide phosphide complexes with secondary phosphide ligands, there are also some actinide complexes containing primary phosphide ligands, which could potentially be used to access actinide phosphorus multiple bonds.For example, in 2015 Walensky and co-workers reported the thorium bisphosphide complex [Th(Cp*) 2 (PHTripp

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Chemical Science the resonances for uranium complexes are signicantly shied owing to the paramagnetic nature of 5f 2 uranium(IV).These phosphanide complexes have proven to be key precursors to access actinide-phosphorus multiple bonds (see below).

Actinide phosphinidiide complexes
Phosphinidiide complexes can be viewed as polynuclear forms of phosphinidene complexes where the phosphinidene ligands are bridged between two or more electropositive metal ions to enhance the stability of the reactive moiety.As discussed above, phosphinidiide complexes are also more common than terminal phosphinidenes for both lanthanides and actinides, but some phosphinidiide complexes can still have metalphosphorus multiple bonding character.
The rst crystallographically characterised actinide phosphinidiide complex was isolated in 1984 by Marks, Day and coworkers, as a parent (HP) 2− unit bridging two uranium(IV) centres.Mixing three equivalents of [U(Cp*) 2 (Me) 2 ] with one equivalent of P(OCH 3 ) 3 and excess hydrogen gave [{U(Cp*) 2 (-OMe)} 2 (m-PH)] (56) in yields of 42%, Scheme 16. 85 The authors additionally prepared [{Th(Cp*) 2 (OMe)} 2 (m-PH)] in an analogous manner.Marks and Day postulated that the mechanism for the synthesis of 56 proceeded via an actinide hydride.To conrm this, additional reactions were conducted in the absence of hydrogen or utilising [An(Cp*) 2 (H) 2 ] 2 as starting materials.The former showed no detectable reaction, whilst the latter gave 56.Complex 56 exhibits two U-P distances of 2.743(1) Å and a U-P-U angle of 157.7(2)°.The IR spectrum of 56 conrmed the presence of the parent phosphinidiide, with a PH stretching mode at 2193 cm −1 (y P-H /y P-D = 1.39).Liddle, Scheer, and co-workers also reported diactinide parent phosphinidiide complexes supported by triamidoamine ligands, 86 which will be discussed in the phosphido section below for comparison purposes. In

Actinide phosphinidene complexes
Although uranium imido chemistry is well-developed, 18 actinide complexes containing heaver pnictogen An = PnR (An = actinide; Pn = P, As, Sb, Bi) multiple bonds are scarce. 20To date, there are few complexes containing An]PnR double bonds outside of cryogenic matrix isolation conditions, but only with P or As ligands, not for Sb and Bi ligands.Also, there is no example of a terminal actinide heavy pnictido An^Pn triple bond isolated under ambient conditions.Even for d-block metals there are relatively few structurally characterised terminal metal pnictinidene complexes (∼50), far fewer than their imido counterparts (>3000). 94Actinide pnictinidene complexes normally require kinetic stabilisation by sterically demanding ancillary ligands, coupled with bulky pnictinidene substituents (cf.59 and 61).
In 1996, Burns and co-workers reported the rst example of a terminal uranium phosphinidene complex utilising the sterically demanding Cp* supporting ligand.The salt elimination/protonolysis reaction of one equivalent each of [U(Cp*) 2 (Me)(Cl)] and KHP(Mes*) in the presence of OPMe 3 gave [U(Cp*) 2 (PMes*)(OPMe 3 )] ( 62), in yields of up to 62%, Scheme 19 (top). 95Complex 62 exhibits a U-P distance of 2.562(3) Å, which is shorter than the bridging phosphinidiide U-P distance in 56 or, for example, the U-P distance in the phosphide complex [U(Cp*) 2 {P(SiMe 3 ) 2 }(Cl)] (2.789(4) Å). 96 The authors postulated that the U-P-C Ar angle of 143.7(3)°in 62 was due to crystal packing forces, or a result of the combination of heavy main group elements generally adopting bent geometries in addition to the preferred linear geometry required to minimise P-U p-overlap.
In 2018, using sterically demanding cyclopentadienyl ligands, Walter, Ding, Zi and co-workers reported a base-free terminal thorium phosphinidene [Th(Cp ttt ) 2 (PMes*)] (63, Cp ttt = 1,2,4-t Bu 3 -C 5 H 2 ), Scheme 19 (bottom). 97Complex 63 was prepared by a similar salt metathesis/protonolysis reaction that was employed to prepare 62; reaction of one equivalent each of [Th(Cp ttt ) 2 (Me)(I)] and KPH(Mes*) produced 63 in high yield (80%), with elimination of one equivalent each of methane and potassium iodide.The molecular structure of 63 further conrmed a base-free terminal phosphinidene species with a short Th-P bond distance (2.536(2) Å).The 31 P{ 1 H} NMR spectrum of 63 shows one resonance at 145.7 ppm, which is close to those for 59 and 61.Calculations probing the Th]P bonding of 63 and the theoretical monomeric model of 63 suggest more covalency in this linkage than related imido complexes.The authors additionally treated the thorium complexes 63 with a wide range of unsaturated substrates to probe the reactivity of Th-P double bond.[100][101][102][103][104][105] From the above examples, it is clear that sterically demanding cyclopentadienyl ligands have proven effective at stabilising actinide phosphinidiide and phosphinidene complexes, but sterically demanding triamidoamine ligands  86 The authors reported that the treatment of 54U with one equivalent each of KCH 2 Ph and 2.2.2-cryptand gave the contact ion pair phosphinidiide complex [{U(Tren TIPS )(m-PH)}{K(2.2.2-cryptand)}] (66). 82The solid-state structures revealed that the terminal phosphinidene complexes 64M and phosphinidiide 66 exhibit U-P distances ranging from 2.613(2) to 2.685(2) Å, which are shorter than that in the parent phosphide precursor 54U (2.883(2) Å).The authors reported that the IR spectrum of 64K exhibits a P-H stretch of 2360 cm −1 .The U-P bond lengths in 64M are longer than those observed for 62 (2.562(3) Å) lying between the sum of the covalent single and double bond radii for uranium and phosphorus (2.81 Å and 2.36 Å, respectively); this reects the sterically demanding nature of Tren TIPS and indicates polarised covalent U]P interactions that is conrmed by DFT calculations.
In 2016, Liddle, Scheer, and co-workers utilised the Tren TIPS ligand framework to stabilise terminal thorium parent phosphinidene analogues.The two methodologies used to synthesise the uranium complexes 64M were adapted to prepare the terminal phosphinidene thorium complex [Na(12C4) 2 ] [Th(Tren TIPS )(PH)] (67), Scheme 21. 83 Complex 67 could be synthesised either through deprotonation of 54Th with one equivalent of NaCH 2 Ph and two equivalents of 12C4, or by the reaction of the thorium cyclometallate complex [Th{N(CH 2 -CH 2 NSi i Pr 3 ) 2 (CH 2 CH 2 NSi i Pr 2 C(H)MeCH 2 )}] (65Th) with one equivalent of NaPH 2 and two equivalents of 12C4 in yields of up to 38%.Complex 67 is isostructural with the uranium phosphinidene complex 64Na.
The Th-P bond length of 2.758(2) Å in 67 is ca.0.22 Å shorter than that of the Th-PH 2 bond in 54Th (2.982(2) Å), but is longer than the U]P double distance of 2.613(2) Å in 64K, suggesting a more polarised double bond interaction for Th]PH linkage.This is in accord with a smaller Th-P Mayer bond order of 1.67 in 67 than that of 1.92 in 64K.The Th]P-H angle of 67.45(8)°in 67 indicates an 'agostic-type' interaction between the metal ion and the electron density of the P-H bond, whereas this interaction was not observed in 64K, which has a U]P-H angle of 118.8(9)°.The 31 P NMR spectrum for 67 has a doublet resonance at 198.8 ppm due to P-H coupling, further conrming the presence of [PH] 2− group at thorium.By contrast, because of the strong paramagnetic shielding from the uranium(IV) centre, no resonance was observed in the 31 P NMR spectra for 64M.
Protonation of reactive actinide-carbon bonds has proven to be an effective strategy for constructing actinide-pnictogen multiple bonds.In 2022, Liddle, Scheer, and co-workers developed a bulky Tren

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Review phosphinidene ligand was observed in both structures, with An-P-H angles of 65.83(17)°and 65.23(13)°, respectively.In addition, absorptions corresponding to P-H stretches at 2072 and 2070 cm −1 for 69Th and 69U, respectively, were observed in their ATR-IR spectra.The An]P vibrations are also observed in the Raman spectra of 69Th and 69U at 306 and 296 cm −1 , respectively.The 31 P NMR spectrum for 69U exhibits a broad resonance at 2629 ppm due to the paramagnetic uranium(IV) centre, while this is not observed in 64M.Similar to 67, 69Th exhibits a doublet resonance at 266.2 ppm in its 31 P NMR spectrum due to P-H coupling.
Reecting the basic and nucleophilic nature of 69An, Liddle, Scheer, and co-authors additionally found that treatment of 69An with [HNEt 3 ][BPh 4 ], as a proton source, in THF resulted in the isolation of the phosphanide complexes 55An in good yields, Scheme 22, 84 which have similar bond metrics to 54An. 82lternatively, 55U could be prepared by oxidation of 69U with AgBPh 4 in benzene.The formation of 55U in these oxidation reactions may involve a transient U(V)]PH species (or valence isomer, e.g.U(IV)]PcH), which then abstracts a proton (or Hc) in the reaction mixture due to the HSAB mismatch of U and P.This reactivity contrasts to the disproportionation observed for Tren TIPS -supported U(V)]NH chemistry, which produces U(IV)-NH 2 amide and U(VI)^N nitride products. 108These reactivity outcomes reect the periodic differences between nitrogen and phosphorus, and that when the latter is paired with electropositive metals P-based electrons can become involved in redox reactions as found in the reactivity of 16.

Actinide phosphido complexes
Actinide phosphido complexes remain exceeding rare.There are only a few bridging dinuclear complexes isolated in recent years with no examples of terminal actinide heavy pnictido An^Pn triple bond isolated under ambient conditions to date. 20n 2016, Liddle, Scheer, and co-workers reported the rst example of an actinide-phosphido complex [Na(12C4) 2 ] [{Th(Tren TIPS )} 2 (m-P)] ( 70), where the phosphido ligand bridges two thorium centres, Scheme 23. 83 Complex 70 was also the rst such f-element phosphido species, and was prepared by the reaction of two equivalents of 65Th with NaPH 2 in the presence of two equivalents of 12C4 in up to 57% yield.Alternatively, 70 can be synthesised by the stoichiometric reaction of 67 with 65Th.The authors additionally reported the synthesis of the bridging phosphinidiide complex [{Th(Tren TIPS )} 2 (m-PH)] (71Th) in a 40% yield, either by treatment of 54Th with one equivalent of 65Th, or the reaction of two equivalents each of 65Th with NaPH 2 , eliminating one equivalent of 'Na 2 PH'.However, attempts to prepare 70 by deprotonation of 71Th were unsuccessful.Complex 70 has a symmetrical ThPTh core with Th-P bond distances of 2.740(2) and 2.735(2) Å, which are shorter than those in 71Th (2.898(2) Å) but compares well to that of the terminal phosphinidene 67 (2.7584(18) Å), and lies between the sum of the covalent single and double bond radii for thorium and phosphorus (2.86 and 2.45 Å, respectively), suggesting multiple bonding interactions in the ThPTh linkage.The 31 P NMR spectra for 71Th and 70 exhibit doublet and singlet resonances at 145.7, and 553.5 ppm, conrming the presence of (HP) 2− and P 3− , respectively.
Subsequently, in 2017 Liddle, Scheer, and co-authors reported that addition of one equivalent of KCH 2 Ph to a mixture of 54U and [U(Tren TIPS )(THF)][BPh 4 ] gave the bridging diuranium phosphinidiide complex [{U(Tren TIPS )} 2 (m-PH)] (71U) 86 that is isostructural to 71Th, Scheme 24.Importantly, the authors found that deprotonation of 71U with benzyl potassium in the presence of two equivalents of B15C5 produced the diuranium(IV/IV) phosphido compound [K(B15C5) 2 ] [{U(Tren TIPS )} 2 (m-P)] (72K), Scheme 24; 86 recall that this method did not work for 71Th. 84Complex 72K was found to rapidly decompose, which was attributed to steric overload, and so, Scheme 23 Synthesis of bridging thorium phosphinidiide and phosphido complex 70 and 71Th.Unlike 70 which is stable, 72K and 73Na are to various extents unstable in solution, in line with the paucity of such phosphido species, which resulted in both complexes being isolated in relatively low yields of 29% and <5% for 73Na and 72K, respectively.Interestingly, 73Na exhibits an asymmetric UPU core, with two U-P bond distances of 2.657(2) and 2.713(2) Å, and the shorter U-P distance is the Tren TIPS ligated unit.In contrast, 72K has a symmetric core with statistically indistinguishable U-P bond lengths of 2.653(4) and 2.665(4) Å, which are shorter than those in the phosphinidiide 71U (2.8187 (12)  and 2.8110(12) Å).Calculations on 72K and 73Na suggested that the U-P bond are less covalent than the terminal phosphinidene species 64M.This is reected by smaller U-P Mayer bond orders of 1.41/1.43and 1.44/1.66for 72K and 73Na, respectively, when compared to [64K] − (1.92).DFT calculations also suggest that the Th-P bonds in 70 are more polarised and ionic than the U-P bonds in 72K and 73Na, as evidenced by smaller Th-P Mayer bond orders of 1.26 and 1.28 for [70] − .However, these An-P Mayer bond orders are nearly twice that of the single U-N amide bonds (∼0.71), suggesting that these An-P bonds are polarised multiple bond interactions.

Actinide 2-phosphaethynolate complexes
Transition metal 2-phosphaethynolate (OCP) − complexes have proven to be useful precursors to prepare phosphido complexes via redox or photolysis process. 31Recently, Liddle, Scheer, and co-workers reported the two new actinide-OCP complexes [An(Tren TIPS )(OCP)] (74An, An = Th and U) and their reduction chemistry. 109,110 75), Scheme 25. 109 Although there was no phosphido species isolated from this reduction, the coordination mode of this trapped OCP-ligand is unique, and derives from a novel highly reduced and bent carbene-like form of this ligand with a bridging P-centre and the most acute P-C-O angle of ∼127°in any complex to date.The mixed valence diuranium(III/IV) formulation is supported by the characterisation data and DFT calculations, where back-bonding from uranium gives a highly reduced form of the OCP unit that is perhaps best described as a uranium stabilised (OCP)  110 In addition, this hexathorium complex can be converted to the oxo complex [{Th(Tren TIPS )(m-ORb)} 2 ] and the known cyclometallated complex 65Th at 80 °C, via an otherwise hidden example of reductive cycloaddition reactivity in the chemistry of 2-phosphaethynolate.From the above examples it can be seen that the reduction chemistry of 2phosphaethynolate for actinides can be quite complicated.

Actinide polyphosphorus complexes
Similar to most polyphosphorus complexes, actinide polyphosphorus complexes are most oen prepared from P 4 and low-valent actinide compounds. 20

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spectrum for 77 exhibits resonances at 125.3, 18.4 and −41.9 ppm.In contrast, the 31 P NMR spectrum for 78 shows temperature-dependent features; at room temperature, there is one resonance at −75.7 ppm, however, at 193 K, two resonances at −69.7 and −94.5 ppm were observed, indicating two unique phosphorus environments at low temperature.This area progressed little over 20 years until in 2011 Cloke, Green and co-workers isolated the cyclo-P 4 uranium complex [{U(Cp*)(C 8 H 6 (Si i Pr 3 ) 2 )} 2 (m 2 -h 4 -P 4 )] (79), Fig. 8, from the reaction of the U(III) complex [U(Cp*)(C 8 H 6 (Si i Pr 3 ) 2 )(THF)] with half an equivalent of P 4 . 112The molecular structure of 79 revealed a diuranium structure where each of the uranium metal centres interacts with the cyclo-P 4 ligand in an h 2 -fashion.The P 4 ligand forms a chair-like structure with the two uranium atoms, with U-P bond distances of 2.9763(12) and 2.9773(12) Å, respectively, which are typical single bond character.In addition, the P-P bond distances of 2.152(2) and 2.149(2) Å within the bridging cyclo-P 4 unit suggest the dianionic charge of the P 4 ligand.There is only a single resonance at 718 ppm in the 31 P{ 1 H} NMR spectrum of 79.In 2016, Mills and co-workers reported another actinide cyclo-P 4 complex, [{Th(Cp ′′ ) 3 } 2 (m 2 -h 2 -P 4 )] ( 80), Fig. 8, by reacting the Th(III) complex [Th(Cp ′′ ) 3 ] with P 4 . 113The solid-state structure of 80 revealed a planar (P 4 ) 2− ligand bridging two thorium centres via two individual h 1 bonding interactions.The Th-P bond distances of 2.919(4) Å indicates Th-P single bond interactions and the short P-P bond distances of only 2.051(9) Å are 0.1 Å shorter than those in 79, suggesting that the bonding within the cyclo-P 4 ligand is closer to double bond character, which might be due to the h 1 coordination mode and the lack of Th-P p-bonding.The 1 H NMR spectrum was diagnostic of a diamagnetic complex, reecting that the reduction of the P 4 arises from the oxidation of the Th(III) precursor to two Th(IV) centres; the 31 P NMR spectrum of 80 suggests the presence of two different phosphorus environments with two triplet signals at 227.59 and 328.86 ppm, respectively, with a P-P coupling constant of ca.400 Hz.
In 2013, Liddle and co-workers reported that reaction of the diuranium(V) arene complex [{U(Ts Tol )} 2 (m 2 -h 6 :h 6 -C 6 H 5 CH 3 )] (Ts tol = HC(SiMe 2 NAr), Ar = 4-MeC 6 H 4 ) with one equivalent of P 4 resulted in the formation and isolation of the triuranium Zintl cluster [{U(Ts Tol )} 3 (m 3 -h 3 -P 7 )] (81), 114 Fig. 8.The solid-state structure of 81 revealed a P 7 3− trianion cluster capped on three of its faces by three TS Tol -uranium cation fragments.The U-P bond lengths of 81 are in the range of 2.949(2)-3.031(2)Å, which are comparable to the U-P cluster distances discussed above.Interestingly, 81 reacts with a variety of halide reagents, such as Me 3 -SiCl, LiCl, MeI, and PhI, to afford P 7 R 3 products with subsequent reduction of the U component closing the synthetic cycle.
Extending the reactivity of P 4 with other U(III) complexes, Liddle and co-workers reported that treatment of [U(Tren TIPS )] with 0.25 equivalents of P 4 reproducibly gives the actinide inverted sandwich cyclo-P 5 complex [{U(Tren TIPS )} 2 (m-h 5 :h 5cyclo-P 5 )] (82), 115 Fig. 8.All previous examples of cyclo-P 5 complexes were stabilised by transition metals, but the isolation of 82 indicates that cyclo-P 5 can also be stabilised by hard actinide ions.Moreover, the characterisation data are consistent with 82 being a diuranium(IV) complex, and thus the cyclo-P 5 unit in 82 is formally a radical dianion rather than the usual monoanion form.The molecular structure of 82 revealed quite long U-P bond distances spanning the range 3.250(6)-3.335(6)Å, which are longer than the sum of the single bond covalent radii of U and P (2.81 Å), perhaps owing to the sterically demanding nature of Tren TIPS ligands combined with the bridging h 5 -bound (per U) nature of the cyclo-P 5 unit in 82.DFT studies on 82 indicates the principal bonding in the U(P 5 )U unit is polarised d-bonding, whilst the isolobal cyclopentadienyl ligand normally interacts with metals via sand p-bonding interactions with minimal d-interaction.In a related study, Zhu, Maron, and co-workers investigated the reactivity of P 4 with U(III) supported by a tertiary phosphine-appended Tren ligand, resulting in a diuranium product containing a P 4 chain. 116n 2021, Liddle and co-workers reported the synthesis and structure of a side-on bound diphosphorus U(IV) complex, [{U(Tren TIPS )} 2 (m-h 2 :h 2 -P 2 )] (83), Scheme 26, by reacting the 7l 3 -(dimethylamino)phosphadibenzonorbornadiene P-atom transfer reagent (anthracene-PNMe 2 ) with [U(Tren TIPS )]. 117The byproduct, [U(Tren TIPS )(NMe 2 )], was isolated from the reaction mixture by fractional crystallisation, accounting for the fate of the NMe 2 unit.Complex 83 is the rst diphosphorus complex for any f-element complex, coming aer the rst f-element dinitrogen complex in 1988. 118The molecular structure of 83 revealed P-P bond distances of 2.036(2) Å, indicative of P]P double bond character and hence a dianionic P 2 2− , which is consistent with a diuanium(IV) formulation conrmed by the characterisation data.The U-P distances of 2.9441(12) and 2.9446 (12) Å are longer than the sum of the single bond covalent radii of U and P (2.81 Å), reecting the side-on bridging mode of the P 2 unit in 83.Computational results indicated that within the UP 2 U motif the in-plane U-P p-bonding dominates with a very weak d-interaction.It was subsequently found that oxidation of 64M with AgBPh 4 also produces 83 (along with 54U), which suggests the formation of a transient U(V)]PH linkage that disproportionates to 54U and U(VI)^P, the latter of which could dimerise to give the

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Chemical Science more stable P-P coupled 83.In addition, a preliminary reactivity study demonstrated that 83 can be converted to uranium cyclo-P 3 complexes [M(arene) 4 ][{U(Tren TIPS ) 2 (m-h 3 :h 3 -P 3 )] (M = K, Rb, Cs; arene = toluene or benzene); these reactions are low yielding, implying the presence of reactive phosphido intermediates. 107FT calculations indicate that uranium moves from p-bonding to P 2 and cyclo-P 3 to d-bonding with cyclo-P 5 , highlighting the exibility of the chemical bonding of uranium.

Lanthanide heavier pnictogen complexes
Upon descending group 15, the number of f-element heavier pnictogen bonds falls away rapidly, and lanthanide complexes containing heavier pnictogen ligands from As to Bi are much rarer than P analogues.Furthermore, such complexes tend to form multi-centre clusters with bridging pnictogen ligands.Thus, well-dened mononuclear species are sparse.To the best of our knowledge, there are no lanthanide complexes isolated to date with multiple bonding to As, Sb, or Bi.

Lanthanide arsenic complexes
The rst crystallographically characterised complex containing a lanthanide-arsenic bond was reported in 1988 by Schumann

Chemical Science Review
2.977(2)-3.019(2)Å; Dy, 2.984(2)-3.009(2)Å).Computational studies of 88Y indicated that the Y-As bonding is ionic with a small amount of covalent character, which is greater than the component seen for the yttrium arsenide precursor.The authors reported that 88Dy demonstrated SMM behaviour at low temperatures (<5 K) with a U eff value of 23(2) cm −1 and magnetic hysteresis observed up to 1.8 K.Although the structurally authenticated terminal phosphinidene complex 40 was reported very recently, an analogous lanthanide terminal arsinidene complex still remains elusive.Due to synthetic difficulties, the chemistry of lanthanide polyarsenide complexes progressed rather slowly until the last decade.In 2016 Roesky and co-workers showed that treatment of [Fe(Cp*)(h 5 91) by using different solvents, Scheme 28. 124The solid-state structures of 90 and 91 revealed hetero-trinuclear and -dinuclear 3d/4f clusters with the As 7 and As 4 ligands bridged between multiple metal centres.Complex 90 exhibits a norbornadienelike structure with two short As-As bonds in the scaffold, whilst 91 is also the rst 3d/4f-triple decker sandwich complex with a purely inorganic ligand middle deck.DFT calculations and physical characterisation data indicated that the central As 4 ligand is dianionic, which is isolobal with the 6p-aromatic cyclobutadiene dianion [C 4 H 4 ] 2− .This is consistent with the As-As bond distances within the cyclo-As 4 ligand in 91, which were found to be in between an As-As single and double bond.
More recent work from the Roesky group has proved that a redox strategy, using [Fe(Cp*)(h 5 -As 5 )] as an arsenic ligand source to react with low-valent lanthanide starting materials, is an effective way to synthesise new lanthanide polyarsenide complexes.Three new 3d/4f polyarsenide complexes in the separate ion pair form [K(18C6)][Ln(Cp ′′ ) 2 (m-h 4 :h 4 -As 5 )Fe(Cp*)] (92Ln, Ln = La, Ce, Nd) were prepared by the reduction of [Fe(Cp*)(h 5 -As 5 )] with formally low-valent bridging arene lanthanide compounds in moderate yields, Scheme 28. 125The molecular structures of 92Ln are very similar to each other, containing highly reduced As 5 units with an envelope shape.In 92Ln the Fe centre is h 4 -coordinated by the cyclo-As 5 unit and the shortest As-As bond distance of 2.3781(6) Å is observed, while the rest of the As-As bonds range from 2.3928(5) Å to 2.4325(5) Å.As expected, Ln-As bond distances vary over a wide range because of the steric constraints and anisotropic charge distribution with in the As 5 ligand.In other work, Roesky and co-workers found that reacting the arsenic source [{Co(Cp ′′′ )} 2 (mh 2 :h 2 -As 2 ) 2 ] with samarocenes produced two new mixed 3d/4f polyarsenic complexes [{Co(Cp ttt )} 2 (m-As) 4 Sm(Cp Me4R ) 2 ] (R = Me, n-propyl), which represent the rst examples of lanthanide complexes with open chain-like polyarsenic ligands. 126ore recently, Roesky and co-workers found that yellow arsenic (As 4 ) is also a useful source to introduce polyarsenic ligands to lanthanides.Due to the unstable nature of this As 4 allotrope under ambient conditions, the authors used freshlyprepared As 4 in solution to react with the divalent precursor [Sm(DippForm) 2 (THF) 2 ]; aer workup, red crystals of the cyclo-As 4 complex [{Sm(DippForm) 2 } 2 (m-h 4 :h 4 -As 4 )] (93), 61 Scheme 29, were obtained as a minor product, which is essentially isostructural to the lanthanide cyclo-P 4 complex 48.Unfortunately, the presence of non-removable impurities hampered further characterisation on 93 because of the instability of As 4 in solution.In parallel work, the authors also explored the reactivity of As 4 with [Sm(Cp*) 2 ].As both reagents are highly reactive and light sensitive, this reaction was performed with the exclusion of light; aer work-up, a few single crystals of [{Sm(Cp*) 2 } 2 (m-h 2 :h 2 -As 2 )] (94), Scheme 29, with inseparable side products were isolated. 127The solid-state structure of 94 revealed a rare diarsenic lanthanide species with the As 2 ligand side-on bound to two samarium centres.The As-As bond distance of 2.278(2) Å indicates As]As double bond character, and hence a dianionic charge on the As 2 2− unit.The Sm-As distances of 3.014(1) Å are statistically the same because of the presence of an inversion centre in 94.
The practical difficulties of working with As 4 solutions has spurred the development of new arsenic starting materials for construction of polyarsenic complexes.Roesky and co-workers reported the preparation for arsenic nanoparticles using a reductive synthetic method developed by Feldmann and coworkers. 128The elemental As 0 nanoparticles (nano-As 0 , d = 7.2 ± 1.8 nm) can be formed by the reduction of AsI 3 with a freshly prepared solution of lithium naphthalenide at 0 °C in THF.The nano-As 0 can be isolated in a pure form as the LiI by-product is sufficiently soluble in THF and Et 2 O to be washed away; this has Scheme 29 Synthesis of lanthanide polyarsenide complexes 93 and 94.
Scheme 30 Synthesis of lanthanide polyarsenic complexes 95-99., and As 14 4− ligands, respectively, that were previously not accessible in molecular lanthanide chemistry, Scheme 30. 129The As 14 4− unit in 97 is the largest organo-lanthanidepolyarsenic complex to date.The authors reported that the two different trianionic fragments feature disorder similar to the tin analogue 113Ln, with several of the atomic positions in the cluster being occupied by Bi, Ga and GaH.Formulation of each anionic cluster was determined by the examination of a combination of techniques, namely single crystal diffraction, energy dispersive X-ray and electrospray ionisation mass spectrometry (ESI-MS), which indicated that the two fragments were present in a ratio of 92% to 8% for [Sm@Ga 2 HBi 11 ] and [Sm@Ga 3 H 3 -Bi 10 ], respectively.The Sm-Bi/Ga distances in 116 span 3.0464(6)-3.4134(6)Å, which is a similar to the other 13-vertex lanthanide cluster complexes 114Ln and 115.
In 2015, Dehnen and co-workers reported the synthesis of a wide range of lead bismuth cages.The separate reactions of the in situ-generated Zintl lead precursor [K(2.2.

Actinide heavier pnictogen complexes
In parallel with lanthanide chemistry, actinide complexes containing heavier pnictogen ligands (from As to Bi) are much rarer than actinide-phosphorus congeners.Again, these complexes tend to form multi-nuclear clusters with the heavier pnictogen ligands bridged by two or more metal centres, thus well-dened mononuclear species are sparse.However, signicant advances have been achieved in actinide arsenic multiple bonding chemistry that are discussed in this section.In contrast, there are currently no examples of actinide complexes featuring multiple bonding interactions with antimony or bismuth ligands.

Review
Chemical Science shorter than those in 133 (2.9619(6)/3.0286(6)Å).The authors reported that attempts to prepare a terminal Tren TIPS thorium arsenido Th^As species using the similar double deprotonation method for 132 proved unsuccessful; indeed, closely related work attempting to construct thorium nitrides supported by Tren TIPS consistently resulted in the formation of parent imido complexes.In 2022, Liddle, Scheer, and co-workers showed that a terminal parent arsinidene at thorium (and uranium for comparison) could be isolated using the bulky Tren TCHS ligand, Scheme 45.Specically, reaction of 68An with KAsH 2 in the presence of 2.  152 All of these results demonstrate the challenges of using the OCAs ligand to synthesise actinide-arsenic bonds and also that the synthetic methods and ancillary ligands drive the (OCAs) − bond cleavage chemistry in very different directions.

Chemical Science Review
systems; the latter results in more orthogonal, and presumably weaker, binding of the stibide ligand.The analogous U-P and U-As complexes were also prepared in that study, revealing increasingly pyramidalised pnictide centres as the group is descended.

Actinide bismuth complexes
In the same publication describing the work in Section 7.3, Liddle, Scheer, and co-workers also reported the synthesis and characterisation of the rst two-centre-two-electron (2c-2e) U-Bi bond, [U(Tren DMBS ){Bi(SiMe 3 ) 2 }] (139), by the reaction of [U(Tren DMBS )(THF)][BPh 4 ] with KBi(SiMe 3 ) 2 in THF, Scheme 47. 153 It was found that the U-Bi bond could not be stabilised using the bulkier Tren TIPS ligands, likely due to steric overload.Complex 139 exhibits U-Bi distances of 3.3208(4) Å which is longer than the above An-Sb bonds.Whilst the U-Bi bond in 139 was isolable, no Th-Bi bond could be isolated with Tren TIPS or Tren DMBS , underscoring the fragility of these linkages.
Complex 139 is the only monomeric actinide complex containing a discrete bond to Bi to date., in the presence of 2.2.2-cryptand in 1,2-ethlyenediamine afforded [K(2.2.2-cryptand)] 4 [Th@Bi 12 ]$2en (143) as black, prismatic crystals, Scheme 49. 155 The structure of 143 was conrmed by single-crystal X-ray diffraction and the Th : Bi ratio within the cluster was veried by micro-X-ray uorescence spectroscopy.The molecular structure revealed Bi-Bi bond distances over a relatively small range (3.0420( 14 ).This is much larger than in 6p-aromatic benzene (11.4 nAT −1 ), but close to that in 26p-aromatic porphine (25.3 nAT −1 ), despite the much smaller number of 2p-electrons involved.The aromatic nature of [Th@Bi12] 4− extends aromaticity to the heaviest all-metal inorganic system.

Conclusions and outlook
Although f-element heavy pnictogen chemistry initially progressed quite slowly for many decades, with the resurgence of non-aqueous f-element chemistry momentum in this area has increased signicantly in recent years, as evidenced by the burgeoning array of novel metal-heavy-pnictogen bond types that are now known.These well-characterised compounds have enabled us to secure new structural motifs and probe the nature of f-element ligand chemical bonds to further deepen our understanding of chemical bonding with increasingly heavy ions in non-relativistic to relativistic regimes.Overall, f-element heavier pnictogen chemistry is developing well but there are certainly numerous opportunities to advance knowledge and understanding in this burgeoning eld.
Looking forward, there are several appealing directions for researchers to explore in this area: (1) in general terms, felement-phosphorus chemistry is maturing, but arsenic and especially antimony and bismuth are poorly developedbringing the latter three to the same level of maturity as phosphorus will do much to elucidate periodic trends; (2) there are still relatively few actinide-pnictidene/ido multiple bond complexes and even fewer lanthanide analogueshowever, the reports of isolated complexes to date suggests that there is ample scope to secure new Ln and An double and triple bonds to P, As, Sb, and Bi if the right supporting ligands can be identied and coupled with suitable synthetic approaches; (3) heavy analogues of dinitrogen are now known and even as radical species, but examples remain few in numberdeveloping better synthetic approaches would open the area up and provide interesting electronic and physicochemical properties and potential atom-transfer methodologies; and, (4) though relatively few in number, it is already clear that f-element pnictogen clusters can exhibit novel magnetic and aromaticity propertiesexpanding the range of such compounds can only enhance our understanding of these fundamental phenomena.

Chemical Science
Review
Scheme 13 Synthesis of a terminal Y-phosphinidene complex 40.
Scheme 16 Synthesis of the U-phosphinidiide complex 56.
Scheme 25 Reduction of actinide-OCP complexes 74An to produce actinide complexes 75 and 76.
Scheme 39 Synthesis of samarium polybismuth Zintl anionic clusters 116.Sm-Bi/A interactions are omitted for clarity.
Scheme 45 Synthesis of terminal actinide parent arsinidene and arsenide complexes 136An and 137An.

Table 1
31P NMR chemical shifts of reported f-element complexes with phosphorus ligands in this review

Table 1 (
Contd. ) 1 J PH values of 144.0 and 201.0 Hz, respectively.Phosphanide complexes featuring the (PH 2 ) − ligand are exceedingly rare for f elements.Although actinide phosphanide complexes with terminal An-PH 2 linkages are known (see below), isostructural analogues still remain rare for lanthanides.The only relevant example is the yttrium complex 63.The phosphido ions in 41-45 lie in the plane of the four equatorial yttrium ions with near-linear mean Y eq -P-Y eq bond angles This class of lanthanide complexes are normally synthesised by the reactions of reducing low valent lanthanide precursors with P 4 or transition metal polyphosphorus compounds via redox methods.26In2009, Roesky and co-workers reported the synthesis and molecular structure of the rst molecular lanthanide metal polyphosphorus complex [{Sm(Cp*) 2 } 4 P 8 ] (46)63prepared by diffusion of P 4vapour into a toluene solution of solvent-free samarocene involving a four electron transfer process, Fig.6.The solid-state structure of 46 revealed a very rare structure that can be seen as a realgar-type P 8 4− ligand, which is isoelectronic with the P 4 S 4 molecule, trapped in a cage of four [Sm(Cp*) 2 ] cations.The Sm-P bond distances of 46 range from 2.997 Bu) 3 } 3− ) was prepared 86 by the reaction of one equivalent each of 64Na with the cyclometallate complex [U{N(CH 2 CH 2 -NSiMe 2 t Bu) 2 (CH 2 CH 2 NSi(Me)(CH 2 )( t Bu))}], which is a less sterically demanding Tren ligand compared to Tren TIPS .