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The salt metathesis reactions of the yttrium methanediide iodide complex [Y(BIPM)(I)(THF)2] (BIPM = {C(PPh2NSiMe3)2}) with the group 1 silanide ligand transfer agents MSiR3 (M = Na, R3 = tBu2Me or tBu3; M = K, R3 = (SiMe3)3) gave the yttrium methanediide silanide complexes [Y(BIPM)(SitBu2Me)(THF)] (1), [Y(BIPM)(SitBu3)(THF)] (2) and [Y(BIPM){Si(SiMe3)3}(THF)] (3). Complexes 1-3 provide rare examples of structurally authenticated rare earth metal-silicon bonds, and were characterized by single crystal X-ray diffraction, multinuclear NMR and ATR-IR spectroscopy, and elemental analysis. Density functional theory calculations were performed on 1-3 to probe their electronic structures further to reveal predominantly ionic Y–Si bonding. The computed Y–Si bonds show lower covalency than Y=C bonds, which are in turn best represented by Y+–C– dipolar forms due to the strong σ-donor properties of the silanide ligands investigated; these observations are in accord with experimentally-obtained 13C{1H} and 29Si{1H} NMR data for 1-3 and related Y(III) BIPM alkyl complexes in the literature. Preliminary reactivity studies were performed, with complex 1 treated separately with benzophenone, azobenzene and N,N′-dicyclohexyl-carbodiimide. 29Si{1H} and 31P{1H} NMR spectra of these reaction mixtures indicated that 1,2-migratory insertion of the unsaturated substrate into the Y–Si bond is favored, whilst for the latter substrate a [2 + 2] cycloaddition reaction also occurs at the Y=C bond to afford [Y{C(PPh2NSiMe3)2[C(NCy)2]-κ4C,N,N′,N′}{C(NCy)2(SitBu2Me)-κ2N,N′}] (4); these reactivity profiles complement and contrast with those of Y(III) BIPM alkyl complexes.

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe

Treatment of [U IV(N 3)(Tren TIPS)] (1, Tren TIPS = {N(CH 2CH 2NSiPr i 3) 3} 3−) with excess Li resulted in the isolation of [{U IV(μ-NLi 2)(Tren TIPS)} 2] (2), which exhibits a diuranium(iv) ‘diamond-core’ dinitride motif. Over-reduction of 1 produces [U III(Tren TIPS)] (3), and together with known [{U V(μ-NLi)(Tren TIPS)} 2] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{U IV(Tren TIPS)} 2(μ-NH)(μ-NLi 2)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin-orbit coupled nature of the ground multiplet of uranium(iv). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of −4.1 and −3.4 cm −1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium-uranium bonding was raised, but in-depth computational analysis reveals that if any uranium-uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide-actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the ‘diamond-core’ dinitride of 2 and matrix-isolated binary U 2N 2

We report the synthesis and characterisation of a series of M(IV) substituted cyclopentadienyl hypersilanide complexes of the general formula [M(CpR)2{Si(SiMe3)3}(X)] (M = Hf, Th; CpR = Cp′, {C5H4(SiMe3)} or Cp′′, {C5H3(SiMe3)2-1,3}; X = Cl, C3H5). The separate salt metathesis reactions of [M(CpR)2(Cl)2] (M = Zr or Hf, CpR = Cp′; M = Hf or Th, CpR = Cp′′) with equimolar K{Si(SiMe3)3} gave the respective mono-silanide complexes [M(Cp′)2{Si(SiMe3)3}(Cl)] (M = Zr, 1; Hf, 2), [Hf(Cp′′)(Cp′){Si(SiMe3)3}(Cl)] (3) and [Th(Cp′′)2{Si(SiMe3)3}(Cl)] (4), with only a trace amount of 3 presumably formed via silatropic and sigmatropic shifts; the synthesis of 1 from [Zr(Cp′)2(Cl)2] and Li{Si(SiMe3)3} has been reported previously. The salt elimination reaction of 2 with one equivalent of allylmagnesium chloride gave [Hf(Cp′)2{Si(SiMe3)3}(η3-C3H5)] (5), whilst the corresponding reaction of 2 with equimolar benzyl potassium yielded [Hf(Cp′)2(CH2Ph)2] (6) together with a mixture of other products, with elimination of both KCl and K{Si(SiMe3)3}. Attempts to prepare isolated [M(CpR)2{Si(SiMe3)3}]+ cations from 4 or 5 by standard abstraction methodologies were unsuccessful. The reduction of 4 with KC8 gave the known Th(III) complex, [Th(Cp′′)3]. Complexes 2-6 were characterised by single crystal XRD, whilst 2, 4 and 5 were additionally characterised by 1H, 13C{1H} and 29Si{1H} NMR spectroscopy, ATR-IR spectroscopy and elemental analysis. In order to probe differences in M(IV)–Si bonds for d- and f-block metals we studied the electronic structures of 1-5 by density functional theory calculations, showing M–Si bonds of similar covalency for Zr(IV) and Hf(IV), and less covalent M–Si bonds for Th(IV).

Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies. Determining the covalency of actinide chemical bonding is a fundamentally important challenge. Here, the authors report a 15N nuclear magnetic resonance spectroscopy study of a terminal uranium-nitride, revealing exceptional NMR properties and covalency that redefine 15N NMR parameter space and actinide chemical bonding.

Metal–metal bonding is a widely studied area of chemistry1–3, and has become a mature field spanning numerous d transition metal and main group complexes4–7. By contrast, actinide–actinide bonding, which is predicted to be weak8, is currently restricted to spectroscopically detected gas-phase U2 and Th2 (refs. 9,10), U2H2 and U2H4 in frozen matrices at 6–7 K (refs. 11,12), or fullerene-encapsulated U2 (ref. 13). Furthermore, attempts to prepare thorium–thorium bonds in frozen matrices have produced only ThHn (n = 1–4)14. Thus, there are no isolable actinide–actinide bonds under normal conditions. Computational investigations have explored the probable nature of actinide–actinide bonding15, concentrating on localized σ-, π-, and δ-bonding models paralleling d transition metal analogues, but predictions in relativistic regimes are challenging and have remained experimentally unverified. Here, we report thorium–thorium bonding in a crystalline cluster, prepared and isolated under normal experimental conditions. The cluster exhibits a diamagnetic, closed-shell singlet ground state with a valence-delocalized three-centre-two-electron σ-aromatic bond16,17 that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide σ-aromatic bonding adds to main group and d transition metal analogues, extending delocalized σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborate actinide–actinide bonding. A crystalline cluster exhibits thorium–thorium bonding, adding to our knowledge of actinide–actinide bonding.

Summary A fundamental part of characterizing any metal complex is understanding its electronic ground state, for which magnetometry provides key insight. Most uranium(IV) complexes exhibit low-temperature magnetic moments tending to zero, consistent with a non-degenerate spin-orbit ground state. However, there is a growing number of uranium(IV) complexes with low-temperature magnetic moments ≥1 μB, suggesting a degenerate ground state, but the electronic structure implications and origins have been unclear. We report uranium(IV)-oxo and -imido complexes with low-temperature magnetic moments (ca. 1.5–1.6 μB) and show that they exhibit near-doubly degenerate spin-orbit ground states. We determine that this results from the strong point-charge-like donor properties of oxo and imido anions generating pseudosymmetric electronic structures and that traditional crystal field arguments are useful for understanding electronic structure and magnetic properties of uranium(IV). This suggests that a significant number of uranium(IV) complexes might benefit from a close re-evaluation of the nature of their spin-orbit ground states.

We report the direct synthesis of the terminal pnictidenes [An(Tren

We report the synthesis of the lanthanide-(bis)boryloxide complex [Dy{OB(NArCH)2}2(THF)4][BPh4] (2Dy, Ar = 2,6-Pri2C6H3), with idealised D4h@Dy(III) point-group symmetry. Complex 2Dy exhibits single-molecule magnetism (SMM), with one of the highest energy barriers (Ueff = 1565(298) K) of any six-coordinate lanthanide-SMM. Complex 2Dy validates electrostatic model predictions, informing the future design of lanthanide-SMMs.

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal–ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)–mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ u > π u energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium–mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal–ligand bond can be stabilized and studied in isolation. [Figure not available: see fulltext.]

A series of Ni complexes [Ni(Cl)2(PR3)2] with R = Me (1-Me), nPr (1-nPr), and nBu (1-nBu) and nickelocenes [Ni(η5-C5H4R′)2] with R′ = H (2-H), Me (2-Me), and SiMe3 (2-SiMe3) were synthesized and ch...

Despite the vast array of ηn-carbocyclic C5–8 complexes reported for actinides, cyclobutadienyl (C4) remain exceedingly rare, being restricted to six uranium examples. Here, overcoming the inherent challenges of installing highly reducing C4-ligands onto actinides when using polar starting materials such as halides, we report that reaction of [Th(η8-C8H8)2] with [K2{C4(SiMe3)4}] gives [{Th(η4-C4[SiMe3]4)(μ-η8-C8H8)(μ-η2-C8H8)(K[C6H5Me]2)}2{K(C6H5Me)}{K}] (1), a new type of heteroleptic actinocene. Quantum chemical calculations suggest that the thorium ion engages in π- and δ-bonding to the η4-cyclobutadienyl and η8-cyclooctatetraenyl ligands, respectively. Furthermore, the coordination sphere of this bent thorocene analogue is supplemented by an η2-cyclooctatetraenyl interaction, which calculations suggest is composed of σ- and π-symmetry donations from in-plane in- and out-of-phase C Created by potrace 1.16, written by Peter Selinger 2001-2019 C 2p-orbital combinations to vacant thorium 6d orbitals. The characterisation data are consistent with this being a metal–alkene-type interaction that is integral to the bent structure and stability of this complex., We report the first thorium–cyclobutadienyl complex, a new type of heteroleptic actinocene, that exhibits ‘an alkene-like’ thorium–η2-cyclooctatetraenyl interaction.

Reaction of [Li2{C4(SiMe3)4}(THF)2] (1) with [U(η5-C5Me5)I2(THF)] (2) produced the oxo-centered homotrimetallic uranium–pentamethylcyclopentadienyl complex [{U(η5-C5Me5)(μ-I)2}3{μ3-O}{Li(THF)3}0.5]...

We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}-, {Si(SiMe2H)3}-, and {SiPh3}-substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, ?Si, span a wide (?225 ppm) range when the metal is kept constant, and direct, linear correlations are found between ?Si and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The ?Si is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Despite continuing and burgeoning interest in f-block complexes and their bonding chemistry in recent years, investigations of the electronic structures and oxidation states of heterobimetallic complexes, and their bonding features between transition-metals (TMs) and f-elements remain relatively less explored. Here, we report a quantum chemical computational study on the series of TM-actinide and -neodymium complexes [TMAn(L)] and [TMNd(L)] [An = U, Np, Pu; TM = Cr, Mn, Fe; L = {N(o-[NCH2P(CH3)2]C6H4)3}3-] in order to explore periodic trend, generalities and differences in the electronic structure and metal-metal bonding between f-block and d-block elements. Based on the calculations, we find up to five-fold covalent multiple bonding between actinide and transition metal ions, which is in sharp contrast with a single bond between neodymium and transition metals. From a comparative study, a general trend of strength of the An-TM interaction emerges in accordance with the atomic number of the actinide metal, which relates to the nature, energy level, and spatial arrangement of their frontier orbitals. The trend presents a valuable insight for future experimental endeavour searching for isolable complexes with strong and multiple An-TM bonding interactions, especially for the experimentally challenging transuranic elements that require targeted research due to their radioactive nature.

We report magnetic hysteresis studies of three Dy(III) single-molecule magnets (SMMs). The three compounds are [Dy(tBuO)Cl(THF)5][BPh4] (1), [K(18-crown-6-ether)(THF)2][Dy(BIPM)2] (2, BIPM = C{PPh2NSiMe3}2), and [Dy(Cpttt)2][B(C6F5)4] (3), chosen as they have large energy barriers to magnetisation reversal of 665, 565, and 1223 cm-1, respectively. There are zero-field steps in the hysteresis loops of all three compounds, that remain in magnetically dilute samples and in samples that are isotopically enriched with 164Dy, which has no nuclear spin. These results demonstrate that neither dipolar fields nor nuclear hyperfine coupling are solely responsible for the quantum tunnelling of magnetisation at zero field. Analysing their vibrational modes, we find that the modes that most impact the first coordination sphere occur at the lowest energies for 1, at intermediate energies for 2 and at higher energies for 3, in correlation with the hysteresis coercive fields. Therefore, we suggest that the efficiency of quantum tunnelling of magnetisation is related to molecular flexibility.

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ

The first isolation and structural characterization of an f-element dinitrogen complex was reported in 1988, but an f-element complex with the first heavier group 15 homologue diphosphorus has to date remained unknown. Here, we report the synthesis of a side-on bound diphosphorus complex of uranium(IV) using a 7λ3-(dimethylamino)phosphadibenzonorbornadiene-mediated P atom transfer approach. Experimental and computational characterization reveals that the diphosphorus ligand is activated to its dianionic (P2)2- form and that in-plane U-P π-bonding dominates the bonding of the U(μ-η2:η2-P2)U unit, which is supplemented by a weak U-P interaction of δ symmetry. A preliminary reactivity study demonstrates conversion of this diphosphorus complex to unprecedented uranium cyclo-P3 complexes, suggesting in situ generation of transient, reactive phosphido species.

Separate reactions of [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSi(Me)(But)(-CH2)]2 (1) with [Re(5-C5H5)2(H)] (2) or [Ru(5-C5H5)(H)(CO)2] (3) produced, by alkane elimination, [Th(TrenDMBS)Re(5-C5H5)2] (ThRe, TrenDMBS = {N(CH2CH2NSiMe2But)3}3-), and [Th(TrenDMBS)Ru(5-C5H5)(CO)2] (ThRu), which were isolated in crystalline yields of 71 and 62%, respectively. Complex ThRe is the first example of a molecular Th-Re bond to be structurally characterised, and ThRu is only the second example of a structurally authenticated Th-Ru bond. By comparison to isostructural U-analogues, quantum chemical calculations, which are validated by IR and Raman spectroscopic data, suggest that the Th-Re and Th-Ru bonds reported here are more ionic than the corresponding U-Re and U-Ru bonds.

Room temperature reaction of the uranium(IV)-carbene [U{C(SiMe3)(PPh2)}(BIPMTMS)(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5]2 (1, BIPMTMS = C(PPh2NSiMe3)2) with white phosphorus (P4) produces the organo-P5 compound [P5{C(SiMe3)(PPh2)}2][Li(TMEDA)2] (2) and the uranium(IV)-methanediide [U{BIPMTMS}{Cl}{μ-Cl}2{Li(TMEDA)}] (3). This is an unprecedented example of cooperative metal-carbene P4 activation/insertion into a metal-carbon double bond and also an actinide complex reacting with P4 to directly form an organophosphorus species. Conducting the reaction at low temperature permits the isolation of the diuranium(IV) complex [{U(BIPMTMS)([μ-η2:η2-P2]C[SiMe3][PPh2])}2] (4), which then converts to 2 and 3. Thus, surprisingly, in contrast to all other actinide P4 reactivity, although this reaction produces catenation overall it proceeds via P4 cleavage to functionalised P2 units. Hence, this work establishes a proof of concept synthetic cycle for direct fragmentation, catenation, and functionalisation of P4.

Treatment of the new methanediide–methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh2S)2}2−) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)2][Dy(SCS)2(K(DME)2)2] (2Dy), [Dy(SCS)2][Na(DME)3] (3Dy) and [Dy(SCS)2][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)2][K(DB18C6)(THF)(toluene)] (5Dy, NCN = {C(PPh2NSiMe3)2}2−, DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy–5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (∼14 Oe s−1). Fitting the alternating current magnetic susceptibility data for 2Dy–5Dy gives energy barriers to magnetic relaxation (Ueff) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy–4Dy join a privileged group of SMMs with Ueff values of ∼1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy–4Dy, the Dy-components can be grouped into 2Dy–cation/4Dy and 2Dy–anion/3Dy, where the former have almost linear C Created by potrace 1.16, written by Peter Selinger 2001-2019 DyC units with short average DyC distances, and the latter have more bent CDyC units with longer average DyC bonds. Both Ueff and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M–L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between Ueff barrier and axial linearity in structurally comparable systems., Studies on equatorial donor and CDyC angle variation effects on energy barriers to the slow relaxation of magnetisation are reported.

We report the synthesis of the lanthanide-(bis)boryloxide complex [Dy{OB(NArCH)2}2(THF)4][BPh4] (2Dy, Ar = 2,6-Pri2C6H3), with idealised D4h@Dy(iii) point-group symmetry. Complex 2Dy exhibits single-molecule magnetism (SMM), with one of the highest energy barriers (Ueff = 1565(298) K) of any six-coordinate lanthanide-SMM. Complex 2Dy validates electrostatic model predictions, informing the future design of lanthanide-SMMs.

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., In this perspective we review the molecular chemistry of f-element silicon and heavy tetrel complexes.

We report the synthesis and characterisation of isostructural thorium(IV)- and uranium(IV)- silanide complexes, providing the first structurally authenticated Th-Si bond and a rare example of a molecular U-Si bond. These complexes therefore present the first opportunity to directly compare the chemical bonding of Th-Si and U-Si bonds. Quantum chemical calculations show significant and surprisingly similar 7s, 6d, and 5f orbital contributions from both actinide (An) elements in polarised covalent An-Si bonds

The reaction of [UO2(μ-Cl)4{K(18-crown-6)}2] with [{N(CH2CH2NSiPri3)3}Li3] gives [{UO(μ-NCH2CH2N[CH2CH2NSiPri3]2)}2] (1), [{(LiCl)(KCl)(18-crown-6)}2] (2), and [LiOSiPri3] (3) in a 1:2:2 ratio. The formation of the oxo-imido 1 involves the cleavage of a N-Si bond and the activation of one of the usually robust U═O bonds of uranyl(VI), resulting in the formation of uranium(VI)-imido and siloxide linkages. Notably, the uranium oxidation state remains unchanged at +6 in the starting material and product. Structural characterization suggests the dominance of a core RN═U═O group, and the dimeric formulation of 1 is supported by bridging imido linkages in a highly asymmetric U2N2 ring. Density functional theory analyses find a σ > π orbital energy ordering for the U═N and U═O bonds in 1, which is uranyl-like in nature. Complexes 1-3 were characterized variously by single crystal X-ray diffraction, multinuclear NMR, IR, Raman, and optical spectroscopies; cyclic voltammetry; and density functional theory.

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC)5W-W(CO)5][K(18-crown-6)(THF)2]2 (1), completing the group 6 dianion triad over half a century since the area began. The W-W bond is long [3.2419(8) Å] and, surprisingly, in the solid-state the dianion adopts a D4h eclipsed rather than D4d staggered geometry, the latter of which dominates the structural chemistry of binary homobimetallic carbonyls. Computational studies at levels of theory from DFT to CCSD(T) confirm that the D4d geometry is energetically preferred in the gas-phase, being ∼18 kJ mol-1 more stable than the D4h form, since slight destabilisation of the degenerate W-CO π 5dxz and 5dyz orbitals is outweighed by greater stabilisation of the W-W σ-bond orbital. The gas-phase D4h structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D4h isomer directly to the D4d forms, which are produced by rotation around the W-W bond by ±45°. It is therefore concluded that the gas-phase transition state becomes a minimum on the potential energy surface when subjected to crystal packing in the solid-state.

Since its inception in the 1950s, the field of organoactinide chemistry has developed and is still burgeoning today. A plethora of molecular actinide cyclopentadienyl (C5), arene (C6), cycloheptatrienyl (C7), and cyclooctatetraenyl (C8) complexes are known. However, the first f‐element cyclobutadienyl complex, a uranium derivative, was only reported as recently as 2013, which contrasts to transition metal chemistry where the first cyclobutadienyl derivatives were realised in the 1950s. A small but growing number of uranium and thorium cyclobutadienyl complexes are now known, so now is an opportune time to review progress to date. This Minireview addresses bonding considerations for the cyclobutadienyl ligand, surveys synthetic routes to crystallographically authenticated actinide cyclobutadienyl complexes and their novel bonding features, and highlights future directions that merit development.