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The actinide-actinide bonding in tri-actinide clusters [An₃Cl₆]ᶻ (An = Ac-Pu, z = 1-6) and [An₃Cl₆Cp₃]ᶻ (z =-2-+3; Cp = (η5-C5H5)) is studied using density functional theory. We find 3-centre bonding similar to the tri-thorium cluster [{Th(η⁸-C₈H₈)(μ₃-Cl)₂}₃{K(THF)₂}₂]∞, as we previously reported (Nature 2021, 598, 72-75). The population of 3-centre molecular orbitals (3c-MOs) by zero, one or two electrons correlates with shortening of the An-An bond lengths, which also decrease with increasing actinide atomic number, consistent with the contraction of the actinide valence atomic orbitals. Mulliken analyses indicate that these 3c-MOs predominantly involve An 6d and 5f orbitals. Various methods evidence the presence of An-An bonding in most systems with populated 3c-MOs, including bond orders (Mayer and Wiberg), quantum theory of atoms in molecules metrics (ρ, ∇²ρ, -G/V, H, delocalization indices), electron localization function, and electron density assessments. Additionally, we explore the effect of Cp ligand substitution on uranium complexes, finding that bulkier Cp ligands can induce U-U bond distortions and result in slightly longer U-U bonds. Overall, this study advances our understanding of metal-metal bonding in tri-actinide clusters, highlighting its effects on geometric and electronic structures., (© 2024 Wiley‐VCH GmbH.)

Reaction of the cesium antimonide complex [Cs(18C6) 2 ][SbH 2 ] ( 1 , 18C6 = 18-crown-6 ether) with the triamidoamine actinide separated ion pairs [An(Tren TIPS )(L)][BPh 4 ] (Tren TIPS = {N(CH 2 CH 2 NSi i Pr 3 ) 3 } 3- ; An/L = Th/DME ( 2Th ); U/THF ( 2U )) affords the triactinide undeca-antimontriide Zintl clusters [{An(Tren TIPS )} 3 (μ 3 -Sb 11 )] (An = Th ( 3Th ), U ( 3U )) by dehydrocoupling. Clusters 3Th and 3U provide two new examples of the Sb 11 3- Zintl trianion and are unprecedented examples of molecular Sb 11 3- being coordinated to anything since all previous reports featured isolated Sb 11 3- Zintl trianions in separated ion quadruple formulations with noncoordinating cations. Quantum chemical calculations describe dominant ionic An-Sb interactions in 3Th and 3U , though the data suggest that the latter exhibits slightly more covalent An-Sb linkages than the former. Complexes 3Th and 3U have been characterized by single crystal X-ray diffraction, NMR, IR, and UV/vis/NIR spectroscopies, elemental analysis, and quantum chemical calculations.

Reactions between [(Tren TIPS )U VI N] (1, Tren TIPS = {N(CH 2 CH 2 NSiPr i 3 ) 3 } 3- ) and [M II (η 5 -C 5 R 5 ) 2 ] (M/R = Cr/H, Mn/H, Fe/H, Ni/H) were intractable, but M/R = Co/H or Co/Me afforded [(Tren TIPS )U V N-(η 1 :η 4 -C 5 H 5 )Co I (η 5 -C 5 H 5 )] (2) and [(Tren TIPS )U IV -NH 2 ] (3), respectively. For M/R = V/H [(Tren TIPS )U IV -NV IV (η 5 -C 5 H 5 ) 2 ] (4), was isolated. Complexes 2-4 evidence one-/two-electron uranium reductions, nucleophilic nitrides, and partial N-atom transfer.

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1 H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29 Si NMR study of an isostructural series of locally D 3 h -symmetric early f-block metal(III) tris -hypersilanide complexes, [M{Si(SiMe 3 ) 3 } 3 (THF) 2 ] ( 1-M ; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe 3 signal was observed in the 29 Si ssNMR spectra of 1-M , while two SiMe 3 signals were seen in solution 29 Si NMR spectra of 1-La and 1-Ce . This is attributed to dynamic averaging of the SiMe 3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3 v -symmetric complexes [M{Si(SiMe 3 ) 3 } 3 (THF) n ] ( n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29 Si NMR data for diamagnetic 1-La , suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe 3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29 Si nuclei could only be reproduced for diamagnetic 1-La , showing the current limitations of pNMR calculations for larger nuclei., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

We introduce the boryloxide ligand {(HCNDipp) 2 BO} - (NBO Dipp , Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe 3 ) 2 } 3 ] with 3 equiv of NBO Dipp H produced the uranium(III) tris(boryloxide) complex [U(NBO Dipp ) 3 ] ( 1 ). In contrast, treatment of UCl 4 with 3 equiv of NBO Dipp K in THF at room temperature or reflux conditions produced only [U(NBO Dipp ) 2 (Cl) 2 (THF) 2 ] ( 2 ) with 1 equiv of NBO Dipp K remaining unreacted. However, refluxing the mixture of 2 and unreacted NBO Dipp K in toluene instead of THF afforded the target complex [U(NBO Dipp ) 3 (Cl)(THF)] ( 3 ). Two-electron oxidation of 1 with AdN 3 (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBO Dipp ) 3 (NAd)] ( 4 ). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene δ-back-bonding with approximately equal donation into the arene π 4 and π 5 δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f | m l , m s ⟩ states and m j projections. Complexes 1 - 4 have been variously characterized by single-crystal X-ray diffraction, 1 H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Comparison of bonding and electronic structural features between trivalent lanthanide (Ln) and actinide (An) complexes across homologous series' of molecules can provide insights into subtle and overt periodic trends. Of keen interest and debate is the extent to which the valence f- and d-orbitals of trivalent Ln/An ions engage in covalent interactions with different ligand donor functionalities and, crucially, how bonding differences change as both the Ln and An series are traversed. Synthesis and characterization (SC-XRD, NMR, UV-vis-NIR, and computational modeling) of the homologous lanthanide and actinide N-heterocyclic carbene (NHC) complexes [M(C 5 Me 5 ) 2 (X)(I Me4 )] {X = I, M = La, Ce, Pr, Nd, U, Np, Pu; X = Cl, M = Nd; X = I/Cl, M = Nd, Am; and I Me4 = [C(NMeCMe) 2 ]} reveals consistently shorter An-C vs Ln-C distances that do not substantially converge upon reaching Am 3+ /Nd 3+ comparison. Specifically, the difference of 0.064(6) Å observed in the La/U pair is comparable to the 0.062(4) Å difference observed in the Nd/Am pair. Computational analyses suggest that the cause of this unusual observation is rooted in the presence of π-bonding with the valence d-orbital manifold in actinide complexes that is not present in the lanthanide congeners. This is in contrast to other documented cases of shorter An-ligand vs Ln-ligand distances, which are often attributed to increased 5f vs 4f radial diffusivity leading to differences in 4f and 5f orbital bonding involvement. Moreover, in these traditional observations, as the 5f series is traversed, the 5f manifold contracts such that by americium structural studies often find no statistically significant Am 3+ vs Nd 3+ metal-ligand bond length differences.

Organoplutonium chemistry was established in 1965, yet structurally authenticated plutonium-carbon bonds remain rare being limited to π-bonded carbocycle and σ-bonded isonitrile and hydrocarbyl derivatives. Thus, plutonium-carbenes, including alkylidenes and N-heterocyclic carbenes (NHCs), are unknown. Here, we report the preparation and characterization of the diphosphoniomethanide-plutonium complex [Pu(BIPM TMS H)(I)(μ-I)] 2 ( 1Pu , BIPM TMS H = (Me 3 SiNPPh 2 ) 2 CH) and the diphosphonioalkylidene-plutonium complexes [Pu(BIPM TMS )(I)(DME)] ( 2Pu , BIPM TMS = (Me 3 SiNPPh 2 ) 2 C) and [Pu(BIPM TMS )(I)(I Me4 ) 2 ] ( 3Pu , I Me4 = C(NMeCMe) 2 ), thus disclosing non-actinyl transneptunium multiple bonds and transneptunium NHC complexes. These Pu-C double and dative bonds, along with cerium, praseodymium, samarium, uranium, and neptunium congeners, enable lanthanide-actinide and actinide-actinide comparisons between metals with similar ionic radii and isoelectronic 4f 5 vs 5f 5 electron-counts within conserved ligand fields over 12 complexes. Quantum chemical calculations reveal that the orbital-energy and spatial-overlap terms increase from uranium to neptunium; however, on moving to plutonium the orbital-energy matching improves but the spatial overlap decreases. The bonding picture that emerges is more complex than the traditional picture of the bonding of lanthanides being ionic and early actinides being more covalent but becoming more ionic left to right. Multiconfigurational calculations on 2M and 3M (M = Pu, Sm) account for the considerably more complex UV/vis/NIR spectra for 5f 5 2Pu and 3Pu compared to 4f 5 2Sm and 3Sm . Supporting the presence of Pu═C double bonds in 2Pu and 3Pu , 2Pu exhibits metallo-Wittig bond metathesis involving the highest atomic number element to date, reacting with benzaldehyde to produce the alkene PhC(H)═C(PPh 2 NSiMe 3 ) 2 ( 4 ) and "PuOI". In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4 .

Diphosphonioalkylidene dianions have emerged as highly effective ligands for lanthanide and actinide ions, and the resulting formal metal-carbon double bonds have challenged and developed conventional thinking about f-element bond multiplicity and covalency. However, f-element-diphosphonioalkylidene complexes can be represented by several resonance forms that render their metal-carbon double bond status unclear. Here, we report an experimentally-validated 13 C Nuclear Magnetic Resonance computational assessment of two cerium(iv)-diphosphonioalkylidene complexes, [Ce(BIPM TMS )(ODipp) 2 ] (1, BIPM TMS = {C(PPh 2 NSiMe 3 ) 2 } 2- ; Dipp = 2,6-diisopropylphenyl) and [Ce(BIPM TMS ) 2 ] (2). Decomposing the experimental alkylidene chemical shifts into their corresponding calculated shielding ( σ ) tensor components verifies that these complexes exhibit Ce[double bond, length as m-dash]C double bonds. Strong magnetic coupling of Ce[double bond, length as m-dash]C σ/π* and π/σ* orbitals produces strongly deshielded σ 11 values, a characteristic hallmark of alkylidenes, and the largest 13 C chemical shift tensor spans of any alkylidene complex to date (1, 801 ppm; 2, 810 ppm). In contrast, the phosphonium-substituent shielding contributions are much smaller than the Ce[double bond, length as m-dash]C σ- and π-bond components. This study confirms significant Ce 4f-orbital contributions to the Ce[double bond, length as m-dash]C bonding, provides further support for a previously proposed inverse-trans-influence in 2, and reveals variance in the 4f spin-orbit contributions that relate to the alkylidene hybridisation. This work thus confirms the metal-carbon double bond credentials of f-element-diphosphonioalkylidenes, providing quantified benchmarks for understanding diphosphonioalkylidene bonding generally., Competing Interests: The author declares no competing interests., (This journal is © The Royal Society of 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., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report the use of solution and solid-state 31 P Nuclear Magnetic Resonance (NMR) spectroscopy combined with Density Functional Theory calculations to benchmark the covalency of actinide-phosphorus bonds, thus introducing 31 P NMR spectroscopy to the investigation of molecular f-element chemical bond covalency. The 31 P NMR data for [Th(PH 2 )(Tren TIPS )] ( 1 , Tren TIPS = {N(CH 2 CH 2 NSiPr i 3 ) 3 } 3- ), [Th(PH)(Tren TIPS )][Na(12C4) 2 ] ( 2 , 12C4 = 12-crown-4 ether), [{Th(Tren TIPS )} 2 (μ-PH)] ( 3 ), and [{Th(Tren TIPS )} 2 (μ-P)][Na(12C4) 2 ] ( 4 ) demonstrate a chemical shift anisotropy (CSA) ordering of (μ-P) 3- > (═PH) 2- > (μ-PH) 2- > (-PH 2 ) 1- and for 4 the largest CSA for any bridging phosphido unit. The B3LYP functional with 50% Hartree-Fock mixing produced spin-orbit δ iso values that closely match the experimental data, providing experimentally benchmarked quantification of the nature and extent of covalency in the Th-P linkages in 1 - 4 via Natural Bond Orbital and Natural Localized Molecular Orbital analyses. Shielding analysis revealed that the 31 P δ iso values are essentially only due to the nature of the Th-P bonds in 1 - 4 , with largely invariant diamagnetic but variable paramagnetic and spin-orbit shieldings that reflect the Th-P bond multiplicities and s-orbital mediated transmission of spin-orbit effects from Th to P. This study has permitted correlation of Th-P δ iso values to Mayer bond orders, revealing qualitative correlations generally, but which should be examined with respect to specific ancillary ligand families rather than generally to be quantitative, reflecting that 31 P δ iso values are a very sensitive reporter due to phosphorus being a soft donor that responds to the rest of the ligand field much more than stronger, harder donors like nitrogen.

The tri-thorium cluster [{Th(η 8 -C 8 H 8 )(μ 3 -Cl) 2 } 3 {K(THF) 2 } 2 ] ∞ (Nature 2021, 598, 72-75) was reported to feature intriguing σ-aromatic bonding between the thorium atoms, a mode of metal-metal bonding unique in the actinide series. However, the presence of this bonding motif has since been challenged by others. Here, we computationally explore electron delocalisation in a molecular cluster fragment of [{Th(η 8 -C 8 H 8 )(μ 3 -Cl) 2 } 3 {K(THF) 2 } 2 ] ∞ and examine its responses to an applied magnetic field using a variety of methods. We also discuss the importance of the choice of basis set for the Th atoms and issues regarding locating QTAIM bond critical points. When taken together, the computed data consistently suggest the presence of delocalised Th-Th bonding and Th 3 σ-aromaticity., (© 2023 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

We report the synthesis and characterisation of a series of M(IV) substituted cyclopentadienyl hypersilanide complexes of the general formula [M(Cp R ) 2 {Si(SiMe 3 ) 3 }(X)] (M = Hf, Th; Cp R = Cp', {C 5 H 4 (SiMe 3 )} or Cp'', {C 5 H 3 (SiMe 3 ) 2 -1,3}; X = Cl, C 3 H 5 ). The separate salt metathesis reactions of [M(Cp R ) 2 (Cl) 2 ] (M = Zr or Hf, Cp R = Cp'; M = Hf or Th, Cp R = Cp'') with equimolar K{Si(SiMe 3 ) 3 } gave the respective mono-silanide complexes [M(Cp') 2 {Si(SiMe 3 ) 3 }(Cl)] (M = Zr, 1; Hf, 2), [Hf(Cp'')(Cp'){Si(SiMe 3 ) 3 }(Cl)] (3) and [Th(Cp'') 2 {Si(SiMe 3 ) 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(SiMe 3 ) 3 } has been reported previously. The salt elimination reaction of 2 with one equivalent of allylmagnesium chloride gave [Hf(Cp') 2 {Si(SiMe 3 ) 3 }(η 3 -C 3 H 5 )] (5), whilst the corresponding reaction of 2 with equimolar benzyl potassium yielded [Hf(Cp') 2 (CH 2 Ph) 2 ] (6) together with a mixture of other products, with elimination of both KCl and K{Si(SiMe 3 ) 3 }. Attempts to prepare isolated [M(Cp R ) 2 {Si(SiMe 3 ) 3 }] + cations from 4 or 5 by standard abstraction methodologies were unsuccessful. The reduction of 4 with KC 8 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 1 H, 13 C{ 1 H} and 29 Si{ 1 H} 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).

The salt metathesis reactions of the yttrium methanediide iodide complex [Y(BIPM)(I)(THF) 2 ] (BIPM = {C(PPh 2 NSiMe 3 ) 2 }) with the group 1 silanide ligand-transfer reagents MSiR 3 (M = Na, R 3 = t Bu 2 Me or t Bu 3 ; M = K, R 3 = (SiMe 3 ) 3 ) gave the yttrium methanediide silanide complexes [Y(BIPM)(Si t Bu 2 Me)(THF)] ( 1 ), [Y(BIPM)(Si t Bu 3 )(THF)] ( 2 ), and [Y(BIPM){Si(SiMe 3 ) 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 spectroscopies, and elemental analysis. Density functional theory calculations were performed on 1-3 to probe their electronic structures further, revealing 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 13 C{ 1 H} and 29 Si{ 1 H} 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. 29 Si{ 1 H} and 31 P{ 1 H} NMR spectra of these reaction mixtures indicated that 1,2-migratory insertion of the unsaturated substrate into the Y-Si bond is favored, while for the latter substrate, a [2 + 2]-cycloaddition reaction also occurs at the Y═C bond to afford [Y{C(PPh 2 NSiMe 3 ) 2 [C(NCy) 2 ]-κ 4 C , N , N ', N '}{C(NCy) 2 (Si t Bu 2 Me)-κ 2 N , N '}] ( 4 ); these reactivity profiles complement and contrast with those of Y(III) BIPM alkyl complexes.

We report the direct synthesis of the terminal pnictidenes [An(Tren TCHS )(PnH)][M(2,2,2-cryptand)] (Tren TCHS ={N(CH 2 CH 2 NSiCy 3 ) 3 } 3- ; An/Pn/M=Th/P/Na 5, Th/As/K 6, U/P/Na 7, U/As/K 8) and their conversion to the pnictides [An(Tren TCHS )(PnH 2 )] (An/Pn=Th/P 9, Th/As 10, U/P 11, U/As 12). Use of the super-bulky Tren TCHS ligand was essential to accessing complete families, and 6 is an unprecedented example of a terminal thorium-arsinidene complex and only the second structurally authenticated parent terminal arsinidene complex of any metal. Comparison of the terminal Th=AsH unit of 6 to the bridging ThAs(H)K linkage in structurally analogous [Th(Tren TIPS ){μ-As(H)K(15-crown-5)}] (Tren TIPS ={N(CH 2 CH 2 NSiPr i 3 ) 3 } 3- ) reveals a stronger Th-As bond in the former compared to the latter, and a large response overall to the nature of the Th=AsH bonding upon removal of the electrostatically-bound K-ion; the σ-bond changes little but the π-bond is significantly perturbed., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

We report the synthesis of the U(iii) bis(cyclopentadienyl) hypersilanide complex [U(Cp'') 2 {Si(SiMe 3 ) 3 }] (Cp'' = {C 5 H 3 (SiMe 3 ) 2 -1,3}), together with isostructural lanthanide and group 4 M(iii) homologues, in order to meaningfully compare metal-silicon bonding between early d- and f-block metals. All complexes were characterised by a combination of NMR, EPR, UV-vis-NIR and ATR-IR spectroscopies, single crystal X-ray diffraction, SQUID magnetometry, elemental analysis and ab initio calculations. We find that for the [M(Cp'') 2 {Si(SiMe 3 ) 3 }] (M = Ti, Zr, La, Ce, Nd, U) series the unique anisotropy axis is conserved tangential to ; this is governed by the hypersilanide ligand for the d-block complexes to give easy plane anisotropy, whereas the easy axis is fixed by the two Cp'' ligands in f-block congeners. This divergence is attributed to hypersilanide acting as a strong σ-donor and weak π-acceptor with the d-block metals, whilst f-block metals show predominantly electrostatic bonding with weaker π-components. We make qualitative comparisons on the strength of covalency to derive the ordering Zr > Ti ≫ U > Nd ≈ Ce ≈ La in these complexes, using a combination of analytical techniques. The greater covalency of 5f 3 U(iii) vs. 4f 3 Nd(iii) is found by comparison of their EPR and electronic absorption spectra and magnetic measurements, with calculations indicating that uranium 5f orbitals have weak π-bonding interactions with both the silanide and Cp'' ligands, in addition to weak δ-antibonding with Cp''., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report the synthesis and characterisation of a series of rare-earth mesoionic carbene complexes, [RE{N(SiMe 3 ) 2 } 3 {CN(Me)C(Me)N(Me)CH}] (3RE, RE=Sc, Ce, Pr, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), greatly expanding the limited library of f-block mesoionic carbene complexes. These complexes were prepared by treatment of the parent RE-triamides with an N-heterocyclic olefin (NHO), where an NHO backbone proton undergoes a formal 1,4-proton migration to the NHO-methylene group. For all RE(III) metals, as expected, quantum chemical calculations suggest only a σ-component to the metal-carbene bonding, in contrast to a previously reported uranium(III) congener where the 5f 3 metal engages in a weak π-back-bond to the MIC. All complexes were characterised by static variable-temperature magnetic measurements, and dynamic magnetic measurements reveal that 3Dy and 3Er are field-induced single-molecule magnets (SMMs), with U eff energy barriers of 35 and 128 K, respectively. Complex 3Dy is, as expected, a poorly performing SMM, but conversely 3Er performs unexpectedly well., (© 2022 Wiley-VCH GmbH.)

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe 3 ) 2 } 2 (CH 2 SiMe 2 NSiMe 3 ){MIC}] (An = U, 4U and Th, 4Th ; MIC = {CN(Me)C(Me)N(Me)CH}), which represent rare examples of actinide mesoionic carbene linkages and the first example of a thorium mesoionic carbene complex. Complexes 4U and 4Th were prepared via a C-H activation intramolecular cyclometallation reaction of actinide halides, with concomitant formal 1,4-proton migration of an N -heterocyclic olefin (NHO). Quantum chemical calculations suggest that the An-carbene bond comprises only a σ-component, in contrast to the uranium(III) analogue [U{N(SiMe 3 ) 2 } 3 (MIC)] ( 1 ) where computational studies suggested that the 5f 3 uranium(III) ion engages in a weak one-electron π-backbond to the MIC. This highlights the varying nature of actinide-MIC bonding as a function of actinide oxidation state. In solution, 4Th exists in equilibrium with the Th(IV) metallacycle [Th{N(SiMe 3 ) 2 } 2 (CH 2 SiMe 2 NSiMe 3 )] ( 6Th ) and free NHO ( 3 ). The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of Δ G 298.15K = 0.89 kcal mol -1 . Energy decomposition analysis (EDA-NOCV) of the actinide-carbon bonds in 4U and 4Th reveals that the former is enthalpically stronger and more covalent than the latter, which accounts for the respective stabilities of these two complexes., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Since the advent of organotransuranium chemistry six decades ago, structurally verified complexes remain restricted to π-bonded carbocycle and σ-bonded hydrocarbyl derivatives. Thus, transuranium-carbon multiple or dative bonds are yet to be reported. Here, utilizing diphosphoniomethanide precursors we report the synthesis and characterization of transuranium-carbene derivatives, namely, diphosphonio-alkylidene- and N -heterocyclic carbene-neptunium(III) complexes that exhibit polarized-covalent σ 2 π 2 multiple and dative σ 2 single transuranium-carbon bond interactions, respectively. The reaction of [Np III I 3 (THF) 4 ] with [Rb(BIPM TMS H)] (BIPM TMS H = {HC(PPh 2 NSiMe 3 ) 2 } 1- ) affords [(BIPM TMS H)Np III (I) 2 (THF)] ( 3Np ) in situ, and subsequent treatment with the N -heterocyclic carbene {C(NMeCMe) 2 } (I Me4 ) allows isolation of [(BIPM TMS H)Np III (I) 2 (I Me4 )] ( 4Np ). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPM TMS )Np III (I)(DME)] ( 5Np , BIPM TMS = {C(PPh 2 NSiMe 3 ) 2 } 2- ). Analogously, addition of benzyl potassium and I Me4 to 4Np gives [(BIPM TMS )Np III (I)(I Me4 ) 2 ] ( 6Np ). The synthesis of 3Np - 6Np was facilitated by adopting a scaled-down prechoreographed approach using cerium synthetic surrogates. The thorium(III) and uranium(III) analogues of these neptunium(III) complexes are currently unavailable, meaning that the synthesis of 4Np - 6Np provides an example of experimental grounding of 5f- vs 5f- and 5f- vs 4f-element bonding and reactivity comparisons being led by nonaqueous transuranium chemistry rather than thorium and uranium congeners. Computational analysis suggests that these Np III ═C bonds are more covalent than U III ═C, Ce III ═C, and Pm III ═C congeners but comparable to analogous U IV ═C bonds in terms of bond orders and total metal contributions to the M═C bonds. A preliminary assessment of Np III ═C reactivity has introduced multiple bond metathesis to transuranium chemistry, extending the range of known metallo-Wittig reactions to encompass actinide oxidation states III-VI.

Treatment of [U IV (N 3 )(Tren TIPS )] (1, Tren TIPS = {N(CH 2 CH 2 NSiPr 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 2 N 2 .

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., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Metal-metal bonding is a widely studied area of chemistry 1-3 , and has become a mature field spanning numerous d transition metal and main group complexes 4-7 . By contrast, actinide-actinide bonding, which is predicted to be weak 8 , is currently restricted to spectroscopically detected gas-phase U 2 and Th 2 (refs. 9,10 ), U 2 H 2 and U 2 H 4 in frozen matrices at 6-7 K (refs. 11,12 ), or fullerene-encapsulated U 2 (ref. 13 ). Furthermore, attempts to prepare thorium-thorium bonds in frozen matrices have produced only ThH n (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 bonding 15 , 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 bond 16,17 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., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

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 15 N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15 N 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 15 N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies., (© 2021. The Author(s).)

Disproportionation, where a chemical element converts its oxidation state to two different ones, one higher and one lower, underpins the fundamental chemistry of metal ions. The overwhelming majority of uranium disproportionations involve uranium(III) and (V), with a singular example of uranium(IV) to uranium(V/III) disproportionation known, involving a nitride to imido/triflate transformation. Here, we report a conceptually opposite disproportionation of uranium(IV)-imido complexes to uranium(V)-nitride/uranium(III)-amide mixtures. This is facilitated by benzene, but not toluene, since benzene engages in a redox reaction with the uranium(III)-amide product to give uranium(IV)-amide and reduced arene. These disproportionations occur with potassium, rubidium, and cesium counter cations, but not lithium or sodium, reflecting the stability of the corresponding alkali metal-arene by-products. This reveals an exceptional level of ligand- and solvent-control over a key thermodynamic property of uranium, and is complementary to isolobal uranium(V)-oxo disproportionations, suggesting a potentially wider prevalence possibly with broad implications for the chemistry of uranium., (© 2021. The Author(s).)

We report the use of 29 Si 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(Si t Bu 3 ) 2 (THF) 2 (THF) x ] ( 1-M : M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(Si t Bu 2 Me) 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 29 Si 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(SiMe 3 ) 3 } - -, {Si(SiMe 2 H) 3 } - -, and {SiPh 3 } - -substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29 Si NMR isotropic chemical shifts, δ Si , span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δ 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 δ 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.

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.

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 7λ 3 -(dimethylamino)phosphadibenzonorbornadiene-mediated P atom transfer approach. Experimental and computational characterization reveals that the diphosphorus ligand is activated to its dianionic (P 2 ) 2- form and that in-plane U-P π-bonding dominates the bonding of the U(μ-η 2 :η 2 -P 2 )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 -P 3 complexes, suggesting in situ generation of transient, reactive phosphido species.

Treatment of the new methanediide-methanide complex [Dy(SCS)(SCSH)(THF)] ( 1Dy , SCS = {C(PPh 2 S) 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(PPh 2 NSiMe 3 ) 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 ( U eff ) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy-4Dy join a privileged group of SMMs with U eff 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[double bond, length as m-dash]Dy[double bond, length as m-dash]C units with short average Dy[double bond, length as m-dash]C distances, and the latter have more bent C[double bond, length as m-dash]Dy[double bond, length as m-dash]C units with longer average Dy[double bond, length as m-dash]C bonds. Both U eff 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 U eff barrier and axial linearity in structurally comparable systems., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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 reduction chemistry of the newly emerging 2-phosphaethynolate (OCP) - is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(Tren TIPS )(OCP)] (2, Tren TIPS =[N(CH 2 CH 2 NSiPr i 3 )] 3- ), with RbC 8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(Tren TIPS )} 6 (μ-OC 2 P 3 ) 2 (μ-OC 2 P 3 H) 2 Rb 4 ] (5) featuring four five-membered [C 2 P 3 ] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(Tren TIPS )(μ-ORb)} 2 ] (6) and the known cyclometallated complex [Th{N(CH 2 CH 2 NSiPr i 3 ) 2 (CH 2 CH 2 SiPr i 2 CHMeCH 2 )}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2-phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP) - , rather than cycloaddition products that then decompose with [Th(Tren TIPS )O] - essentially acting as a protecting then leaving group. In contrast, when KC 8 or CsC 8 were used the phosphinidiide C-H bond activation product [{Th(Tren TIPS )}Th{N(CH 2 CH 2 NSiPr i 3 ) 2 [CH 2 CH 2 SiPr i 2 CH(Me)CH 2 C(O)μ-P]}] (3) and the oxo complex [{Th(Tren TIPS )(μ-OCs)} 2 ] (7) were isolated., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

We report the synthesis and characterisation of isostructural thorium(iv)- and uranium(iv)-silanide actinide (An) complexes, providing an opportunity to directly compare Th-Si and U-Si chemical bonds. Quantum chemical calculations show significant and surprisingly similar An%:Si%, 7s-, 6d-, and 5f-orbital contributions from both elements in polarised covalent An-Si bonds, and marginally greater covalency in the U-Si vs. Th-Si linkages.

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., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Treatment of [Ph 3 EMe][I] with [Na{N(SiMe 3 ) 2 }] affords the ylides [Ph 3 E=CH 2 ] (E=As, 1As; P, 1P). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P, confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E + -C - dipolar resonance form (sp 3 -C) over the E=C ene π-bonded form (sp 2 -C), as group 15 is descended. The uranium(IV)-cyclometallate complex [U{N(CH 2 CH 2 NSiPr i 3 ) 2 (CH 2 CH 2 SiPr i 2 CH(Me)CH 2 )}] reacts with 1As and 1P by α-proton abstraction to give [U(Tren TIPS )(CHEPh 3 )] (Tren TIPS =N(CH 2 CH 2 NSiPr i 3 ) 3 ; E=As, 2As; P, 2P), where 2As is an unprecedented structurally characterised arsonium-carbene complex. The short U-C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U-C distance is found for 2As than 2P, consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report the synthesis and structural authentication of the ditungsten decarbonyl dianion in [(OC) 5 W-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 D 4h eclipsed rather than D 4d 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 D 4d geometry is energetically preferred in the gas-phase, being ∼18 kJ mol -1 more stable than the D 4h form, since slight destabilisation of the degenerate W-CO π 5d xz and 5d yz orbitals is outweighed by greater stabilisation of the W-W σ-bond orbital. The gas-phase D 4h structure displays a single imaginary vibrational mode, intrinsic reaction coordinate analysis of which links the D 4h isomer directly to the D 4d 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.

The reaction of [UO 2 (μ-Cl) 4 {K(18-crown-6)} 2 ] with [{N(CH 2 CH 2 NSiPr i 3 ) 3 }Li 3 ] gives [{UO(μ-NCH 2 CH 2 N[CH 2 CH 2 NSiPr i 3 ] 2 )} 2 ] ( 1 ), [{(LiCl)(KCl)(18-crown-6)} 2 ] ( 2 ), and [LiOSiPr i 3 ] ( 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 U 2 N 2 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.

Despite the vast array of η n -carbocyclic C 5-8 complexes reported for actinides, cyclobutadienyl (C 4 ) remain exceedingly rare, being restricted to six uranium examples. Here, overcoming the inherent challenges of installing highly reducing C 4 -ligands onto actinides when using polar starting materials such as halides, we report that reaction of [Th(η 8 -C 8 H 8 ) 2 ] with [K 2 {C 4 (SiMe 3 ) 4 }] gives [{Th(η 4 -C 4 [SiMe 3 ] 4 )(μ-η 8 -C 8 H 8 )(μ-η 2 -C 8 H 8 )(K[C 6 H 5 Me] 2 )} 2 {K(C 6 H 5 Me)}{K}] ( 1 ), a new type of heteroleptic actinocene. Quantum chemical calculations suggest that the thorium ion engages in π- and δ-bonding to the η 4 -cyclobutadienyl and η 8 -cyclooctatetraenyl ligands, respectively. Furthermore, the coordination sphere of this bent thorocene analogue is supplemented by an η 2 -cyclooctatetraenyl interaction, which calculations suggest is composed of σ- and π-symmetry donations from in-plane in- and out-of-phase C[double bond, length as m-dash]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., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. Here, we report hydrogenolysis of a terminal uranium(V)-nitride under mild conditions even though it is electron-poor and not low-coordinate. Two divergent hydrogenolysis mechanisms are found; direct 1,2-dihydrogen addition across the uranium(V)-nitride then H-atom 1,1-migratory insertion to give a uranium(III)-amide, or with trimesitylborane a Frustrated Lewis Pair (FLP) route that produces a uranium(IV)-amide with sacrificial trimesitylborane radical anion. An isostructural uranium(VI)-nitride is inert to hydrogenolysis, suggesting the 5f 1 electron of the uranium(V)-nitride is not purely non-bonding. Further FLP reactivity between the uranium(IV)-amide, dihydrogen, and triphenylborane is suggested by the formation of ammonia-triphenylborane. A reactivity cycle for ammonia synthesis is demonstrated, and this work establishes a unique marriage of actinide and FLP chemistries.

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C 5-8 ), cyclobutadienyl (C 4 ) complexes remain exceedingly rare. Here, we report that reaction of [Li 2 {C 4 (SiMe 3 ) 4 }(THF) 2 ] (1) with [U(BH 4 ) 3 (THF) 2 ] (2) gives the pianostool complex [U{C 4 (SiMe 3 ) 4 }(BH 4 ) 3 ][Li(THF) 4 ] (3), where use of a borohydride and preformed C 4 -unit circumvents difficulties in product isolation and closing a C 4 -ring at uranium. Complex 3 is an unprecedented example of an f-element half-sandwich cyclobutadienyl complex, and it is only the second example of an actinide-cyclobutadienyl complex, the other being an inverse-sandwich. The U-C distances are short (av. 2.513 Å), reflecting the formal 2- charge of the C 4 -unit, and the SiMe 3 groups are displaced from the C 4 -plane, which we propose maximises U-C 4 orbital overlap. DFT calculations identify two quasi-degenerate U-C 4 π-bonds utilising the ψ 2 and ψ 3 molecular orbitals of the C 4 -unit, but the potential δ-bond using the ψ 4 orbital is vacant., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Little is known about the chemistry of the 2-arsaethynolate anion, but to date it has exclusively undergone fragmentation reactions when reduced. Herein, we report the synthesis of [U(Tren TIPS )(OCAs)] (2, Tren TIPS =N(CH 2 CH 2 NSiiPr 3 ) 3 ), which is the first isolable actinide-2-arsaethynolate linkage. UV-photolysis of 2 results in decarbonylation, but the putative [U(Tren TIPS )(As)] product was not isolated and instead only [{U(Tren TIPS )} 2 (μ-η 2 :η 2 -As 2 H 2 )] (3) was formed. In contrast, reduction of 2 with [U(Tren TIPS )] gave the mixed-valence arsenido [{U(Tren TIPS )} 2 (μ-As)] (4) in very low yield. Complex 4 is unstable which precluded full characterisation, but these photolytic and reductive reactions testify to the tendency of 2-arsaethynolate to fragment with CO release and As transfer. However, addition of 2 to an electride mixture of potassium-graphite and 2,2,2-cryptand gives [{U(Tren TIPS )} 2 {μ-η 2 (OAs):η 2 (CAs)-OCAs}][K(2,2,2-cryptand)] (5). The coordination mode of the trapped 2-arsaethynolate in 5 is unique, and derives from a new highly reduced and bent form of this ligand with the most acute O-C-As angle in any complex to date (O-C-As ∠ ≈128°). The trapping rather than fragmentation of this highly reduced O-C-As unit is unprecedented, and quantum chemical calculations reveal that reduction confers donor-acceptor character to the O-C-As unit., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

During our attempts to prepare paddlewheel actinide-molybdenum complexes of the type [(X)An(MesNPR 2 ) 3 Mo(CO) 3 ] (Mes= 2,4,6-trimethylphenyl; X = Cl or I; An = U or Th; R = i Pr or Ph) we have found that under certain conditions acetonitrile insertion reactions occur to give the heterobimetallic bridging ketimido species [ClAn(μ-MesNP i Pr 2 ) 2 (μ-MesNP i Pr 2 {μ-NCMe})Mo(CO) 3 ] (An = U, 1 ; Th, 2 ), [ClAn(μ-MesNPPh 2 ) 2 (μ-MesNPPh 2 {μ-NCMe})Mo(CO) 3 ] (An = U, 3 ; An = Th, 4 ), and [IAn(η 2 -MesNP i Pr 2 )(μ-MesNP i Pr 2 ){μ-NC(Me)N(Mes)P i Pr 2 }Mo(CO) 3 ] (An = U, 5 ; Th, 6 ). Structural and spectroscopic data confirm the assignment of a ketimido ligand bridging An(IV) and Mo(0) centers. The isolation of 1 - 6 is in contrast to our previously reported preparations of [(X)An(MesNPPh 2 ) 3 Mo(CO) 3 ] (An = U or Th; X= Cl or I; Chem. Commun. 2018 , 54 , 13515-13518) with the difference in reactivity being attributable to a combination of ancillary phosphino-amide, reaction solvent, and temperature variation. Complexes 1 - 5 represent the first examples of structurally characterized ketimido-bridged actinide-transition metal linkages and demonstrate the profound differences in reaction outcomes that can occur from relatively minor experimental changes.

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-[NCH 2 P(CH 3 ) 2 ]C 6 H 4 ) 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.

Although the chemistry of uranium-ligand multiple bonding is burgeoning, analogous complexes involving other actinides such as thorium remain rare and there are not yet any terminal thorium nitrides outside of cryogenic matrix isolation conditions. Here, we report evidence that reduction of a thorium-azide produces a transient Th≡N triple bond, but this activates C-H bonds to produce isolable parent imido derivatives or it can be trapped in an N-heterocycle amine. Computational studies on these thorium-nitrogen multiple bonds consistently evidences a σ > π energy ordering. This suggests pushing-from-below for thorium, where 6p-orbitals principally interact with filled f-orbitals raising the σ-bond energy. Previously this was dismissed for thorium, being the preserve of uranium-nitrides or the uranyl dication. Recognising that pushing-from-below perhaps occurs with thorium as well as uranium, and with imido ligands as well as nitrides, suggests this phenomenon may be more widespread than previously thought.

A fundamental bonding model in coordination and organometallic chemistry is the synergic, donor-acceptor interaction between a metal and a neutral π-acceptor ligand, in which the ligand σ donates to the metal, which π back-bonds to the ligand. This interaction typically involves a metal with an electron-rich, mid-, low- or even negative oxidation state and a ligand with a π* orbital. Here, we report that treatment of a uranium-carbene complex with an organoazide produces a uranium(V)-bis(imido)-dinitrogen complex, stabilized by a lithium counterion. This complex, which was isolated in a crystalline form, involves an electron-poor, high-oxidation-state uranium(V) 5f 1 ion that is π back-bonded to the poor π-acceptor ligand dinitrogen. We propose that this is made possible by a combination of cooperative heterobimetallic uranium-lithium effects and the presence of suitable ancillary ligands that render the uranium ion unusually electron rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-O fragmentations or couplings. Here, we report that reduction of [U(Tren TIPS )(OCP)] (Tren TIPS =N(CH 2 CH 2 NSiPr i 3 ) 3 ) with KC 8 /2,2,2-cryptand gives [{U(Tren TIPS )} 2 {μ-η 2 (OP):η 2 (CP)-OCP}][K(2,2,2-cryptand)]. The coordination mode of this trapped 2-phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P-C-O angle in any complex to date (P-C-O ∡ ≈127°). The characterisation data support a mixed-valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P-C-O unit that is perhaps best described as a uranium-stabilised OCP 2-. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P-C-O unit, which engages in a weak donor-acceptor interaction with one of the uranium ions., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)