Despite there being numerous examples of f‐element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C5–8), cyclobutadienyl (C4) complexes remain exceedingly rare. Here, we report that reaction of [Li2{C4(SiMe3)4}(THF)2] (1) with [U(BH4)3(THF)2] (2) gives the pianostool complex [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3), where use of a borohydride and preformed C4‐unit circumvents difficulties in product isolation and closing a C4‐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 C4‐unit, and the SiMe3 groups are displaced from the C4‐plane, which we propose maximises U−C4 orbital overlap. DFT calculations identify two quasi‐degenerate U−C4 π‐bonds utilising the ψ2 and ψ3 molecular orbitals of the C4‐unit, but the potential δ‐bond using the ψ4 orbital is vacant.

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Unprecedented silyl‐phosphino‐carbene complexes of uranium(IV) are presented, where before all covalent actinide–carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPMTMS)(Cl)(μ‐Cl)2Li(THF)2] (1, BIPMTMS=C(PPh2NSiMe3)2) into [U(BIPMTMS)(Cl){CH(Ph)(SiMe3)}] (2), and addition of [Li{CH(SiMe3)(PPh2)}(THF)]/Me2NCH2CH2NMe2 (TMEDA) gave [U{C(SiMe3)(PPh2)}(BIPMTMS)(μ‐Cl)Li(TMEDA)(μ‐TMEDA)0.5]2 (3) by α‐hydrogen abstraction. Addition of 2,2,2‐cryptand or two equivalents of 4‐N,N‐dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe3)(PPh2)}(BIPMTMS)(Cl)][Li(2,2,2‐cryptand)] (4) or [U{C(SiMe3)(PPh2)}(BIPMTMS)(DMAP)2] (5). The characterisation data for 3–5 suggest that whilst there is evidence for 3‐centre P−C−U π‐bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a true uranium alkylidene yet outside of matrix isolation experiments.

The reactions of anionic aluminium or gallium nucleophiles {K­[E­(NON)]}2 (E = Al, 1; Ga, 2; NON = 4,5-bis­(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp2) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be–Al or Be–Ga bonds (NON)­EBeCp (E = Al, 3; Ga, 4). The Be–Al bond in the beryllium–aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be–Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium–aluminyl complex with N,N′-diisopropylcarbodiimidethe electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium–diaminocarbene complex.

The complex diberyllocene, CpBeBeCp (Cp, cyclopentadienyl anion), has been the subject of numerous chemical investigations over the past five decades yet has eluded experimental characterization. We report the preparation and isolation of the compound by the reduction of beryllocene (BeCp 2 ) with a dimeric magnesium(I) complex and determination of its structure in the solid state by means of x-ray crystallography. Diberyllocene acts as a reductant in reactions that form beryllium-aluminum and beryllium-zinc bonds. Quantum chemical calculations indicate parallels between the electronic structure of diberyllocene and the simple homodiatomic species diberyllium (Be 2 ).

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 bond...

Back-bonding between an electron-poor, high-oxidationstate metal and poor -acceptor ligand in a uranium(V)-dinitrogen complex . Back-bonding between an electron-poor, high-oxidation-state metal and poor -acceptor ligand in a uranium(V)-dinitrogen complex. Nature Chemistry,11,[806][807][808][809][810][811] Back-bonding between an electron-poor, high-oxidation-state metal and poor π-acceptor ligand in a uranium(V)-dinitrogen complex Abstract A fundamental bonding model in coordination and organometallic chemistry is the synergic, donor-acceptor interaction between a metal and a neutral π-acceptor ligand where the ligand σdonates 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 organo-azide produces a uranium(V)-bis(imido)-dinitrogen complex, stabilised by a lithium counter-ion. This complex, which has been isolated in 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 rendering the uranium ion unusually electron-rich. This electron-poor back-bonding could have implications for the field of dinitrogen activation.

We report the first thorium–cyclobutadienyl complex, a new type of heteroleptic actinocene, that exhibits ‘an alkene-like’ thorium–η²-cyclooctatetraenyl interaction.