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The reaction chemistry of an unprecedented 'inorganic cumulene' - featuring a five-atom BNBNB chain - towards C[double bond, length as m-dash]O (and related) multiple bonds is disclosed. In marked contrast to related all-carbon systems, the intrinsic polarity of the BNBNB chain (featuring electron-rich nitrogen and electron-deficient boron centres) enables metathesis chemistry with electrophilic heteroallenes such as CO 2 and with organic carbonyl compounds. Transfer of the borylimide unit to [CO], [CS], [PP{(NDippCH 2 ) 2 }] and [C(H)Ph] moieties generates (boryl)N[double bond, length as m-dash]C[double bond, length as m-dash]X systems (X = O, S, PP{(NDippCH 2 ) 2 }, C(H)Ph), driven thermodynamically by B-O bond formation. Pairwise exchange of O and {(HCDippN) 2 }BN fragments occurs via consecutive [2+2] cyclo-addition/cyclo-reversion steps. An isolable complex of stoichiometry K[(boryl)NB(O)OC(H)Ph], formed via [2+2] cycloaddition to [(boryl)N[double bond, length as m-dash]B[double bond, length as m-dash]O] - can be shown to be an intermediate in the formation of (boryl)N[double bond, length as m-dash]C(H)Ph, and provides corroborating evidence for a DFT-calculated mechanism proceeding via a 'bora-Wittig' mechanism., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report a convenient synthesis of the zinc-boryl complex (Nacnac Mes )ZnBpin under mild conditions via formal disproportionation of bis(pinacolato)diboron(4), aided thermodynamically by B-O bond formation. This species can be isolated both base-free and as the DMAP adduct (Nacnac Mes )Zn(DMAP)Bpin and crystallographically characterised in the latter form. Onward reactivity of the base free zinc boryl complex with CO 2 occurs reductively, yielding further B-O bonds as well as CO gas. The strength of this driving force can then be harnessed in the reaction between (Nacnac Mes )ZnBpin and a Zn-O bonded species (Nacnac Mes )ZnOB{(NDippCH) 2 }, which yields the metal-metal bonded dimer [(Nacnac Mes )Zn] 2 , the overall formation of which utilizes a diboron(4) species as the stoichiometric reductant of Zn(ii) to Zn(i)., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The synthesis of a series of isostructural organometallic complexes featuring Ae-Tr bonds (Ae = Be, Mg; Tr = Al, Ga, In) has been investigated, and their electronic structures probed by quantum chemical calculations. This systematic study allows for comparison, not only of the metal-metal bonding chemistries of the two lightest alkaline earth (Ae) elements, beryllium and magnesium, but also of the three triel (Tr) elements, aluminium, gallium, and indium. Computational analyses (NBO, QTAIM, EDA-NOCV) reveal that Be-Tr bonding is more covalent than Mg-Tr bonding. More strikingly, these calculations predict that the beryllium-indyl complex - featuring the first structurally characterised Be-In bond - should act as a source of nucleophilic beryllium. This has been confirmed experimentally by its reactivity towards methyl iodide, which yields the Be-Me functionality. By extension, the electrophilic character of the beryllium centre in the beryllium-gallyl complex contrasts with the umpoled, nucleophilic behaviour of the beryllium centre in both the -indyl and -aluminyl complexes., Competing Interests: The authors declare no competing interests., (This journal is © The Royal Society of Chemistry.)