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 CO2 and with organic carbonyl compounds. Transfer of the borylimide unit to [CO], [CS], [PP{(NDippCH2)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{(NDippCH2)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.
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