Borolides (BC 4 2− ) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr 2 NBC 4 Ph 2 ) 2 (1) features an ∼90°angle between the Ni−B−N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni 2+ species bound to two monoanionic radical (BC 4•− ) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

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