Nickel-Borolide Complexes and Their Complex Electronic Structure

Borolides (BC₄^2-) 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(iPr₂NBC₄Ph₂)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 Ni²⁺ species bound to two monoanionic radical (BC₄^•-) 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.