Unmasking the constitution and bonding of the proposed lithium nickelate “Li₃NiPh₃(solv)₃”: revealing the hidden C₆H₄ligand

Abstract

More than four decades ago, a complex identified as the planar homoleptic lithium nickelate "Li₃NiPh₃(solv)₃" was reported by Taube and co-workers. This and subsequent reports involving this complex have lain dormant since; however, the absence of an X-ray diffraction structure leaves questions as to the nature of the Ni-PhLi bonding and the coordination geometry at Ni. By systematically evaluating the reactivity of Ni(COD)₂ with PhLi under different conditions, we have found that this classical molecule is instead a unique octanuclear complex, [{Li₃(solv)₂Ph₃Ni}₂(μ-η²:η²-C₆H₄)] (5). This is supported by X-ray crystallography and solution-state NMR studies. A theoretical bonding analysis from NBO, QTAIM, and ELI perspectives reveals extreme back-bonding to the bridging C₆H₄ ligand resulting in dimetallabicyclobutane character, the lack of a Ni-Ni bond, and pronounced σ-bonding between the phenyl carbanions and nickel, including a weak σ_C-Li → s_Ni interaction with the C-Li bond acting as a σ-donor. Employing PhNa led to the isolation of [Na₂(solv)₃Ph₂NiCOD]₂ (7) and [Na₂(solv)₃Ph₂(NaC₈H₁₁)Ni(COD)]₂ (8), which lack the benzyne-derived ligand. These findings provide new insights into the synthesis, structure, bonding and reactivity of heterobimetallic nickelates, whose prevalence in organonickel chemistry and catalysis is likely greater than previously believed.