σ-Noninnocence: Masked Phenyl-Cation Transfer at Formal Ni^IV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of −2 at a transition-metal center. For a series of formal high-valent Ni^IV complexes, aryl–CF₃ bond-forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni^IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ-noninnocence. A direct consequence is that the elimination of aryl–CF₃ products occurs in an essentially redox-neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF₃ group is transferred to an electrophilic aryl group. The uncovered role of σ-noninnocence in metal–ligand bonding, and of an essentially redox-neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.