Reactivity of Transition Metal Complexes in Excited States: C−O Bond Coupling Reductive Elimination of a Ni(II) Complex Is Elicited by the Metal-to-Ligand Charge Transfer State

Reactivity of electronically excited base transition metals is an emerging frontier wherein mechanistic understanding is highly desired but mostly lacking. To reveal how C−O bond coupling reductive elimination (RE) is stimulated by excited NiII (Welin, E. R.; Le, C.; Arias-Rotondo, D. M.; McCusker, J. K.; MacMillan, D. W. C. Science, 2017, 355, 380), we report here high-level theoretical modellings based on a combined ab initio protocol (CASSCF, CASPT2, DLPNO-CCSD(T)). In contrast to the experimental proposal of d-d excited state, we find that metal-to-ligand charge transfer (MLCT) excited state is most likely to stimulate the C−O coupling RE. This unprecedented assignment of the reactive excited state not only obviates the known thermo-dynamic prohibition of C−O coupling by ground state NiII, but also matches the experimental triplet energy requisite for energy transfer. In addition, the enhanced RE reactivity in excited NiII can be well rationalized by NiIII character of the MLCT state. The resolution o...