X-ray Spectroscopy Characterization of Electronic Structure and Metal-Metal Bonding in Dicobalt Complexes

Developing multimetallic complexes with tunable metal-metal interactions has long been a target of synthetic inorganic chemistry efforts due to the unique properties that such compounds can exhibit. However, understanding relationships between metal-metal bonding and chemical properties is challenging due to system-dependent factors that influence metal-metal and metal-ligand interactions, including ligand identity, coordination geometry, and metal-metal distance. In this work, we apply X-ray absorption and emission spectroscopy and quantum chemical calculations to describe electronic structure and bonding in a series of dicobalt complexes. The compounds with silane ligands and pseudo-octahedral coordination geometry exhibit Co-Co σ and multicentered bonding character, which we characterize from both the occupied and vacant perspectives via their contributions to the Co X-ray emission and absorption spectra, respectively. In contrast, the dicobalt complexes with a pseudotetrahedral coordination environment do not exhibit Co-Co bonding due to symmetry constraints on orbital overlap. We extend these insights to the theoretical evaluation of related dicobalt complexes to explain how ligand coordination and symmetry dictate the presence or absence of a Co-Co bond. This work highlights how fundamental insights into electronic structure and bonding through X-ray spectroscopy uncover important factors governing metal-metal interactions and guide the rational design of multimetallic complexes with tunable metal-metal bonds.