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 and desirable properties that such compounds can exhibit. However, understanding the relationship between metal-metal bonding and chemical properties in multimetallic compounds is challenging due to system-dependent factors that can influence metal-metal and metal-ligand interactions including ligand identity, coordination geometry, and metal-metal distance. In this work we apply a combination of X-ray absorption and emission spectroscopy and quantum chemical calculations to describe the electronic structure and bonding properties in a series of dicobalt complexes supported by expanded pincer PNNP ligands. The compounds with silane ligands and a pseudo-octahedral coordination geometry exhibit Co-Co σ and multicentered bonding character, which we characterize from both the occupied and vacant perspectives via their strong contributions to the Co K-edge X-ray emission and absorption spectra, respectively. In contrast, the dicobalt complexes with a pseudo-tetrahedral coordination environment do not exhibit Co-Co bonding, due to symmetry constraints on orbital overlap and the 3d orbital occupancies of Co2+ ions in a tetrahedral ligand field. We extend the spectroscopically driven insights to theoretical evaluation of related dicobalt complexes with silane and hydride ligands to explain the presence or absence of a Co-Co bond in these species based on ligand coordination and symmetry arguments. 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.