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 has been challenging due to system-dependent factors that can influence metal-metal and metal-ligand interactions including ligand identity, coordination geometry, and metal-metal distance. Moreover, experimental investigations often provide only indirect information about metal-metal bonding while direct experimental insight into orbital overlap is lacking. 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. In the compounds with a pseudo-octahedral coordination geometry, a single Co-Co σ-bond forms, for which we directly characterize both the σ-bonding and σ*-antibonding molecular orbitals 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 Co(II) ions in a tetrahedral ligand field. We extend the spectroscopically-driven insights into Co-Co bonding to diiron(II) (d6) complexes to develop conditions necessary for metal-metal σ-bonding to occur. We show that strong overlap of atomic orbitals with appropriate symmetry (e in tetrahedral ligand field, eg in octahedral ligand field) is a necessary condition for metal-metal σ-bonding to occur. Finally, we demonstrate how the orbital overlap arguments can be applied to related dicobalt complexes to resolve uncertainties regarding the presence or absence of a Co-Co bond in these species. 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.