Two-Center/Three-Electron Sigma Half-Bonds in Main Group and Transition Metal Chemistry

ConspectusFirst proposed in a classic Linus Pauling paper, the two-center/three-electron (2c/3e) σ half-bond challenges the extremes of what may or may not be considered a chemical bond. Two electrons occupying a σ bonding orbital and one electron occupying the antibonding σ* orbital results in bond orders of ∼0.5 that are characteristic of metastable and exotic species, epitomized in the fleetingly stable He2+ ion.In this Account, I describe the use of coordination chemistry to stabilize such fugacious three-electron bonded species at disparate ends of the periodic table. A recent emphasis in the chemistry of metal–metal bonds has been to prepare compounds with extremely short metal–metal distances and high metal–metal bond orders. But similar chemistry can be used to explore metal–metal bond orders less than one, including 2c/3e half-bonds. Bimetallic compounds in the Ni2(II,III) and Pd2(II,III) oxidation states were originally examined in the 1980s, but the evidence collected at that time suggested that they did not contain 2c/3e σ bonds. Both classes of compounds have been re-examined using EPR spectroscopy and modern computational methods that show the unpaired electron of each compound to occupy a M–M σ* orbital, consistent with 2c/3e Ni–Ni and Pd–Pd σ half-bonds.Elsewhere on the periodic table, a seemingly unrelated compound containing a trigonal bipyramidal Cu3S2 core caused a stir, leaving prominent theorists at odds with one another as to whether the compound contains a S–S bond. Due to my previous experience with 2c/3e metal–metal bonds, I suggested that the Cu3S2 compound could contain a 2c/3e S–S σ half-bond in the previously unknown oxidation state of S23–. By use of the Cambridge Database, a number of other known compounds were identified as potentially containing S23– ligands, including a noteworthy set of cyclopentadienyl-supported compounds possessing diamond-shaped Ni2E2 units with E = S, Se, and Te. These compounds were subjected to extensive studies using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, density functional theory, and wave function-based computational methods, as well as chemical oxidation and reduction. The compounds contain E–E 2c/3e σ half-bonds and unprecedented E23– "subchalcogenide" ligands, ushering in a new oxidation state paradigm for transition metal–chalcogen chemistry.