The Nature of One-Electron σ-Bonds Probed by X-Ray Quantum Crystallography

Two-center one-electron (2c-1e) σ bond is a unique class of bonding that departs from the century-old shared electron-pair model of covalent bonds. To resolve their bonding features, it is essential to capture the subtle features of electron density localized between the two centers. Here, we have employed X-ray quantum crystallography (QCr)-based models for a quantitative evaluation of this elusive class of bonds across a diverse series of compounds, which apparently exhibit C•C, C•B, B•B, and Cu•M (M = B, Al, Ga) one-electron bonds. The electron density at the bond critical points of these bonds is notably low and comparable to that typically observed for π···π and other noncovalent interactions. In contrast, the bond orders span a wide range, from ~0.6-approximately half of a single bond-to values ~1. Combined bonding analyses and residual electron density maps obtained from QCr models indicate that these interactions are more appropriately described as weak, noncovalent interactions rather than a distinct subclass of covalent bonds.