The Whole Story of the Two-Electron Bond, with the δ Bond as a Paradigm

Abstract
It is shown that the δ bond, as found particularly in the Re26+ and Mo24+ cores of hundreds of compounds, provides a paradigm for the behavior of two-electron bonds of all types. By control of the angle of twist around the M−M axis, the strength of the bond can be systematically varied. By means of conventional electronic spectroscopy, nuclear magnetic resonance spectroscopy, and two-photon excitation spectroscopy, the entire picture of the manifold of four states for two electrons bonding two atoms, as first described by Coulson and Fischer in 1949, has been confirmed.