The whole story of the two-electron bond, with the δ bond as a paradigm

It is shown that the delta bond, as found particularly in the Re-2(6+) and Mo-2(4+) 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 twophoton 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.