The whole story of the two-electron bond, with the delta 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 behavi or of two-electron bonds of all types. By control of the angle of twist aro und the M-M axis, the strength of the bond can be systematically varied By means of conventional electronic spectroscopy, nuclear magnetic resonance s pectroscopy, and twophoton excitation spectroscopy, the entire picture of t he manifold of four states for two electrons bonding two atoms, as first de scribed by Coulson and Fischer in 1949, has been confirmed.