The delta bond - an old story with a new twist
The nature of the two electron 6-bond is examined in detail, with attention to the four states that constitute the 6- manifold, viz., 'Al, (ground state), 3A2u, 'A2, and 'Al,*, in a species, M2X8"-, with D4, symmetry. The effects of increasing the M-M distance, twisting from D4, through D4 to D4d, and configuration interaction are then discussed. Experimental measurements of the lA1, - 3A2u separations in MO,C~,(P-P)~ complexes with twist angles between 17 and 40" as well as CASSCF calculations for the model system. Mo2C14(PH3), are discussed. It is concluded that the 6-bond energy is 12 (Exp.) - 17 (theor.) kcal mol-l and the rotational barrier is 24.5 kcal mol-l.