THE DELTA BOND - AN OLD STORY WITH A NEW TWIST

The nature of the two electron delta-bond is examined in detail, with attention to the four states that constitute the delta-manifold, viz., 1A1g (ground state), 3A2u, 1A2u and 1A1g*, in a species, M2X8n- with D4h symmetry. The effects of increasing the M-M distance, twisting from D4h through D4 to D4d, and configuration interaction are then discussed. Experimental measurements of the 1A1g - 3A2u separations in Mo2Cl4(P-P)2 complexes with twist angles between 17 and 40-degrees as well as CASSCF calculations for the model system. Mo2Cl4(PH3)4 are discussed. It is concluded that the delta-bond energy is 12 (Exp.) - 17 (theor.) kcal mol-1 and the rotational barrier is 24.5 kcal mol-1.