OPTIMIZED STRUCTURES OF CR-2(CO)(6)(-FUNCTIONAL CALCULATION() WITH 3 DIFFERENT SYMMETRIES BY DENSITY)

Density functional calculations have been made on a binuclear metal ca rbonyl ion Cr-2(CO)(6)(+) found in our laser ablation-molecular beam ( LAMB) experiment. Optimized structures are calculated for three differ ent conformations: T33 of D-3d symmetry with three terminal carbonyl g roups on each chromium atom, B2T22 of D-2h symmetry with two bridging carbonyl groups and two terminal carbonyl groups on each chromium atom , and B4T11 of D-4h symmetry with four bridging carbonyl groups and on e terminal carbonyl group on each chromium atom. The most stable confo rmation is T33 which is 36.76 and 286.44 kJ mol(-1) lower in energy th an B2T22 and B4T11, respectively. The difference of conformation exert s a significant influence on the internuclear distance between chromiu m and the carbon of terminal CO, but hardly on the Cr-Cr bond length. For B2T22 and B3T11, longer C-O distances for bridging carbonyls compa red with those for terminal ones indicate effective pi-back donation from the chromium atom to the bridging carbonyl groups. Furthermore, t he relative abundance of Cr-2(CO)(n)(+) (n = 0-6) observed in our prev ious experimental study can be explained qualitatively by comparison o f the excess energy produced in the formation of a Cr+-Cr bond with th e CO dissociation energy of Cr-2(CO)(6)(+). (C) 1998 John Wiley & Sons , Ltd.