Decarbonylation of As2Co2(CO)6, a binuclear cobalt carbonyl derivative of diarsenic

The structures and relative energies of the As2Co2(CO)n (n = 6, 5, 4) derivatives are predicted by density functional theory to be analogous to those of the corresponding H2C2Co2(CO)n derivatives. Thus As2Co2(CO)6 is predicted to have three carbonyls on one cobalt atom eclipsed relative to the three carbonyls on the other cobalt atom. The corresponding As2Co2(CO)6 structure with a staggered rather than eclipsed arrangement of the Co(CO)3 units is a transition state rather than a genuine minimum. For As2Co2(CO)5 the structure in which an equatorial group is removed from the As2Co2(CO)6 structure and a singly bridged As2Co2(CO)4(μ-CO) structure are predicted to have essentially the same energies, within <2 kcal/mol. A higher energy As2Co2(CO)5 structure by 9 ± 2 kcal/mol is derived from the As2Co2(CO)6 structure by removal of an axial carbonyl group. The two unbridged As2Co2(CO)5 structures correspond to those observed experimentally in the photolysis of As2Co2(CO)6 in Nujol matrices at low temperatures. In such photolysis experiments the higher energy isomer is produced initially and then converted to the lower energy isomer upon annealing. A singly bridged structure was found for As2Co2(CO)4. The analogous structure was not observed in the previous work with H2C2Co2(CO)4. However, such a H2C2Co(CO)3(μ-CO) structure is found here for the acetylene complex. This singly bridged structure is predicted to lie 1.9 kcal/mol below the H2C2Co2(CO)4 4–1S structure by the BP86 method but 3.5 kcal/mol above the latter by the B3LYP method. In addition to the singly bridged As2Co2(CO)4 structure, the same six unbridged structures were located for As2Co2(CO)4 that were previously found for H2C2Co2(CO)6.