Decarbonylation of As 2Co 2(CO) 6, a binuclear cobalt carbonyl derivative of diarsenic

Abstract

The structures and relative energies of the As 2Co 2(CO) n ( n = 6, 5, 4) derivatives are predicted by density functional theory to be analogous to those of the corresponding H 2C 2Co 2(CO) n derivatives. Thus As 2Co 2(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 As 2Co 2(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 As 2Co 2(CO) 5 the structure in which an equatorial group is removed from the As 2Co 2(CO) 6 structure and a singly bridged As 2Co 2(CO) 4(μ-CO) structure are predicted to have essentially the same energies, within <2 kcal/mol. A higher energy As 2Co 2(CO) 5 structure by 9 ± 2 kcal/mol is derived from the As 2Co 2(CO) 6 structure by removal of an axial carbonyl group. The two unbridged As 2Co 2(CO) 5 structures correspond to those observed experimentally in the photolysis of As 2Co 2(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 As 2Co 2(CO) 4. The analogous structure was not observed in the previous work with H 2C 2Co 2(CO) 4. However, such a H 2C 2Co(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 H 2C 2Co 2(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 As 2Co 2(CO) 4 structure, the same six unbridged structures were located for As 2Co 2(CO) 4 that were previously found for H 2C 2Co 2(CO) 6.