Effects of the strongly electron-withdrawing trifluoromethyl group in cobalt carbonyl chemistry

The neutral CF3 group is a strongly electron-withdrawing one-electron donor ligand found in the known cobalt complex Co(CF3)(CO)(4) with properties similar to Fe(CO)(5). Density functional theory predicts an axially-substituted trigonal bipyramidal structure for Co(CF3)(CO)(4). The lowest energy structures for the unsaturated Co(CF3)(CO)(n) (n = 3, 2, 1) derivatives can be derived from this Co(CF3)(CO)(4) structure by the successive removal of CO groups. In the binuclear Co-2(CF3)(2)(CO)(n) (n = 7, 6, 5, 4) derivatives the CF3 groups are always terminal CF3 groups. The lowest energy Co-2(CF3)(2)(CO) structures have two bridging CO groups. For Co-2(CF3)(2)(CO)(7) the lowest energy structure is an unsymmetrical doubly bridged structure with both CF3 groups bonded to the same cobalt atom. Higher energy Co-2(CF3)(2)(CO)(7) structures include a triply bridged structure analogous to the known Fe-2(CO)(9) structure as well as a singly bridged structure, both with a symmetrical distribution of the CF3 groups. For the unsaturated Co-2(CF3)(2)(CO)(n) (n = 6, 5, 4) systems, structures with coordinately unsaturated cobalt atoms having 16-electron configurations are energetically preferred over structures with cobalt-cobalt multiple bonds. (C) 2013 Elsevier B.V. All rights reserved.