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 Co2(CF3)2(CO)n (n = 7, 6, 5, 4) derivatives the CF3 groups are always terminal CF3 groups. The lowest energy Co2(CF3)2(CO)n structures have two bridging CO groups. For Co2(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 Co2(CF3)2(CO)7 structures include a triply bridged structure analogous to the known Fe2(CO)9 structure as well as a singly bridged structure, both with a symmetrical distribution of the CF3 groups. For the unsaturated Co2(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.

The lowest energy structure of the known Co(CF3)(CO)4 is a trigonal bipyramid with the CF3 group in an axial position. The lowest energy Co2(CF3)2(CO)n structures have two bridging CO groups. For the unsaturated Co2(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.Figure optionsDownload as PowerPoint slideHighlights
► An axially-substituted trigonal bipyramidal structure is predicted for Co(CF3)(CO)4.
► The CF3 groups are always terminal CF3 groups.
► The lowest energy Co2(CF3)2(CO)n structures have two bridging CO groups.
► For Co2(CF3)2(CO)7 the lowest energy structure has both CF3 groups bonded to the same cobalt atom.
► Higher energy Co2(CF3)2(CO)7 structures include a triply bridged structure analogous to Fe2(CO)9.