X-ray crystal structure of the trifluoroacetylcobalt complex CF3COCo(CO)3(PPh3) – Implications for the relationship between structure and reactivity toward migratory insertion of carbon monoxide in cobalt alkyl complexes

In order to gain insight into the bonding in perfluoroalkyl and perfluoroacyl complexes of transition metals, the X-ray crystal structure of CF3COCo(CO)3(PPh3) has been determined. Comparison of this structure with that previously reported by us for CF3Co(CO)3(PPh3) and several other acyl/alkyl pairs reported by others highlights the cobalt–carbon bond contraction in the perfluoroalkyl complex and provides an explanation for why such complexes do not undergo migratory insertion of CO. Comparisons of cobalt–carbon bond lengths in hydrocarbon alkyl and acyl complexes show that the acyl complexes exhibit uniformly shorter bonds than the alkyl complexes, consistent with their ability to undergo CO insertion under mild conditions, and in contrast to the shorter Co–C bond length in the CF3 complex relative to that in the COCF3 species. Several other unique features of the bonding in the CF3 complex become evident upon comparison with the CF3CO complex and several hydrocarbon alkyl and acyl complexes. Other interesting comparisons include carbonyl stretching frequencies in the IR spectra of these complexes.