Abstract
An e.s.r. study of Co3(CO)9S (S=½) in solution and doped in single crystals of the diamagnetic host FeCo2(CO)9S has shown from a detailed analysis of the hyperfine interaction that the unpaired electron is in a non-degenerate molecular orbital of a2 symmetry comprised primarily of an anti-bonding combination of d orbitals essentially localized in the plane of the cobalt atoms. These definitive results firmly substantiate the previous conclusions based on X-ray measurements of Co3(CO)9S and FeCo2(CO)9S that the unpaired electron in Co3(CO)9S has an appreciable conjugative destabilization effect (or antiaromatic character) in drastically altering the tricobalt geometry of Co3(CO)9S. The bonding in Co3(CO)9S and in the closely related Ni3(C5H5)3(CO)2 complex is discussed in light of these observations.

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