Paramagnetic transition metal carbonyls : IV. ESR study of anions derived from (CO)5MnPbPh3 and (CO)4CoPbPh3 by high energy radiation

Exposure of (CO)₅MnPbPh₃ to ⁶⁰Co γ-ray at 77 K gave one major paramagnetic species detectable by ESR spectroscopy. This exhibited an anisotropic hyperfine interaction with ⁵⁵Mn, near free-spin g-values, and a small, almost isotropic coupling to ²⁰⁷Pb. The form of the A(⁵⁵Mn) and g-tensor components suggest an orbital of d_z² symmetry on manganese for the unpaired electron, but this cannot be directed along the MnPb bond since the ²⁰⁷Pb hyperfine coupling indicates a very low spin-density on lead. We suggest that the centre is formed by electron addition to manganese to give a formal d⁷ centre, with concomitant loss of one equatorial carbonyl ligand. We defind z as the direction of the lost ligand. A second centre, detected at high gain, having a large hyperfine coupling to ²⁰⁷Pb and a 31 G coupling to ⁵⁵Mn is tentatively identified as the parent cation.In marked contrast, the molecule (CO)₄CoPbPh₃ gave a single centre having comparable ⁵⁹Co hyperfine and g-tensor components, but also a very large hyperfine coupling to ²⁰⁷Pb (ca. 3300 G). Thus, in this case, an electron gain centre (d⁹) has been formed, the electron being accomodated in the highest MO having a large d_z² component on cobalt (z being now the CoPb direction).Reasons for the adoption of these different structures are discussed.