Paramagnetic transition-metal carbonyls. Part 6. Derivatives of dicarbonyl(η5-cyclopentadienyl)cobalt(I)

Abstract

Exposure of solutions of the title complex [Co(cp)(CO)₂] to ⁶⁰Co γ-rays at 77 K has produced a radical identified as the parent anion by its e.s.r. spectrum. Electron addition has been confirmed by photoionisation of the electron source NNN′N′-tetramethyl-p-phenylenediamine. Analysis of the ⁵⁹Co hyperfine structure and g-tensor components suggests a d¹_yz structure, where z bisects the OC–Co–CO angle and y lies in the OC–Co–CO plane. This σ* orbital is well removed from the remaining filled d orbitals. In methanolic solutions, and on annealing a methyltetrahydrofuran (mthf) solution above 77 K, a second species, with similar magnetic properties but exhibiting an extra 13 G doublet splitting, is also observed. This extra splitting is assigned to hyperfine coupling with a single proton, probably attached to one of the carbonyl groups. Other spectra observed in solutions and in the pure solid are attributed to the parent cation and to a dimeric cation respectively. The results for the anion are compared with solid-state data for the dimer anion [{Co(cp)(CO)}₂]^– and its isoelectronic analogue [(cp)Co(NO)(CO)Co(cp)], both previously studied only in the liquid phase. Our results for these dimers show that the two cobalt atoms are identical in the solid state, that the extra electron is in a π* orbital having ca. 77% cobalt character, and that reduction to the dimer proceeds via the parent anion. A stable trimeric cobalt radical is formed when [Co(cp)(CO)₂] in mthf solution is γ-irradiated at room temperature.