An electron spin resonance study of isomerism in a Co0 acetylene π-complex

Abstract

The e.s.r. spectrum of (Ph₂C₂)Co(CO)[P(OEt)₃]₂, obtained by electrolytic reduction of (Ph₂C₂)Co₂(CO)₆ in THF solution in the presence of an excess of P(OEt)₃, is reported. The isotropic spectrum shows coupling to ⁵⁹Co and to two equivalent ³¹P nuclei [〈g〉= 2.065, 〈A^Co〉, 〈A^P〉=(43, 105)× 10^–4 cm^–1]. The spectrum of the frozen solution at 120 K shows the presence of two isomeric forms of the radical. One isomer has two equivalent ³¹P couplings, 〈A^P〉= 64 × 10^–4 cm^–1, while the other has two very different couplings, (168 and 39)× 10^–4 cm^–1. The radicals have similar approximately axial ⁵⁹Co hyperfine matrices which are consistent with semi-occupied molecular orbitals either cobalt d_x²–y² or d_z² in character, but the g-matrix and the magnitudes of the ³¹P couplings are consistent only with the latter choice. A d_z² SOMO suggests a radical with pseudo-C_3ν symmetry with two isomeric forms: axial–equatorial and equatorial–equatorial.