Dithiolenes revisited: an electron spin resonance study of some five-co-ordinate cobalt complexes and the crystal structures of [Co{S2C2(CF3)2}2{P(OPh)3] and [Co{S2C2(CF3)}2(PPh3)]

Abstract

Isotropic and frozen-solution ESR spectra in CH₂Cl₂–ClCH₂CH₂Cl have been recorded for the five-co-ordinate, formally cobalt(IV) complexes [Co(S₂C₂R₂)₂L][R = CN, L = PEt₃; R = CF₃or Ph, L = PPh₃or P(OPh)₃; R = C₆H₄Me-4, L = PPh₃or PEt₃; R = C₆H₄OMe-4, L = PPh₃]. The spectra can be described by approximately axial g and ⁵⁹Co hyperfine matrices with g_∥=g_x≈ 1.99, g_⊥≈ 2.03 and A_∥=A_y≈ 61 × 10^–4cm ^–1, A_⊥≈ 7 × 10^–4cm^–1 These parameters are interpreted to show that the cobalt electronic structure is best regarded as low-spin d⁵, formally Co^IV, but the singly occupied molecular orbital is extensively delocalized with only about 25% cobalt 3d_xzcharacter. The g and A principal axes are displaced in the xy plane by the angle α which varies from 2 to 31°, increasing with the steric bulk of R and L. This angle is related to the degree of d_xz/d_yz, hybridization resulting from molecular distortion from C_2v, symmetry. The structures of [Co{S₂C₂(CF₃)₂}₂L][L = P(OPh)₃ or PPh₃] were determined by X-ray diffraction methods Both complexes are approximately square pyramidal, but for L = P(OPh)₃(diamagnetic in the solid state) the molecules are packed in the crystal as face-to-face pairs (Co ⋯ Co 4.11 Å). Extended-Hückel molecular-orbital calculations performed for [Co{S₂C₂-(CF₃)₂}₂{P(OH)₃}] support the interpretation of the ESR results.