Electron spin resonance study of the frozen-solution equilibrium [Co{S2C2(CF3)2}2L]+ L ⇌[Co{S2C2(CF3)2}2L2][L = P(OR)3 or PPh(OMe)2; R = alkyl]

Abstract

The ESR spectra have been recorded for a series of cobalt dithiolene complexes [Co(S₂C₂R₂)₂L][R = CF₃; L = P(OMe)₃, P(OEt)₃, P(OBuⁿ)₃ or PPh(OMe)₂]. In toluene, tetrahydrofuran or CH₂Cl₂–1,2-C₂H₄Cl₂ solutions containing a small excess of L, reversible spectral changes are observed between 165 and 130 K which correspond to the addition of a second ligand to form a six-co-ordinate complex. For L = P(OMe)₃ in toluene, equilibrium constants were estimated from the spectra which lead to ΔH°=–10 kJ mol^–1, ΔS°=–68 J K^–1 mol^–1 for the five-/six-co-ordinate equilibrium. Ligand addition is not observed when CF₃ is replaced by Ph, 4-MeC₆H₄ or 4-MeOC₆H₄ or L = PPh₃, PEt₃, P(OPrⁱ)₃, P(OCH₂CF₃)₃ or P(OPh)₃. Complexes with L =(Ph₂P)₂CH₂, (Ph₂PCH₂)₂ or [(MeO)₂PCH₂]₂ are five-co-ordinate with no evidence for chelation. Thus both steric and electronic effects are critical to the ability of the five-co-ordinate complex to add a sixth ligand. Unlike the five-co-ordinate complexes which are significantly distorted from ideal C_2v symmetry, the six-co-ordinate complexes are highly symmetrical with greater delocalization of the unpaired electron onto the phosphite ligands.