Electron paramagnetic resonance study of the 19-electron Co(CO)3L2 and Co(CO)2L2PPh3 complexes [L2= 2,3-bis(diphenylphosphino)maleic anhydride]

Abstract

Isotropic electron paramagnetic resonance spectra of square pyramidal Co(CO)₃L₂ and trigonal bipyramidal Co(CO)₂PPh₃L₂ in toluene, dichloromethane and THF solutions show small ⁵⁹Co hyperfine couplings and g-values close to the free-electron value, suggesting that the complexes are best described as Co^I coordinated by an L₂ radical anion. The frozen solution spectrum of Co(CO)₃L₂ in toluene or THF shows anisotropic ⁵⁹Co coupling corresponding to a 3d spin density of 0.016. In both cases, the ⁵⁹Co and ³¹P hyperfine couplings are strongly temperature dependent. Complex linewidth effects in the spectra of the PPh₃ derivative suggest a pseudorotation process which interconverts trigonal bipyramidal and square pyramidal conformations, averaging the couplings of the ³¹P nuclei of the L₂ ligand. Analysis of spectra of CH₂Cl₂ solutions leads to activation parameters for pseudorotation: ΔH‡= 14 kJ mol^–1, ΔS‡=–66 J mol^–1 K^–1.