Isotropic Proton Resonance Shifts of Some Bis-(triarylphosphine) Complexes of Cobalt(II) and Nickel(II) Dihalides

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Abstract
Isotropic proton resonance shifts have been observed for some bis-(triarylphosphine) complexes of cobalt(II) and nickel(II) dihalides. The equations for the pseudocontact shifts were extended to include these complexes which possess g tensors with C2v symmetry. The isotropic shifts for the nickel complexes are interpreted as arising solely from a contact interaction with unpaired spin in the π orbital of the ligand. Based on the postulate that unpaired spin will distribute itself in the same way in a given ligand whether attached to cobalt or nickel, the shifts for the cobalt complexes are separated into contact and pseudocontact contributions. The relative magnitudes of the pseudocontact shifts correspond well with the expected relative strength of the C2v distortion of the tetrahedral ligand field of the cobalt complexes. The evaluation of the proportionality constant, QCH3, between the unpaired spin density and the hyperfine interaction constant for the methyl-substituted complexes demonstrates the consistency of the interpretation of the shifts. The difference in the unpaired spin densities on the cobalt and nickel complexes is explained by slightly greater interelectronic respulsion in the t orbitals of the nickel complexes.
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