Isotropic Shifts of Some Ionic Complexes of Cobalt(II) and Nickel(II) : Evidence for Ion Pairing

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Abstract
Isotropic proton resonance shifts have been observed for the ionic complexes, [Bu4N][φ3P)CoI3] and [Bu4N][φ3P)NiI3]. In addition to the phenyl (φ) proton shifts for the complex anions, isotropic shifts were observed for the cation protons as well. These shifts for the tetra‐n‐butylammonium cations are interpreted as arising solely from a pseudocontact interaction with the metal in the complex anion through partial ion pairing in the deuterochloroform solutions. From the magnitudes of the shifts for the butyl protons, an estimate is made for the relative extent of g‐tensor anisotropies for the two anionic complexes. The observed shifts for the phenyl protons are interpreted as arising from both contact and pseudocontact interactions. Based on the postulate that the unpaired spin will distribute itself in a like manner in a given ligand whether attached to cobalt or nickel, and using the estimate of the relative anisotropies of the two metals, the phenyl proton shifts are separated into their respective contact and pseudocontact contributions. The resulting unpaired spin densities on the phenyl rings compare well with those reported for the related bis(triphenylphosphine) complexes.
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