Proton Magnetic Resonance Shifts in Aqueous Cobalt(II) Solutions Containing Various Monodentate Ligands

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Abstract
Isotropic PMR shifts for protons of water in aqueous solutions of cobalt(II) containing a variety of monodentate ligands have been measured. The results indicate that the magnitude of the shift in the bulk water resonance does not invariably decrease as water is diplaced from the primary coordination sphere. A marked increase in the shift upon complexation was observed for the system cobalt(II)—thiocyanate which was studied in detail. These results may be understood in terms of a significant contribution of the dipolar (pseudocontact) interaction to the total observed shift in these magnetically anisotropic cobalt(II) complexes. Relevant data in the literature concerning cobalt(II)—methanol—water complexes may also be interpreted in these terms. The present findings suggest that measurement of solventNMR shifts in cobalt(II) solutions containing potential ligands may not be as useful in determining the average number of solvent molecules displaced from the first coordination sphere as previously suggested.