NUCLEAR MAGNETIC RESONANCE LINE-BROADENING STUDY OF COBALT(II) AND NICKEL(II) IN DIMETHYL SULFOXIDE AND TRIMETHYL PHOSPHATE

The effects of cobalt(II) and nickel (II) on the proton nmr of dimethyl sulfoxide (DMSO) and trimethyl phosphate (TMPA) have been studied. Chemical exchange controlled line broadening was found only for nickel(II) in DMSO, in which case the exchange rate is 4.2 X 103sec-1 at 25° with ΔH≠ = 12.1 (±0.3) kcal mol-1 and ΔS≠ = 1.3 (±0.5) eu. In all other cases the chemical exchange was too fast to measure and only dipolar line broadening was observed. Chemical shifts were measured in all cases. The hyperfine coupling constants (A/h) for Ni2+ in DMSO and TMPA are 7.50 X 104 and 7.16 X 103 Hz, and for Co2+ in DMSO, the constant is 2.08 X 104 Hz. The chemical shift for the Co2+-TMPA system changes from positive to negative as the temperature decreases. This observation, combined with a spectrophotometric study, indicates an octahedral- tetrahedral equilibrium of the type Co(TMPA)42+ + 2TMPA ⇄ Co(TMPA)62+. The position of this equilibrium does not permit the extinction coefficient or hyperfine coupling constant of the tetrahedral species to be determined with any certainty. However if an extinction coefficient of 500 is assumed for Co(TMPA)42+, then a self-consistent fit of the temperature dependence of the absorbance, chemical shift, and line broadening can be obtained. The fit is far from unique however and a range of extinction coefficients from about 400 to 1200 would also fit the data. It has been found that the activation enthalpy for solvent exchange in a number of systems may be predicted by assuming that the activation enthalpy is made up of a general solvation term, which is constant for a given solvent and charge on the metal ion, and a crystal field contribution. © 1969, American Chemical Society. All rights reserved.