EPR Studies at High-Pressures. V. Spin-Exchange Reactions of Nitroxide Radicals with Cobalt and Nickel Complexes

The rate constants k_ex for the spin exchange of nitroxides with various kinds of cobalt(II) and nickel(II) complexes were determined from the EPR line-broadening of nitroxides. The rate constants estimated were in the range of 108-10⁹ M^-1 s^-1 and this reaction was found to be diffusion-controlled. In order to distinguish cases of strong and weak spin exchange, we examined the pressure dependence of the spin exchange between nitroxide and the complexes. It was found that the ligand in the first coordination sphere of the complexes plays an important role for the efficiency of spin exchange. The presence of the coordinated halide ion provides strong exchange, regardless of any screening of the complex's internal atoms by organic molecules. From the pressure dependence of k_ex, the J² f_x values (the exchange integral J and the steric factor f_s) for the spin exchange of nitroxides with cobalt and nickel complexes were estimated to be about 10₂₀ s^-2. Based on the results, the steric hindrance for the reaction of spin exchange is discussed.