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 vari ous kinds of cobalt(II) and nickel(II) complexes were determined from the EPR line-broadening of nitroxides. The rate constants estimated we re in the range of 10(8)-10(9) M-1 s(-1) and this reaction was found t o be diffusion-controlled In order to distinguish cases of strong and weak spin exchange, we examined the pressure dependence of the spin ex change between nitroxide and the complexes. It was found that the liga nd in the first coordination sphere of the complexes plays an importan t role for the efficiency of spin exchange. The presence of the coordi nated halide ion provides strong exchange, regardless of any screening of the complex's internal atoms by organic molecules. From the pressu re dependence of k(ex), the J(2)f(s) values (the exchange integral J a nd the steric factor f(s)) for the spin exchange of nitroxides with co balt and nickel complexes were estimated to be about 10(20) s(-2). Bas ed on the results, the steric hindrance for the reaction of spin excha nge is discussed.