ION-PAIRING EFFECTS ON THE CO-59 ELECTRIC-FIELD GRADIENTS IN THE NMR RELAXATIONS OF TRIPOSITIVE COBALT(III) COMPLEX-IONS

Co-59 NMR relaxation rates for the [Co(chxn)(3)](3+) (chxn = trans-(1R,2R)-1,2-diaminocyclohexane) and [Co(en)(3)](3+)(en = ethylenediamine) ions were measured in the presence of sulfate or oxalate ions. H-1 relaxation rates for the chxn ligands in the same systems were also measured. The ratios of the Co-59 relaxation rates to the H-1 ones indicate that the Co-59 electric field gradient of [Co(chxn)(3)](3+) is appreciably decreased by the formation of the 1:1 ion pair with the sulfate ion. The difference of the concentration dependence of the Co-59 NMR relaxations between the sulfate and the oxalate ions can be attributed to the faster change in the configuration of the 1:1 ion pair of the oxalate ion.