Ion pairing and possible nucleophilic assistance in anation reactions of some aquorhodium(III) complexes

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Abstract
Anation reactions of [Rh(NH3)5OH2]3+ with chloride and bromide have been studied at an ionic strength of 1·5 M. Pseudo-first-order rate constants do not increase linearly with concentration of anion, and this behaviour can be interpreted on the usual basis of rapid pre-formation of an ion-pair followed by a slow interchange reaction. Possible effects of changing composition of the ionic atmosphere, including specific ion interactions and ionpairing by perchlorate, are also considered. Lower limits are derived for the ratio of rate constants for interchange and water exchange, and these are several times greater than the same ratios for the corresponding cobalt(III) reactions. Although this factor can be explained in terms of a greater degree of nucleophilic assistance by chloride and bromide than by water in the reactions of the rhodium(III) complexes, other possible explanations are discussed. Anation reactions of trans-[Rh(en)2(OH2)2]3+ and trans-[Rh(en)2Br(OH2)]2+(en = ethylenediamine) show a similar behaviour, but the extent of specific ion atmosphere and/or ion-pairing effects is considerably greater. These interactions appear, therefore, to depend on the exact nature of the ligands present in the inner sphere of the complex, and the charge on the complex plays a relatively minor role. A kinetic method for determining ion-pair association constants of these complexes with weak donor ligands such as the fluoride ion is demonstrated, and again it is found that the interactions do not decrease with decreasing charge on the complex.

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