The trans-effect in octahedral complexes. Part V. Intrinsic kinetic trans-effect of hydroxide ions in octahedral rhodium(III) complexes

Abstract

Data are presented for the kinetics of the reaction of trans-Rh(en)₂(OH)l⁺(en = ethylenediamine) with hydroxide ions. The observed pseudo-first-order rate constants fit the expression k_obs=k₁+k₂[OH^–], and activation parameters corresponding to k₁ and k₂ have been obtained. The value of ΔH^‡₁ gives a different measure of the kinetic trans-effect of the hydroxide ion, relative to those of ammonia and iodide, from that based on anation reactions, but this ambiguity can be resolved by appropriate allowance for the corresponding values of ΔH⁰ for the reactions. When this allowance is made, a measure of the intrinsic kinetic trans-effect of hydroxide ion in such rhodium(III) complexes is obtained and this is found to be very close to that of chloride. This intrinsic kinetic trans-effect is obtainable even though data for the corresponding reactions of the trans-chloroiodo- and -bromoiodo-complexes, etc., are not available.

The reaction path corresponding to k₂ is the first [OH^–]-dependent path observed for such trans-complexes but it is not very important, even in this case, because the value of ΔS^‡₂ is relatively unfavourable compared with those usually found for base hydrolysis of amine complexes of cobalt(III) and rhodium(III).