Base hydrolysis of trans-bis(ethylenediamine) dihalogenorhodium(III) complexes

Abstract

Base hydrolysis of the complexes trans-[Rh(en)₂X₂]⁺(X = Cl, Br, and I) obeys the rate equation k_obs=K₁+k₂[OH^–] at an ionic strenth of 1·0 mol dm^–3 where k₂/k₁≈ 1 dm³ mol^–1. Activation parameters for the[OH^–]-indepedent path are essentially the same as those for aquation. The [OH^–]-dependent path leads to virtually complete trans→cis isomerisation when X = Cl or Br. and to ca. 50% when X = 1. The values of ΔH₂^‡ and ΔS₂^‡ lie at the upper end of an isokinetic plot that includes data for the trans-[Rh(en)₂(OH)X]⁺ complexes. An excellent ‘pseudo-isokinetic’ plot of ΔH₂^‡–ΔH₁^‡ against ΔS₂^‡– DeltaS₁^‡ has also been obtained. The isokinetic plot lies above the corresponding plot for complexes with co-ordinated NH groups trans to the leaving group, the respective isokinetic temperatures being ca. 100 and 190 °C. It seems probable that the reactions that lead to extensive streochemical rearrangement occur by an S_N1 CB mechanism but that reactions of complexes further down the isokinetic plot may well have a mechanism closer to the S_N1 IP end of an S_N1 CB–S_N1 IP mechanistic spectrum.