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 kobs=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 SN1 CB mechanism but that reactions of complexes further down the isokinetic plot may well have a mechanism closer to the SN1 IP end of an SN1 CB–SN1 IP mechanistic spectrum.