Slolvent effects on rates and equilibria of chelate ring-closure ⇌ ring-opening nucleophilic substitution reactions of octahedral rhodium(III) complexes

Abstract

The solvent effects on rates and equilibria of reversible reactions (i) have been studied [X^n–= SCN^–, mer-[RhL(L′)Cl₂X]^1–n⇌trans-[RhL₂Cl₂]⁺+ X^n–(i) SeCN^–, N₃^–, NO₂^–, or pyridine; L and L′=(o-dimethylaminophenyl)dimethylarsine-NAs and -As respectively]. When X^n– is pyridine the reactions are both kinetically and thermodynamically independent of the nature of the solvent. When X^n– is anionic the equilibria and the rates of ring closure [forward reactions (i)] are strongly affected by the solvent as well as by the leaving group. Linear free-energy relationships of type (ii) exist for the activation and standard free energies of ΔG(m→t)‡=αΔG(m→t)^⊖+β(ii) either reactions of different mer complexes carried out in a given solvent or reactions of a given mer complex carried out in different solvents. An analysis of the solvent effect through Grunwald–Winstein or Kamlet–Taft relationships enables the observed results to be interpreted in terms of a dissociative mechanism for forward reactions (i).