Kinetics and equilibria of reversible chelate ring-opening nucleophilic substitution reactions of [(o-dimethylaminophenyl)dimethylarsine]-rhodium(III) complexes

Abstract

The kinetics and equilibria of the reversible reactions trans-[RhL₂Cl₂]⁺+ X^n–⇌mer-[RhL(L′)Cl₂X]^1–n have been studied in methanol [X^n–= SCN^–, SeCN^–, NO₂^–, N₃^–, or pyridine; L and L′=(o-dimethylaminophenyl)-dimethylarsine-NAs and -As respectively]. The second-order rate constants of the forward reactions are scarcely affected by the nature of the entering group, whereas the first-order rate constants of the reverse reactions are strongly influenced by the leaving group involved. A linear free-energy relationship of the type ΔG_(m→t)‡=αΔG_(m→t)^⊖+β with a value of α of 0.75 exists for the activation and standard free energies of mer→trans reactions where a dissociative mechanism is considered to be operative. When X^n– is SCN^–, N₃^–, or NO₂^–, the mer→trans reactions are acid-catalyzed, through protonation of the leaving group.