Reversible chelate ring-opening and ring-closure substitution reactions of trans-dichlorobis(o-dimethylaminophenyldimethylarsine-NAs)-rhodium(III) ions. A kinetic study in hydroxylic solvents

Abstract

The kinetics of the interconversion (i) have been studied in hydroxylic solvents (X = Cl, Br, I, and occasionally trans-[Rh(L)₂Cl₂]⁺+ X^–⇌mer-[Rh(L)(L′)Cl₂X](i) SCN: L and L′=o-dimethylaminophenyldimethylarsine-NAs and -As respectively). The system obeys rate law –d[Rh(L)₂Cl₂⁺]/dt =k_(t→m)⁰γ_±²[Rh(L)₂Cl₂⁺][X^–]–k_(m→t)[Rh(L)(L′)Cl₂X](ii)(ii). The rate of the forward reaction increases with decreasing polarity of the solvent, as expected for reactions between ions of opposite charge. The reactivity of the trans complex changes with the entering group, an order SCN > I > Br > Cl being observed in methanol, and is explained by the occurrence of some Rh–X bond making during formation of the transition state. The rate of the reverse reaction is also affected both by the solvent and by the nature of the X group. The solvent effect is rationalized in terms of the Grunwald–Winstein relation log k_(t→m)⁰=mY+ constant. The high values of m obtained [1.24 (X = Cl), 1.19(Br), and 0.95(I)] are considered to indicate extensive Rh–X bond rupture in the activated complex of the mer→trnas conversion. The reactivity order with respect to the leaving group. SCN < Cl <Br < I, is interpreted in terms of the energy required to break the Rh–X bond in order to attain the configuration of the activated complex.