The solvolytic aquation and base hydrolysis of halogenopentaammine-rhodium(III) complexes

Abstract

The rates and Arrhenius parameters for the replacement of halogens in halogenopentaamminerhodium(III) complexes both by solvent water and by hydroxide ion have been determined and their relative mobilities of halogens compared with the corresponding observations in organic compounds. These reactions are expected to follow the pattern of S_NAr reactions, viz., F > Cl ≈ Br > I, and, except for the fluoro complex, this is now confirmed experimentally. The kinetic results are discussed and an attempt is made to interpret them in terms of the electronegativity of the halogens, which facilitates bond formation with the incoming nucleophile. Solvation factors are also important in the control of reactivities. For these reactions, the rate order depends on opposing effects of activation energy and entropy, with the former predominating, and in this way they are in complete contrast to S_N2 reactions of octahedral cobalt(III) complexes where the entropy factor alone controls the rate.