Displacement of chelate ligands from planar four-co-ordinate complexes. Part III. Displacement of 5-nitro-1,10-phenanthroline from dichloro(5-nitro-1,10-phenanthroline)gold(III) ion

Abstract

The kinetics of nucleophilic displacement of the chelate 5-nitro-1,10-phenanthroline (5-NO₂phen) from the complex [AuCl₂(5-NO₂phen)]⁺ have been studied in 5% water–methanol solutions at 25 °C. At constant [H⁺] the reaction rate is second order in the chloride-ion concentration, and, at constant [Cl^–], there is a linear dependence on the hydrogen-ion concentration with a finite intercept at [H⁺]= 0. This dependence on [Cl^–] differs from that fround in the displacement of 2,2′-bipyridyl from [Aucl₂(bipy)]⁺ and has been interpreted in terms of a mechanism in which there is no competition between co-ordination of an extra chloride and ring closing (as found in the bipy case). The effect of [H⁺] is smaller than that found in the bipy complex, indicating that in the present case the formation of protonated intermediates with the ligand still bonded to the metal is less favoured.