Displacement of chelate ligands from planar four-co-ordinate complexes. Part II. Preparation and substitution reactions of dichloro(NNN′N′-tetramethylethylenediamine)- and dichloro(NNN′N′-tetraethylethylenediamine)-gold(III) complexes

Abstract

The kinetics of nucleophilic displacement of the chelate ligands NNN′N′-tetraethylethylenediamine (teen) and NNN′N′-tetramethylethylenediamine (tmen) from the complexes [Au(teen)Cl₂]⁺ and [Au(tmen)Cl₂]⁺ 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 for low values of [Cl^–] and tends to a first-order dependence as [Cl^–] increases. On the other hand, at constant [Cl^–] the rate is first order in acid concentration for low values of [H⁺] and approaches a limiting value as [H⁺] increases. The kinetic data fit a rate law for a mechanism in which displacement of the two donor atoms of the bidentate ligand occurs in two consecutive steps, each proceeding through the normal associative nucleophilic attack known for planar complexes. The intermediate in which the ligand is half bonded undergoes reversible protonation at the unco-ordinated nitrogen atom. The low value of the rate constant calculated for the protonation process is explained on the basis of slow interconversion from ‘gauche’ to ‘trans’ configurations of the half-bonded ligand. The rate constant for opening of the chelate ring has also been estimated and compared with substitution rates of unidentate amines in gold complexes.