Equilibrium and kinetic studies of (2,2′∶6′,2″-terpyridine)gold(III) complexes. Preparation and crystal structure of [Au(terpy)(OH)][ClO4]2

Abstract

Kinetic and equilibrium studies of the processes (1) and (2) (terpy = 2,2′∶6′,2″-terpyridine) have been carried out in

water at 25 °C, I = 0.1 mol dm^–3 (LiClO₄). Owing to the high charge of the metal centre, the [Au(terpy)(OH₂)]³⁺ cation behaves as a strong acid (K_a  0.8 mol dm^–3) and dissociates completely into the corresponding hydroxo species, which can be isolated in the solid state as its perchlorate. The crystal structure of [Au(terpy)(OH)][ClO₄]₂ has been determined by the single-crystal X-ray diffraction technique. It consists of SP (square planar) [Au(terpy)(OH)]²⁺ cations having Au–N distances of 2.009(5), 2.008(4) and 1.949(4) Å and an Au–OH distance, the first experimentally determined, of 2.000(4) Å. The SP geometry is expanded to distorted tetragonal bipyramidal (TBPY) by linking the two perchlorate anions with Au–O distances of 3.023(8) and 3.069(8) Å, which are intermediate between bonding and van der Waals interactions. The secondary co-ordination phenomenon in gold(III) SP complexes is reviewed and a possible reason for its occurrence proposed.