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

Kinetic and equilibrium studies of the processes (1) and (2) (terpy = 2,2' :6',2 "-terpyridine) have been carried out in water at 25 degrees 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 greater than or equal to 0.8 mol dm(-3)) and dissociates completely into the corresponding hydroxo species, which can be isolated in the solid st ate as its perchlorate. The crystal structure of [An(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(3) Angstrom and an Au-OH distance, the first experimentally determined, of 2.000(4) Angstrom. 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) Angstrom, 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.