ChemInform Abstract: Formation of Gold(I) Halide and Thiocyanate Complexes in Pyridine and Acetonitrile and the Structures of Gold(I) Solvates in These Solvents. A Thermodynamic and EXAFS Spectroscopic Study

Abstract

The thermodynamics of the formation of gold(I) halide and thiocyanate complexes in pyridine and acetonitrile has been studied by means of potentiometric and calorimetric measurements at 25{degree}C. All solutions contained 0.1 M tetraethylammonium ions; perchlorate was used as a supplementary anion. Changes of free energies, enthalpies, and entropies could be determined for the two mononuclear complexes formed in each system, except for the iodide and thiocyanate complexes in acetonitrile, where only the formation of the second complex could be studied. The solubility was too low for the neutral thiocyanate complex, and gold(I) was reduced when the iodide/gold ratio was <2. The complexes are much less stable in pyridine than in acetonitrile, certainly due to the stronger solvation of Au{sup +} in the former solvent. The formation reactions are characterized by very large entropy gains. An analogous lowering of the coordination number therefore most probably takes place in the case of gold(I), in spite of the fact that the crystallized solvates are two-coordinated. Structural studies of gold(I) solutions by means of the EXAFS technique have fully confirmed this inference for both solvents. The Au-N distances are 2.16 and 2.19 {angstrom} in pyridine and acetonitrile, respectively, which is about 0.2 {angstrom} longermore » than in the corresponding two-coordinated solid solvates. Similar differences in metal-ligand distance are found between two- and four-coordinated complexes of other d{sup 10} acceptors, not only for the monovalent copper(I) and silver(I) but also for the divalent mercury(II). In pyridine and acetonitrile solutions, gold(I) thus certainly exists as a tetrahedral solvate. 50 refs., 7 figs., 5 tabs.« less