An EXAFS study of the co-ordination chemistry of hydrogen hexachloroplatinate(IV): 1. Speciation in aqueous solution

Hydrogen hexachloroplatinate(IV), also called chloroplatinic acid (CPA), is a strong acid that undergoes rapid and extensive hydrolysis. Extended X-ray absorption fine structure (EXAFS) characterization was performed at the Advanced photon source (APS) at Argonne National Laboratory to determine the PtCl and PtO co-ordination chemistry of 200–2000 ppm CPA at pHs of 1.5–12 with different chloride concentrations, light conditions, and time frames. The EXFAS analysis was combined with potentiometric data to postulate the following speciation behavior of the dilute CPA. The initial hydrolysis reaction, aquo ligand exchange of chloride ions, is rapid and reversible, while the latter two reactions, hydroxide ion ligand exchange of chloride and aquo ligands, are relatively slow in acidic solutions but accelerated in the presence of light. Many of the stable Pt complexes in solution are zero valent. High chloride co-ordination is favored at low pH and high chloride concentration. As a result, the [PtCl 6] 2− species is present only in acidic solutions with a moderate excess of chloride ion or in the neutral solutions in a large excess of chloride ion. Hydroxide ligand formation is favored at low pH and suppressed by chloride ion concentration. As a result, full hydrolysis of CPA by hydroxide ions with precipitation of H 2Pt(OH) 6 (or Na 2Pt(OH) 6) is favored only at very low CPA concentrations (ca. 30 ppm).