Studies of platinum electroplating baths Part III. The electrochemistry of Pt(NH 3) 4 − x(H 2O) 2+2 and PtCl 4 − x(H 2O) (2 − x)− x

Abstract

The electrochemistry of the Pt(NH 3) 4 − x (H 2O) 2+ x complexes ( x = 0−2) in an aqueous phosphate buffer was investigated. 195Pt NMR was used to confirm that, in this medium, all are stable in solution even at 370 K. Moreover, their voltammetry shows that none is reduced at room temperature, although all give reduction peaks at above 353 K; a slow chemical change to the Pt(II) species (possibly displacement of NH 3 ligands by H 2O) in solution is essential to produce the electroactive species. The magnitudes of the current densities are always much less than those expected for mass transport control and suggest that the rate of this chemical change increases along the series Pt(NH 3) 2+ 4 < Pt(NH 3) 3(H 2O) 2+ < cis-Pt(NH 3) 2(H 2O) 2+ 2. In contrast, the complexes PtCl 4 − x (H 2O) (2 − x)− x in 1 M HClO 4 all show comparatively simple electrochemistry even at room temperature. Indeed, the complexes ( x = 2−4) show single reduction waves with mass transport-controlled plateaux and very similar half-wave potentials. Furthermore, 195Pt NMR shows that displacement of chloride by water does occur, but only on the time-scale of many hours. The mechanism of these electrodeposition reactions, including the possible role of Pt(H 2O) 2+ 4, is discussed. Although the dissolution of K 2PtCl 4 in water is a facile way to produce a readily reducible Pt(II) species, the presence of free chloride ion in solution leads to corrosion of substrates and this prevents the use of such solutions as practical electroplating baths.