The hydrolysis products of cis-dichlorodiammineplatinum(II) 2. The kinetics of formation and anation of the cis-diamminedi(aqua)platinum(II) cation

Abstract

In basic solution ([OH −] ⩾ 0.01 M), cis-PtCl 2(NH 3) 2 undergoes hydrolysis of both chloro ligands to give cis-Pt(OH) 2(NH 3) 2 as the stable end product. The rate of this reaction has been studied spectrophotometrically in aqueous NaOH solution (0.01–1.0 M) containing NaClO 4 (μ = 0.01–1.0 M) or NaCl (0.1–1.0 M) over a 35 °C temperature range. The reaction rate is almost independent of [OH −], ionic strength and added chloride ion and at 25 °C (μ = 0.1 M, NaClO 4), k 12 OH = 1.90 × 10 −5 s −1, Δ H ‡ = 84.4 kJ mol −1 and Δ S ‡ = −52 J K −1 mol −1. The rate of loss of the chloro ligand from cis-PtCl(OH)(NH 3) 2 (k 2 OH, s −1) generated by basification of cis-PtCl(NH 3) 2(OH 2) + is similar to k 12 OH, with k 2 OH = 1.43 × 10 −5 s −1, Δ H ‡ = 84.3 and Δ S ‡ = −55 at 25 °C (μ = 0.1 M). Acidification of cis-Pt(OH) 2(NH 3) 2 (generated from cis-PtCl 2(NH 3) 2 in 0.01 M NaOH) rapidly forms cis-Pt(NH 3) 2(OH 2) 2+ and this, in turn is slowly anated by the background chloride ion to give an equilibrium cis-PtCl(NH 3) 2(OH 2) +, cis-Pt(NH 3) 2(OH 2) 2 2+ and Cl − mixture. The equilibrium constant ( K 2) has been measured spectrophotometrically to give at 25 °C (μ = 0.1 M, NaClO 4) K 2 = 2.7 × 10 −4. Addition of controlled amounts of Cl − to the in situ generated cis-Pt(NH 3) 2(OH 2) 2 2+ allows the spectrophotometric measurement of the rate of the anation reaction ( k 2). cis-Pt(NH 3) 2(OH 2) 2 2+ + Cl − ▪ At 25 °C, k −2 = 9.09 × 10 −2 M −1 s −1, Δ H ‡ = 72.7 kJ mol −1, Δ S ‡ = −29.2 J K −1 mol −1. A knowledge of K 2 and k −2 allows the calculation of k 2 ( K 2 = k 2/ k −2) and at 25 °C k 2 = 2.5 × 10 −5 s −1. It is difficult to measure k 2 directly by spectrophotometry as, at reasonable cis-PtCl(NH 3) 2(OH 2) + concentrations (> 1 mM), the forward reaction proceeds only to the extent of <40% before equilibrium is established.