Stabilities of Platinum(II) Chloro and Bromo Complexes and Kinetics for Anation of the Tetraaquaplatinum(II) ion by Halides and Thiocyanate

The anations of Pt(H2O)42+ and PtX(H2O)3+ (to trans-PtX(H2O)2) by X− = Cl−, Br−, I− and SCN− and the anation of trans-PtX2(H2O)2) by X− = Cl−, Br− and I− have been studied at 25 °C and 1.00 M perchlorate medium using both stopped-flow and conventional spectrophotometry. For large excess of X−, PtX42− is formed according to the mechanism in Figure 1. The results indicate an entering ligand order of Cl− < Br− < SCN− < I− (1:8:50:350), a kinetic trans-effect order of H2O < Cl− < Br− < I− < SCN− (1:330:3300:∼4 × 104:∼1 × 105) and a kinetic cis-effect order of Br− < Cl− < H2O < I− (0.3:0.4:1:∼2). Acid hydrolysis rate constants for PtX3H2O−, trans-PtX2(H2O)2 and Pt(H2O)3+ (X = Cl, Br) have also been determined. Bromide is about 3 times better as leaving ligand than chloride. The palladium chloro and bromo complexes react 2 × 104 to 3 × 106 times faster than corresponding platinum complexes. Spectrophotometric equilibrium measurements performed as competition experiments with palladium give the stepwise stability constants K1 = [PtX+] [Pt2+]−1 [X−]−1 as (9 ± 2) × 104 M−1 (X = Cl) and (1.9 ± 0.4) × 105 M−1 (X = Br). These values combined with previous literature data enable a calculation of the over-all stability constants for the platinum(II) chloro and bromo complexes. lg (βn/M−n), n = 1,2,3,4, are: 4.97 ± 0.11, 8.97 ± 0.20, 11.89 ± 0.35, and 13.99 ± 0.45 for the chloro complexes and 5.28 ± 0.09, 9.72 ± 0.18, 13.32 ± 0.35 and 16.11 ± 0.45 for the bromo complexes, at 25 °C and for a 0.50 to 1.00 M perchloric acid medium.