Stabilities of platinum(II) chloro and bromo complexes and kinetics for anation of the tetraaquaplatinum(II) ion by halides and thiocyanate

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The anations of Pt(H 2O) 2+ 4 and PtX(H 2O) + 3 (to trans-PtX(H 2O) 2) by X − = Cl −, Br −, I − and SCN − and the anation of trans-PtX(H 2O) 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 −, PtX 2− 4 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 H 2O < Cl − < Br − < I − < SCN − (1:330:3300:∼4 × 10 4:∼1 × 10 5) and a kinetic cis-effect order of Br − < Cl − H 2O < I − (0.3:0.4:1:∼2). Acid hydrolysis rate constants for PtX 3H 2O −, trans-PtX 2(H 2O) 2 and Pt(H 2O) + 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 × 10 4 to 3 × 10 6 times faster than corresponding platinum complexes. Spectrophotometric equilibrium measurements performed as competition experiments with palladium give the stepwise stability constants K 1 = [PtX +] [Pt 2+] −1 [X −] −1 as (9 ± 2) × 10 4 M −1 (X = Cl) and (1.9 ± 0.4) × 10 5 M −1 (X = Br). These values combined with previous literature data enable a calculation of ht over-all stability constants for the platinum(II) chloro and bromo complexes. lg (β n/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.