Formation and characterization of platinum(III)-ammonia complex ions using pulse radiolysis

The reactions of hydroxyl radical with tetraammineplatinum(II) perchlorate and of the hydrated electron with trans-dihydroxotetraammineplatinum(IV) perchlorate have been investigated in aqueous media with the fast-reaction technique of pulse radiolysis in conjunction with conductivity and UV-visible absorption detection. The kinetic studies have been supplemented by final-product analysis on pulse-irradiated solutions for the formation of free ammonia and for changes in the concentration of platinum(II). The hydroxyl radical reaction proceeds by an addition proces to the platinum(II) center, with a rate constant of (6.6 +/- 0.4) x 109 M-1 s-1. The hydrated electron reduces the platinum(IV) complex with a rate constant of (4.9 +/- 0.3) x 1010 M-1 s-1. The nascent products of these reactions are transitory species in which the metal has a formal oxidation state of III, and evidence is presented to support their formulation as Pt(NH3)4(H2O)(OH)2+ and Pt(NH3)4(OH)2+, respectively. Both species exhibit intense absorption bands with peaks near 270 nm, and they undergo reactions to yield three further transients, one of these being Pt(NH3)4(H2O)23+ absorbing in the region of 250 nm. The two additional transients, arising at a secondary level, exhibit absorption bands of lesser intensity with peaks in the region of 340 and 530 nm. The nature of these species and their long-term reactions are discussed. A mechanism is proposed whereby the foregoing intermediates are interrelated by acid-base, ammonia substitution, and water elimination processes