Kinetics and mechanism of the reactions of [Pt(terpy)H2O]2+ with thiols in acidic aqueous solution.: Synthesis and crystal structure of [Pt(terpy)(tu)](ClO4)2 (tu = thiourea)

The kinetics of the complex-formation reactions between [Pt(terpy) H2O](2+), where terpy is 2,2': 6', 2"-terpyridine, with L-cysteine, DL-penicillamine, glutathione and thiourea (tu) were studied in an aqueous 0.10 M perchloric acid medium using variable-temperature and -pressure stopped-flow spectrophotometry. Thiourea is the best nucleophile with a second order rate constant, k(1)(298), of 1.72 x 10(5) M-1 s(-1), whereas glutathione is the strongest nucleophile of the studied thiols, for which k(1)(298) varied between 38 and 583 M-1 s(-1). Activation volumes for the reactions with thiourea, L-cysteine, glutathione and DL-penicillamine, are -6.0 +/- 0.3, -9.3 +/- 0.4, -12.4 +/- 0.6 and -20.6 +/- 1.0 cm(3) mol(-1), respectively. The negative entropies and volumes of activation support a strong contribution from bond making in the transition state of the substitution process. The crystal structure of [Pt(terpy)(tu)](ClO4)(2) was determined by X-ray diffraction. Crystals are monoclinic with the space group P21/c and consist of the distorted square-planar [Pt(terpy)(tu)](2+) complex. The Pt-N (central) bond distance, 1.971( 14), is shorter than the other two Pt-N distances, 2.078(15) and 2.046(15) Angstrom. The Pt-S distance is 2.301(5) Angstrom.