Kinetics and mechanism of the reaction of chelated Pd(II) complexes with thiols in acidic aqueous solution.: Synthesis and crystal structure of [Pd(bpma)Cl]Cl•H2O (bpma = bis(2-pyridylmethyl)amine)

The kinetics of the complex-formation reactions between monofunctional palladium(II) complexes [Pd(N-N-N)-H2O](2+), where N-N-N is 2,2': 6',2"-terpyridine (terpy), diethylenetriamine (dien) or bis(2-pyridylmethyl)amine (bpma), with L-cysteine, DL-penicillamine and glutathione, have been studied in an aqueous 0.10 M perchloric acid medium using variable-temperature and -pressure stopped-flow spectrophotometry. Second-order rate constants, k(1)(298), varied between 2.8 x 10(2) and 4.4 x 10(4) M-1 s(-1). The highest reactivity was observed for the [Pd(terpy)H2O](2+) complex, whereas glutathione is the strongest nucleophile. Activation volumes for these reactions varied between -5.6 +/- 0.3 and -10.7 +/- 1.0 cm(3) mol(-1). 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 [Pd(bpma)Cl]Cl.H2O has been determined by X-ray diffraction at 190 K. Crystals are triclinic with space group P (1) over bar and consist of distorted square-planar [Pd(bpma)Cl](+) cations. The Pd-N distances are all equal to 2.005(7) Angstrom. The Pd-Cl distance is 2.305(3) Angstrom.