Kinetics and mechanism of the reactions of Pd(II) complexes with azoles and diazines.: Crystal structure of [Pd(bpma)(H2O)](ClO4)2•2H2O

Substitution reactions of the complexes [Pd(bpma)(H2O)](2+), [Pd(bpma)Cl](+), [Pd(dien)(H2O)](2+) and [Pd(dien)Cl](+), where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO4 at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H2O)](2+) and [Pd(bpma)Cl](+), are more reactive than the complexes of dien, viz. [ Pd( dien)( H2O)] 2+ and [ Pd( dien) Cl]+. Also, the aqua complexes, [ Pd( bpma)( H2O)] 2+ and [ Pd( dien)( H2O)] 2+, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (Delta S-double dagger, Delta V-double dagger) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H2O)](ClO4)(2) center dot 2H2O was determined by X-ray diffraction. Crystals are monoclinic with the space group P21/c. The coordination geometry of [Pd(bpma)(H2O)](2+) is distorted square-planar. The Pd - N (central) bond distance, 1.958(5) angstrom, is shorter than the other two Pd - N distances, 2.007(5) and 2.009(5) angstrom. The Pd - O distance is 2.043(5) angstrom.