Thermodynamic and Kinetic Studies on Reactions of Pt(II) Complexes with Pyrazole, Pyridazine, and 1,2,4-Triazole

Substitution reactions of the complexes [Pt(dien)H₂O]²⁺ and [PtCl(dien)]⁺, where dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as 1,2,4-triazole, pyrazole, and pyridazine, were studied in an aqueous 0.10 M NaClO₄ at pH = 2.5 using variable-temperature spectrophotometry and ¹H NMR spectroscopy. The second-order rate constants indicate that the aqua complex, [Pt(dien)H₂O]²⁺, is more reactive than the corresponding chloro complex, [PtCl(dien)]⁺. The reactivity of the used ligands follows the order: 1,2,4-triazole > pyridazine > pyrazole. Activation parameters were determined for all reactions and the negative entropies of activation (ΔS ^≠) support an associative ligand substitution mechanism.