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Substitution of the aqua ligand from three platinum(II) complexes, namely [Pt(H2O)(N,N-bis(2-pyridylmethyl)cyclohexylamine](ClO4)2 (Pt1); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](ClO4)4 (Pt2); and [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-ethylenedicyclohexyldiamine)](ClO4)4 (Pt3), by three biologically relevant nucleophiles [viz., glutathione (Glu); DL-penicillamine (Pen); and l-histidine (His)] was studied in aqueous 0.1 M perchloric acid medium. The substitutions were investigated under pseudo-first-order conditions as a function of nucleophile concentration and temperature using UV–visible spectrophotometry. The reactions of these complexes with the nucleophiles proceeded via a single step whose reactivity decreased in the order: Pt1 > Pt3 > Pt2 and was controlled by steric and electronic influences of the complexes. Sulfur donor nucleophiles (Glu and Pen) reacted much faster than the nitrogen bearing His, with Glu appearing to be more nucleophilic than Pen. The large and negative activation entropies and low but positive enthalpies of activation affirm an associative mode of activation.

The substitution of the aqua ligands from six Pt(II) complexes, viz., [Pt(H2O)(N,N-bis(2-pyridylmethyl)cyclohexylamine](ClO4)2 (Pt1); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](ClO4)4 (Pt2); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-methylenedicyclohexyldiamine)](ClO4)4 (Pt3); [Pt(H2O)N,N-bis(2-pyridylmethyl)phenylamine)](ClO4)2 (Pt4); [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,4-phenyldiamine](ClO4)4 (Pt5); and [{Pt(H2O)}2(N,N,N′,N′-tetrakis(2-pyridylmethyl)-4,4′-methylenediphenyldiamine)](ClO4)4 (Pt6) by nitrogen heterocyclic ligands{viz., pyrazole (Pz); 3-methylpyrazole (mPz); 1,2,4-triazole (Tz) and pyrazine (Pzn)} were studied in an aqueous 0.01 M perchloric acid medium. The substitutions were investigated under pseudo first-order conditions as a function of the concentration of nucleophiles and reaction temperature using UV-Visible spectrophotometry. The substitution of the aqua ligands by all the nitrogen donor heterocycles proceeded via a single step whose rate decreased in the respective orders: Pt1 > Pt3 > Pt2 and Pt4 > Pt6 > Pt5 in the two sets of complexes. Of the nucleophiles used in this study, pyrazine was most reactive, and the complete order of the rate of aqua substitution was Pzn >> Pz > Tz > mPz. The large and negative activation entropies and low but positive enthalpies of activation values support a significant contribution from bond-making in the transition state of the substitution process.