A comparative substitution behaviour of isomeric cis/trans-platinum(ii) complexes.

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This study was undertaken to investigate the comparative substitution behavior of mononuclear trans-platinum(II) complexes with symmetric and asymmetric amine ligands. The rate of substitution of the aqua moeities from the complexes trans-[Pt(NH 3) 2 (H 2 O) 2 ](ClO 4) 2 (tPt), trans-[Pt(NH 3)(NH 2 C 2 H 5)(H 2 O) 2 ] (ClO 4) 2 (tPt2H 2 O), trans-[Pt(NH 3)(NH 2 C 3 H 7)(H 2 O) 2 ](ClO 4) 2 (tPt3H 2 O), [trans-Pt(OH 2) 2 (NH 2 CH 3) 2 ](ClO 4) 2 (tPtM), and [trans-Pt(OH 2) 2 {NH 2 CH(CH 3) 2 } 2 ] (ClO 4) 2 (tPtR), by three nucleophiles, namely thiourea (TU), 1,3-dimethylthiourea (DMTU), and 1,1,3,3-tetramethylthiourea (TMTU) was studied under pseudo-first-order conditions as a function of concentration and temperature by stopped-flow spectrophotometry. All the substitution reactions of each of the trans-platinum(II) complexes proceeds by a stepwise mechanism involving rate-determining substitution of the first aqua ligand followed by a fast second substitution step, without any intermediates formed. The reactions were second order overall (rate = k obs [complex] where k obs = k 2 [nucleophile]), first order in both [complex] and [nucleophile]. The reactivity of the complexes was essentially governed by both steric and electronic factors. Comparing the second-order rate constants for the substitution reactions of the mononuclear diaqua trans-platinum(II) complexes with the thiourea-based nucleophiles, the observed trend follows: tPt > tPt2H 2 O > tPtM > tPt3H 2 O > tPtR. This reactivity trend is consistent with the pK a values obtained for the first deprotonation step. The reactivity of the nucleophiles with the complexes decreases with an increase in steric demand in the following order: TU > DMTU > TMTU. The low positive values of activation enthalpy and large negative values of activation entropy indicate an associative mechanism of substitution in all the complexes. The computational modeling using density functional theory calculations was employed to provide theoretical interpretation of kinetic data.

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