摘要:
Reaction of trans-[PtClMe(SMe2)(2)] with the mono anionic ligands azide, bromide, cyanide, iodide and thiocyanate result in substitution of the chloro ligand as the first step. In contrast the neutral ligands pyridine, 4-Me-pyridine and thiourea substitute a SMe2 ligand in the first step as confirmed by H-1 NMR spectroscopy and the kinetic data. Detailed kinetic studies were performed in methanol as solvent by use of conventional stopped-flow spectrophotometry. All processes follow the usual two-term rate law for square-planar substitutions, k(obs) k(1) + k(2)[Y] (where k(1) = k(MeOH)[MeOH]), with k(1) = 0.088 +/- 0.004 s(-1) and k(2) = 1.18 +/- 0.13, 3.8 +/- 0.3, 17.8 +/- 1.3, 34.9 +/- 1.4, 75.3 +/- 1.1 mol(-1) dm(3) s(-1) for Y- = N-3, Br, CN, I and SCN respectively at 298 K. The reactions with the neutral ligands proceed without an appreciable intercept with k(2) = 5.1 +/- 0.3, 15.3 +/- 1.8 and 195 +/- 3 mol(-1) dm(3) s(-1) for Y = pyridine, 4-Me-pyridine and thiourea, respectively, at 298 K. Activation parameters for MeOH, N-3(-), Br-, CN-, I-, SCN-, and Tu are Delta H-not equal = 47.1 +/- 1.6, 49.8 +/- 0.6, 39 +/- 3, 32 +/- 8, 39 +/- 5, 34 +/- 4 and 31 +/- 3 kJ mol(-1) and Delta S-not equal = -107 +/- 5, -77 +/- 2, -104 +/- 9,-113 +/- 28, -85 +/- 18, -94 +/- 14 and -97 +/- 10 J K-1 mol(-1), respectively. Recalculation of k(1) to second-order units gives the following sequence of nucleophilicity: MeOH < N-3(-) < Br- similar to py < 4 - Me - py similar to CN- < I- < SCN- < Tu (1:13:42:57:170:200:390:840:2170) at 298 K. Variation of the leaving group in the reaction between trans-[PtXMe(SMe2)(2)] and SCN- follows the same rate law as stated above with k(2) = 75.3 +/- 1.1, 236 +/- 4 and 442 +/- 5 mol(-1) dm(3) s(-1) for X- Cl, I and N-3, respectively, at 298 K. The corresponding activation parameters were determined as Delta H-not equal = 34 +/- 4, 32 +/- 2 and 39.3 +/- 1.7 kJ mol(-1) and Delta S-not equal = -94 +/- 14, -86 +/- 8 and -68 +/- 6 J K-1 mol(-1). All the kinetic measurements indicate the usual associate mode of activation for square planar substitution reactions as supported by large negative entropies of activation, a significant dependence of the reaction rate on different entering nucleophiles and a linear free energy relationship. (c) 2006 Published by Elsevier B.V.