THE ION-PAIR MECHANISM IN SQUARE-PLANAR SUBSTITUTION - DISPLACEMENT OF DIMETHYL-SULFOXIDE FROM CHLORO(DIMETHYL SULFOXIDE) (N,N,N',N'-TETRAMETHYLDIAMINOETHANE)PLATINUM(II) SALTS BY HALIDE-IONS IN DICHLOROMETHANE SOLUTION

Freshly prepared [Pt(Me4en)(Me2SO)Cl]Cl rapidly loses Me2SO in the solid state to give [Pt(Me4en)CI21. This reaction also occurs in chloroform and dichloromethane solution. The kinetics are first-order with a rate constant (10(4)k1/s-1 = 3.7 al 298 K, DELTAH double dagger = 98 +/- 2 kJ mol-1, DELTAS double dagger = 19 +/- 5 J K-1 mol-1 in CH2Cl2) that is independent of the initial concentration of the complex and of added [Me2SO]. Addition of small amounts of [AsPh4]Cl causes the rate of displacement of dimethyl sulfoxide to increase, but further addition leads to a rate that is independent of the concentration of added nucleophile. Other anions replace the chloride at a rate that is too fast to measure and then replace the dimethyl sulfoxide. The kinetics are consistent with an interchange within ion aggregates. The limiting interchange rate constant depends only slightly on the nature of the entering halide, 10(4)k2/S-1 = 5.3, 4.3, and 11.6 for the entry of Cl-, Br , and I-, respectively, at 298 K in CH2CI2, but it is not possible to separate the cis effects of the different halides from the nucleophilic discrimination of the substrate. Uncharged nucleophiles react either slowly or not at all in these weakly-polar solvents. Other salts of this cation are described, and the ion-association equilibria of the hexafluorophosphate and the tetraphenylborate have been investigated in dichloromethane. In aqueous and methanolic solution, [Pt(Me4en)(Me2SO)Cl]Cl remains unchanged for a long time even in the presence of a large excess of chloride ions, and in aqueous solution the coordinated chloride is replaced by other nucleophiles at a rate that is some 2 orders of magnitude slower than that in the reaction of the analogous diaminoethane complex. The dimethyl sulfoxide is not replaced. The molecular structure of [Pt(Me4-en)(Me2SO)Cl]BPh4 was determined by single-crystal X-ray diffraction analysis. White C32H42N2BClOPtS crystallizes in the orthorhombic Pcab space group with o = 12.088 (4) angstrom, b = 22.738 (6) angstrom, c = 23.246 (9) angstrom, and Z = 8. There are no unusual steric interactions within the cation to account for the unusual lability of the dimethyl sulfoxide.