A NEW REACTION PATHWAY FOR THE GEOMETRICAL ISOMERIZATION OF MONOALKYL COMPLEXES OF PLATINUM(II) - KINETIC-BEHAVIOR OF CIS-[PT(PET3)2(NEOPENTYL)CL]

The title complex converts spontaneously into its trans isomer in protic and in some polar aprotic solvents. The process can be easily followed by H-1 and P-31 NMR spectroscopy and, in transparent solvents, by conventional spectrophotometric techniques. In 2-propanol there are two reaction pathways. The first is controlled by solvolysis and implies the formation of and the rapid cis-to-trans conversion of a cationic [Pt(PEt3)2(neopentyl)(S)]+ (S = 2-propanol) intermediate. At 298.16 K, k(i) = 11.7 x 10(-3) s-1, DELTA-H* = 60.7 +/- 0.7 kJ mol-1, and DELTA-S* = - 78 +/- 3 J mol-1 K-1. By addition of chloride ion, at [Cl-] > 0.1 M, the solvolysis is blocked and a new pathway becomes important with k(i) = 0.76 x 10(-3) s-1 at 298.16 K, DELTA-H* = 100 +/- 2 kJ mol-1, and DELTA-S* = + 31 +/- 7 J mol-1 K-1. In dichloromethane at 298.16 K, k(i) = 0.82 x 10(-4) s-1, DELTA-H* = 108 +/- 3 kJ mol-1 and DELTA-S* = + 32 +/- 9 J mol-1 K-1. The reaction is thought to proceed through the dissociative loss of a phosphine ligand, presumably that in the trans position to the alkyl group, and the conversion of the uncharged [Pt(PEt3)(neopentyl)Cl] 14-electron intermediate. The rate of isomerization is strongly accelerated by the presence in solution of very small amounts of the complex cis-[PtMe2(Me2SO)2], and it is shown that its catalytic efficiency depends on the extent of formation of a coordinatively unsaturated 14-electron [PtMe2-(Me2SO)] species. A mechanism for the catalyzed pathway is proposed.