Kinetics of cis-trans isomerization and reductive elimination in dihydridobis(trimethylphosphine)platinum(II)

Abstract

Solutions of PtH/sub 2/(PMe/sub 3/)/sub 2/ (1) exist as an equilibrium mixture of cis and trans isomers, with the cis isomer dominating in polar solvents. The forward rate constant for trans /leftrightarrow/ cis isomerization was determined by spin-saturation-transfer studies to be /approximately/ 0.1 s/sup /minus/1/ at /minus/60/degree/C. The isomerization may proceed by an associative mechanism involving a phosphine ligand or solvent molecule, or by dissociation of PMe/sub 3/ from the cis and trans isomers to form T-shaped intermediates that slowly interconvert. Above /minus/35/degree/C the phosphine ligands exchange rapidly between the isomers of 1 with rate constants in the range 7-600 s/sup /minus/1/. Addition of a trace of free phosphine to solutions of 1 results in the formation of PtH/sub 2/(PMe/sub 3/)/sub 3/ (2), which exchange phosphine ligands rapidly with trans-1 but not with cis-1. Under a nitrogen atmosphere, 1 decomposes by unimolecular rate-determining reductive elimination of hydrogen with an inverse kinetic isotope effect, k/sub H//k/sub D/ = 0.45 /plus minus/ 0.1. This inversion isotope effect supports the theoretical predictions of nearly complete H-H bond formation in the transition state for reductive elimination. In coordinating THF solvent, reductive elimination of H/sub 2/ occurs slowly, with /Delta/H/double dagger/ = 9.4 /plus minus/more » 1 kcal mol/sup /minus/1/ and /Delta/S/double dagger/ = /minus/41 /plus minus/ 3 cal mol/sup /minus/1/ deg/sup /minus/1/. The large negative entropy of activation reflects solvent reorganization in the transition state. In noncoordinating Me/sub 4/THF solvent elimination of hydrogen proceeds more rapidly, with /Delta/H/double dagger/ = 20.0 /plus minus/ 0.5 kcal mol/sup /minus/1/ and /Delta/S/double dagger/ = /minus/1 /plus minus/ 2 cal mol/sup /minus/1/ deg/sup /minus/1/. 62 refs., 5 figs., 4 tabs.« less