Phosphine exchange at trans-Pt(SnPh_3)_2(PMe_2Ph)_2

摘要

Phosphine exchange at square-planar trans-Pt(SnPh_3)_2(PMe_2Ph)_2 has been studied by ~(31)P NMR spectroscopy as a functionof ligand concentration and temperature in C_6D_6 using magnetisation transfer techniques. Exchange of a specific phosphine takes place according to a second order rate law with k_(et)~(298) = 33.1 +- 3 dm~3 mol~(-1) s~(-1), #DELTA#H~(++) = 24.4 +- 7 kJ mol~(-1) and #DELTA#S~(++) = -131 +- 23 J K~(-1) mol~(-1). The highly negative activationentropy and the rate law indicate an associative mode of activation. In spite of the overall large steric requirements of the four tertiary ligands the activation parameters indicate a fairly closed transition state, possibly due to an electronic stabilisation of the transition state by the #pi#-acidic stannyl ligands