Solvent effects in square planar complexes: Kinetics of substitution at 1-(2-hydroxyphenyl)-3,5-(diphenylformazanato)-platinum(II) complexes

Abstract

A kinetic study at 25 °C is reported of the substitution reaction FoPtX + Y → FoPtY + X (H 2Fo = 1-(2-hydroxyphenyl)-3,5-diphenylformazan), with X = NH 3 and pyridine and Y = thiourea, triphenylphosphine, and the thiocyanate ion, in seven non-aqueous pure solvents. The main reaction route is the direct associative displacement of X by Y. The mutual dependence of reactivity effects of entering and leaving ligands and solvent was interpreted as indicating a synchronous mechanism. Steric effects were also found to be important, probably due to the rigidity of the substrate. Solvent effects on the rates are comparatively small. With the help of transfer functions from measured solubilities a dissection of solvent effects on initial and transition states was accomplished; for one reaction also the final state could be included in the comparison. On the whole non-specific solvation effects dominate and the transfer Gibbs free energy of solvation does not change on going to the transition state. Exceptions to this rule are caused by initial state stabilization of two of the entering ligands in the protic solvent methanol and by transition state labilization for FoPtNH 3 in strong donor solvents as DMSO and DMF. The mechanism for the latter labilization is suggested to be charge transfer to Pt 2+ caused by solvent donor interaction with coordinated ammonia or directly to the metal centre. A similar electronic effect seems to govern the solvent dependence of the 195Pt NMR chemical shift for FoPtNH 3.