Ligand substitution reaction at a binuclear organoplatinum(II) complex

The ligand substitution reactions of the N-donor ligand in the binuclear dimethylplatinum(II) complex of formula cis,cis-[Me2Pt(μ-NN)(μ-dppm)PtMe2], 1, in which dppm = bis(diphenylphosphino)methane and NN = phthalazine, by different nucleophilic phosphorous-donors L, L = P(O–iPr)3 or PPh3 and L2 = dppm, to form the dinuclear complexes 2, cis,cis-[Me2LPt(μ-dppm)PtLMe2] and cis,cis-[Me2Pt(μ-dppm)2PtMe2], respectively, are studied. Complex 1 has a MLCT band in the visible region which was used to easily follow the kinetics of its ligand substitution reactions. These reactions which involve diplatinum(II) complex 1 containing cis Pt–C bonds, proceeded by the normal associative mechanism. In associative reactions of the present work, as expected, the rate of the reactions was depended on the concentration and the nature of the entering group. The nucleophilicity of PPh3 is stronger than P(O–iPr)3 on the basis of its stronger σ-donor ability and its lower solvation and is responsible for the observed 3-fold increase of its rate as compared to that of P(O–iPr)3. Also, the solvation energy involved is suggested to be responsible for the observation of higher rates in benzene than in acetone. The ΔH‡/ΔS‡ compensation plot gives a straight line which suggests the operation of the same mechanism for all entering nucleophiles.