Associative and Dissociative Mechanisms in the Formation of Phthalazine Bridged Organodiplatinum(II) Complexes

The reaction of phthalazine with the binuclear organoplatinum complexes [Me2Pt(μ-SMe2)(μ-dppm)PtR2], R = Me, Ph, 4-tolyl or R2 = (CH2)4, dppm = bis(diphenylphosphino)methane, gives the corresponding complexes [Me2Pt(μ-phthalazine)(μ-dppm)PtR2] by displacement of the bridging dimethylsulfide ligand. The structures of [Me2Pt(μ-SMe2)(μ-dppm)PtMe2] and [Me2Pt(μ-phthalazine)(μ-dppm)PtMe2] have been determined. Kinetic studies show that the reactions occur mostly by a second order reaction when R = Me or R2 = (CH2)4 but entirely by a first order reaction when R = Ph or 4-tolyl. Evidence is presented that the reactions when R = Me or R2 = (CH2)4 can occur by either associative or dissociative mechanisms but that the reactions when R = Ph or 4-tolyl occur only by an unusual dissociative mechanism involving formation of an intermediate with a donor−acceptor Pt−Pt bond.