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RESEARCH PRODUCT

Role of Halide Ion in the Mechanism of Protonolysis of the Pt-C Bond in Pt(II) Alkyl and Aryl Complexes.

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Abstract A mechanistic study is described for the electrophilic cleavage of the Pt C σ bond in complexes cis -[PtPh 2 (PEtPh 2 ) 2 ] and trans -[PtXR(PEt 3 ) 2 ] (X = Cl and Br, R = Me, Et, n-Pr, n-Bu, CH 2 Ph) by the proton in the presence of halide ions in aqueous methanol (MeOH/H 2 O, 91v/v) which yields cis -[PtClPh(PEtPh 2 ) 2 ] and trans -[PtX 2 (PEt 3 ) 2 ], respectively. The reactions are first-order in substrate for both systems and the general bivariate rate law k obs = [H + ]{ k H + k x K [[X − ]}/(1 + K [[X − ]) is obeyed. The proposed mechanism involves a fast pre-equilibrium formation (K) of a platinum(II) anionic intermediate via interaction of the halide with the square-planar substrate, combined with slow, parallel protonation of both the substrate ( k H ) and the intermediate ( k x ), causing the cleavage of the metal carbon σ bond. The intermediate should be conceived of as weak association product, since all attempts at measuring the constant K by UV, visible and 31 P{ 1 H} NMR spectroscopy were unsuccessful. This mechanism will rationalize the previously reported diverse kinetic results for the protonolysis of Pt C σ bonds, within a unified picture which takes into account the electronic and steric properties of the group to be cleaved and of ancillary ligands. The role of the halide ion can be put into evidence by the use of substrates that will promote association through a reduced electron density on the central metal.