Computational study of reductive elimination reactions to form C-H bonds from platinum(II) and platinum(IV) centers with strongly coordinating trimethylphosphine ligands

B3LYP calculations have been performed on the effects of replacing PH3 with PMe3 ligands in the reductive elimination of methane from cis-hydridomethy l-bisphosphine platinum(II) and platinum(IV) model complexes. In both the P t(II) and Pt(IV) complexes, the replacement of PH3 ligands by the more stro ngly basic PMe3 ligands is predicted to favor a direct mechanism for reduct ive elimination over one in which the initial step is phosphine ligand loss . However, the effect of the increased platinum-phosphine binding enthalpy on the ligand-predissociation mechanism was found to be partially canceled by an increase in the barrier height computed for the direct mechanism. It was possible to locate the transition structures for direct reductive elimi nation of methane from several isomers of Cl-2(PMe3)(2)-Pt(CH3)H. In contra st, the lower binding enthalpy of PH3, compared to PMe3, made it impossible to locate the transition structures for direct elimination from the corres ponding isomers of Cl-2(PH3)(2) Pt(CH3)H. Our computational results suggest that destabilizing cis interactions between the atoms of the axial and equ atorial ligands that are coordinated to platinum are responsible for the lo wer phosphine binding enthalpy in six-coordinate Pt(IV), compared to four-c oordinate Pt(II) complexes. The much greater propensity of Pt(IV) than of P t(II) complexes to undergo reductive elimination via a ligand-predissociati on mechanism can be attributed to these interactions. Our calculations sugg est that Pt(TV) complexes with chelating trialkylphosphine ligands should b e good candidates for undergoing C-H reductive eliminations by a direct mec hanism, rather than by a ligand predissociation pathway.