A Computational Study of Reductive Elimination Reactions to Form C−H Bonds from Pt(II) and Pt(IV) Centers. Why Does Ligand Loss Precede Reductive Elimination from Six-Coordinate but Not Four-Coordinate Platinum?

In agreement with the experimental results of Halpern on (PPh₃)₂Pt(CH₃)H, B3LYP and CCSD(T) calculations on the model species (PH₃)₂Pt(CH₃)H (1) find that reductive elimination of methane from this four-coordinate, Pt(II) complex proceeds without prior PH₃ ligand loss. The free energy of activation calculated for reductive elimination of methane from 1 is in good agreement with the value measured for (PPh₃)₂Pt(CH₃)H. In contrast to the case for 1, we were unable to find a genuine pathway for direct reductive elimination of methane, without concomitant ligand loss, from (PH₃)₂Cl₂Pt(CH₃)H (13). This computational finding is in accord with the observation that reductive eliminations from six-coordinate, Pt(IV) complexes almost invariably take place via a pathway involving loss of a ligand, prior to the reductive elimination step. PH₃ ligand loss is found to lower the barrier to reductive elimination by slightly more in the Pt(IV) complexes than in the Pt(II) complexes. However, our calculations indicate that the difference between the preferred pathways for reductive elimination from 1 and 13 can be primarily attributed to a reduction in the enthalpic cost of PH₃ ligand loss from the six-coordinate, Pt(IV) complex (13), compared to the four-coordinate, Pt(II) complex (1).