A computational study of reductive elimination reactions to form C-H bondsfrom 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)(2)Pt(CH3)H, B3LYP and CCSD(T) calculations on the model species (PH3)(2)Pt(CH3)H (1) f ind that reductive elimination of methane from this four-coordinate, Pt(II) complex proceeds without prior PH3 ligand loss. The foe energy of activati on calculated for reductive elimination of methane from 1 is in good agreem ent with the value measured for (PPh3)(2)Pt(CH3)H. In contrast to the case for 1, we were unable to find a genuine pathway for direct reductive elimin ation of methane, without concomitant ligand loss, from (PH3)(2)Cl2Pt(CH3)H (13). This computational finding is in accord with the observation that re ductive eliminations from six-coordinate, Pt(IV) complexes almost invariabl y take place via a pathway involving loss of a Ligand, prior to the seducti ve elimination step. PH3 ligand loss is found to lower the barrier to reduc tive elimination by slightly more in the Pt(TV) 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 prim arily attributed to a reduction in the enthalpic cost of PH, ligand loss fr om the six-coordinate, Pt(IV) complex (13), compared to the four-coordinate , Pt(II) complex (1).