Ab Initio MO Study of the Geometry, η³ ⇄ η¹ Conversion, and Reductive Elimination of a Palladium(II) η³-Allyl Hydride Complex and Its Platinum(II) Analogue

The geometry, η 3 ⇄ η 1 conversion, and reductive elimination of a palladium(II) η 3 -allyl hydride complex and its platinum(II) analogue, MH(η 3 -C 3 H 5 )(PH 3 ) ( 1 ) (M = Pd, Pt), are theoretically investigated with ab initio MO/MP4, SD-CI, and coupled cluster double substitution (CCD) methods. In 1, the M−C bond trans-positioned to H(hydride) is much longer than the other M−C bond, while the C−C bond trans-positioned to H is considerably shorter than the other C−C bond, due to the strong trans-influence of H. Nevertheless, the two C−C bonds of the allyl ligand are conjugated well with each other, and 1 is still considered as an η 3 -allyl complex. The η 1 -isomer, MH(η 1 -C 3 H 5 )(PH 3 ) ( 2 ), is less stable than 1 by 8.6 kcal/mol for M = Pd and 13.0 kcal/mol for M = Pt at the MP4SDQ level (MP4SDQ and SD-CI calculations yield similar energy differences). The C−H reductive elimination of PdH(η 3 -C 3 H 5 )(PH 3 ) proceeds with a much lower activation energy ( E a ) of 5.4 kcal/mol and a significantly higher exothermicity ( E exo ) of 29.4 kcal/mol than those of the Pt analogue ( E a = 20.9 kcal/mol and E exo = 11.1 kcal/mol), where the MP4SDQ values are given. This C−H reductive elimination of 1 requires a somewhat higher E a than that of MH(CH 3 )(PH 3 ). These results are interpreted in terms of bond energies. Changes in bonding characteristics caused by this reductive elimination are discussed in detail, on the basis of orbital mixing.