Theoretical Studies of Oxidative Addition and Reductive Elimination. 2. Reductive Coupling of H–H, H–C, and C–C Bonds from Palladium and Platinum Complexes

Ab initio calculations have been carried out on MR₂ complexes (where M = Pd or Pt and R = H or CH₃) to model concerted reductive coupling from MR₂L₂ complexes (where L is a substituted phosphine). The results of these calculations support the following two conclusions. (1) The difference in the driving force for reductive elimination from Pd(II) and Pt(II) complexes with the same R groups is very close (0–4 kcal/mol) to the difference in the s¹d⁹-d¹⁰ state splittings of these elements (32 kcal/mol). Thus reductive elimination is exothermic from Pd complexes (since Pd prefers d¹⁰) and endothermic from Pt complexes (since Pt prefers s¹d⁹), where the metal product is in its d¹⁰ state. This supports the conclusion, derived from qualitative considerations of generalized Valence bond wave functions, that Pt(II) and Pd(II) complexes have their metal atoms in an s¹d⁹ configuration and the metal atoms in Pt(0) and Pd(0) complexes are in a d¹⁰ configuration. (2) The activation Barriers for C-C coupling are approximately twice that for C-H coupling. There are Essentially no Barriers for processes involving H-H bonds. The origin of this trend is the directionality of the methyl sp³ orbital, which destabilizes the transition state for the case where an M-C bond is being converted to a C-C or C-H bond. Conversely, the spherical H 1s orbital can form multicenter bonds easily, allowing it to break M-H bonds while forming an H-H bond and leading to low intrinsic Barriers. These results are consistent with the experimentally observed trends.