Theoretical Studies of Oxidative Addition and Reductive Elimination (2009)

Chapter 1: Ab initio calculations (Hartree-Fock, generalized valence bond, and configuration interaction), utilizing relativistic core potentials, have been used to follow the oxidative addition of H2 to Pt(PH3)2. We find an activation barrier of 2.3 kcal/mol and an exothermicity of 15.9 kcal/mol. From examination of the geometries and wavefunctions, we find that up to the transition state the H-H bond is still intact. The role of the Pt s1d9 and d10 states in oxidative addition is described, and the effects of including electronic correlation are discussed. The implications for reductive elimination of the dimethyl and hydridomethyl complexes are also discussed. Chapter 2: 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 differences 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 wavefunctions, that Pt(II) and Pd(II) complexes have their metal atoms in a 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 is 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. Chapter 3: Ab initio calculations were carried out on Pt(CH3)2(Cl)2(PH3)2 and on various Mt(R1)(R2)(PH3)2 complexes (where Mt = Pd or Pt; R1, R2 = H or CH3) in order to elucidate the differences in reductive H-C and C-C coupling from Pd(II), Pt(II), and Pt(IV) complexes. These studies explain why (1) reductive C-C coupling is facile for Pd(II), favorable for Pt(IV), and unobserved for Pt(II) systems, while (2) reductive H-C coupling is facile for Pt(II) and Pd(II) systems, and (3) oxidative addition is favorable only for addition of H2 to Pt(0) systems. Chapter 4: Ab initio calculations were carried out on CHx and NHx molecular fragments on small clusters of Ni atoms (Ni13 and Ni14), as a model for chemisorption on the Ni(100) surface. The results presented here show that these species make strong π bonds to the surface which cause methylidyne and imidogen to be the most stable CHx and NHx, species on this surface. The results have also been used to estimate ∆H0f for various intermedates important for methanation and ammonia decomposition on Ni surfaces.