Theoretical studies of oxidative addition and reductive elimination

Citation

Low, John James (1985) Theoretical studies of oxidative addition and reductive elimination. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:10232009-095454256

Abstract

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 H_2 to Pt(PH_3)_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 s^1d^9 and d^(10) 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 MR_2 complexes (where M = Pd or Pt and R = H or CH_3) to model concerted reductive coupling from MR_2L_2 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 s^1d^9-d^(10) state splittings of these elements (32 kcal/mol). Thus reductive elimination is exothermic from Pd complexes (since Pd prefers d^(10)) and endothermic from Pt complexes (since Pt prefers s^1d^9), where the metal product is in its d^(10) state. This supports the conclusion, derived from qualitative considerations of generalized valence bond wavefunctions, that Pt(TT) and Pd(TT) complexes have their metal atoms in a s^1d^9 configuration and the metal atoms in Pt(0) and Pd(0) complexes are in a d^(10) 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 sp^3 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 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(CH_3)_2(Cl)_2(PH_3)_2 and on various Mt(R_1)(R_2)(PH_3)_2 complexes (where Mt = Pd or Pt; R_1, R_2 = H or CH_3) 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 H_2 to Pt(O) systems. Chapter 4: Ab initio calculations were carried out on CH_x and NH_x molecular fragments on small clusters of Ni atoms (Ni_(13) and Ni_(14)), 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 CH_x and NH_x, species on this surface. The results have also been used to estimate ∆H^0_f for various intermedates important for methanation and ammonia decomposition on Ni surfaces.

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