Alkane activation by first, second, and third row transition metal ions : organometallic chemistry in the gas plate

Author: Perry, Jason Kendrick

Year: 1994

Degree: Dissertation (Ph.D.)

Advisor: Goddard, William A., III

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/cs1d-8f56

Abstract

Reactions of the atomic transition metal cations, Co^+, Rh^+, and Ir^+, with H_2, CH_4, and C_2H_6 in the gas phase are considered using high level ab initio techniques. The nature of complexation and oxidative addition, in particular, are discussed. We find that the third row metal, Ir^+, is significantly more reactive toward these small molecules in comparison to the first and second row metals. Ir^+ is capable of inserting into the H—H bond and leads to the facile dehydrogenation of CH_4 to form IrCH^+_2. Co^+ and Rh^+ form only molecular complexes with these molecules. All three metals exothermically dehydrogenate ethane but Co^+ has a barrier which prevents the reaction from being observed at room temperature. The mechanisms for dehydrogenation are distinctly different for each metal. For Co^+, the initial C—H insertion to form Co(H)(C_2 H_5)^+ is rate limiting. This is followed by a multi-center elimination of H_2. For Rh^+, the concerted insertion into two C—H bonds to form Rh(H)_2(C_2H_4)+ is seen without the intermediacy of Rh(H)(C_2H_5)^+. Reductive elimination ofH_2 follows. For Ir^+, a number of reaction pathways are viable with the most favorable being a stepwise oxidative addition/β-H shift mechanism. Much of the difference in the chemistry of these metals stems from two principal factors: the atomic state splittings and the orbital sizes. For Co^+ and Ir^+, both the s^1d^7 and d^8 valence electron configurations are accessible, providing the flexibility needed to adapt to the changing ligation of the reaction profile. For Rh^+, the s^1d^7 state is high in energy, limiting the efficiency of this metal in these reactions. In addition, the s and d orbitals have dramatically different sizes for Co^+, which diminishes the effectiveness of sd hybridization and leads to weaker bonds, particularly in highly ligated complexes. The s and d orbitals of Rh^+ and Ir^+ are more similar in size, providing strong sd hybrid bonds. These two factors compromise the reactivity of Co^+ and Rh^+, leaving only Ir^+ near the ideal.

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