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College Park, Maryland      June 6 - 10 , 2004

T2-B6 (3:00 PM): Single Crystal Analyses of the Structure and Bonding in Transition-Metal Sigma Complexes

A. J. Schultz, T. F. Koetzle, J. A. Cowan, M. E. Miller, X. Wang (Intense Pulsed Neutron Source, Argonne National Lab)

In this presentation we will discuss a series of structures of B-H and Si-H sigma complexes that were recently obtained based on data from the IPNS Single Crystal Diffractometer (SCD). Transition-metal sigma complexes are coordination compounds in which two electrons in an X-H sigma bond form a dative bond with a transition metal. This three-center, two-electron bond can be further stabilized by pi-backbonding from the metal to the X-H sigma* antibonding orbital. Transition metal sigma complexes are typically reactive intermediates that precede oxidative addition of substrates having an X-H bond. Sigma complexes are, therefore, identified as intermediates in catalytic hydrogenation (X = H), activation and functionalization of hydrocarbons (X = C), hydrosilylation (X = Si), and hydroboration (X = B) reactions. Characterizing the structure and bonding of the intermediate species is important to our understanding of these reactions and to improving catalytic processes. Single crystal neutron diffraction has played a critical role in characterizing the structure and bonding of sigma complexes, for which the positional and thermal parameters of the hydrogen atom in close proximity to a metal atom are clearly of utmost importance.

This research was supported by the U.S. DOE, Office of Basic Energy Sciences, under contract W-31-109-ENG-38.

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