Chemistry Department Seminar

Transition-metal hydrides are important intermediates in a large range of catalytic reactions. One of the defining characteristics of these compounds is their ability to act as hydride donors. In fact, it is this property for which they are named. However, reliable methods for experimentally determining the thermodynamic hydride donor ability of these compounds have only been developed in the past five years. The hydricity of a metal complex is defined in a manner strictly analogous to acidity. It is the free energy associated with the heterolytic cleavage of the M-H bond to form a metal fragment and a H- ion in solution. This presentation will describe various methods that have been developed in our group for determining this property. Contrary to previous expectations, the hydricity of a transition-metal hydride does not correlate with bond polarity or with the position of the metal in the periodic chart. Hydricity is not simply the inverse of acidity. In fact, these two properties can be independently controlled. Some of the factors that do control hydricity will be discussed. A knowledge of these factors has been used to: (1) Understand the stability of hydrides. (2) Design a new stoichiometric reaction for the room temperature hydrogenation of coordinated CO. (3) Design electrocatalysts for hydrogen oxidation and production. (4) Demonstrate the feasibility of forming boron-hydrogen bonds from hydrogen using transition metals as hydride transfer agents. Other potential applications include the development of transition-metal hydride catalysts for reactions in very acidic solutions, and the rational development of compounds for hydrogen separation applications. Finally, initial approaches to developing models that predict the thermodynamic properties of M-H bonds will be discussed.