In this project funded within the Inorganic, Bioinorganic and Organometallic Chemistry Program, Robert R. Holmes of the University of Massachusetts will investigate the extent to which phosphorus and silicon nucleophilic displacement reactions are described by a common basis. The study of the substitution reaction mechanisms of these elements relates to a wide range of relevant problems such as phosphoryl transfer reactions in biological processes and the polymerization of silanols and siliconates to form silicates and silicones. The problem will be studied by a combined experimental and theoretical approach which leads to the determination of the energetics and stereochemistry of the likely intermediates in these processes. The theoretical program consists of ab-initio molecular orbital calculations and will treat common nucleophilic displacement reactions for which a series of entering, leaving and non-departing groups are varied. The experimental program involves the synthesis and structural characterization of five-coordinated anionic siliconates such as penta(alkoxy)- ,penta(hydroxy)- and pentahalo-siliconates as well as mixed alkylhalo-, alkoxyhydroxy- and chlorofluoro- penta-substituted siliconates. Single crystal X-ray structural analysis and solid state nuclear magnetic resonance measurements will be conducted to determine structural change between the solid and solution states and to evaluate activation energies for ligand exchange processes.
