Role of Hexacoordinated Silicon Intermediates in the Hydrolysis and Racemization Reactions of Silyl Halides

Potential energy surfaces for systems consisting of SiH 3 F and SiH 3 Cl molecules with addition of two water molecules (as a model for neutral hydrolysis), with one water and one ammonia molecule (base-catalyzed hydrolysis), and with two ammonia molecules (racemization) were investigated by the B3LYP and MP2 methods with 6-31G(d,p) and 6-311+G(d,p) basis sets. It was shown that in addition to global energy minima corresponding to loosely bound complexes with SiO and SiN separations exceeding 2.5 Å, there exist the stationary points for tightly bound, low-entropy complexes with SiO and SiN bonds of length ca. 2.0 Å. Such complexes were predicted earlier by the analysis of experimental data on nucleophile-assisted hydrolysis and racemization. For SiH 3 F structures of these complexes are close to octahedral with NH 3 or H 2 O in axial positions, while those for silyl chloride are closer to trigonal bipyramidal zwitterions formed by the SiH 3 cation with two nucleophiles and the Cl anion separated from Si by ca. 3 Å. The barriers separating these local minima from collapsing into the global minima (with inversion of configuration) are ca. 10 kcal/mol in systems with two ammonia molecules, However, with substitution of ammonia by water molecules these barriers tend to disappear, and shallow minima at the top of the barrier for inversion of configuration develop into transition states. This tendency is more pronounced for silyl chloride. The barrier for this channel providing inversion of configuration for the SiH 3 Cl + 2H 2 O system becomes lower than the competing channel leading to the hydrolysis products. This is in keeping with the observed preference of the inversion channel for the neutral hydrolysis of chlorosilanes.