AB-INITIO SCF STUDY OF THE NATURE OF BONDING IN NEUTRAL AND CATIONIC SILICON HYDRIDES AND ANALOGOUS CARBON-COMPOUNDS

The nature of bonding in a number of neutral and cationic silicon hydr ides has been studied on the basis of atomic charge (q(A)), Valency (V (A)), molecular valency (V(M)) and localised molecular orbitals (LMO). Several structural isomers of each species have been considered. It i s found that the most stable isomer is generally associated with maxim um q(Si) and minimum V(Si) and V(M). Protonation of the neutral hydrid es leads to an increase in the positive charge of Si, and in molecular valency. The bonding characteristics of the most stable isomers of th e silicon hydrides have been compared with that of analogous hydrocarb ons. It has been observed that the difference in bonding stems mainly from the highly electro-positive nature of the Si atom which prefers t o form three-centre rather than multiple bonds. The molecular hardness parameters (eta) have also been calculated. The most stable isomers o f silicon hydrides are generally associated with maximum eta. The HSAB principle has been tested by considering some hydrogenation and proto n transfer reactions. It is qualitatively satisfied in both cases.