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 hydrides 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 is found that the most stable isomer is generally associated with maximum q_si and minimum V_si and V_M. Protonation of the neutral hydrides leads to an increase in the positive charge of Si, and in molecular valency. The bonding characteristics of the most stable isomers of the silicon hydrides have been compared with that of analogous hydrocarbons. It has been observed that the difference in bonding stems mainly from the highly electro-positive nature of the Si atom which prefers to form three-centre rather than multiple bonds. The molecular hardness parameters (η) have also been calculated. The most stable isomers of silicon hydrides are generally associated with maximum η. The HSAB principle has been tested by considering some hydrogenation and proton transfer reactions. It is qualitatively satisfied in both cases.