Quantum-chemical analysis of silicon atom pentacoordination

Abstract

The energy characteristics and dynamics of charge redistribution in the X ax(H 3) eq Si…Y group in changing the Si ← Y coordination bond length and rehybridization of the silicon atom valence shell have been calculated in MNDO approximation. The calculations are carried out with a complete optimization of molecular geometry of C 3y symmetry. The formation of complex anions X = H, F, Cl; Y = H − (I) and molecular complex F ax(H 3) eqSi ← OCHOH (II) is discussed. The main difference between pentacoordinate structures I and II is a weaker perturbation of the FSiH 3 molecule caused by the OCHOH ligand as compared to that of H −. When tetrahedral silicon is involved in the formation of an additional donor-acceptor Si ← Y bond, this atom acquires a higher positive charge as compared with the tetracoordinate state of its valence shell. In this case, the electronic density is transferred to the axial and equatorial atoms attached to silicon, affecting predominantly the substituent X ax charge. The complexing energies calculated are as follows, kJ/mol: −253.9 (X = H), −273.2 (X = F), −398.7 (X = Cl) for I and 72.4 for II.