Analysis of competing bonding parameters. Part 2. The structure of halosilanes and halogermanes (MH4−nXn, n=1-4; M=Si, Ge; X=F, Cl, Br)

A qualitative rationalization of bonding patterns in halosilanes and halogermanes (MH4−nXn, n=1-4; M=Si, Ge; X=F, Cl, Br) is presented. Geometrical and bonding properties in these molecules are discussed on the basis of ab initio molecular orbital calculations employing the natural bond orbital population analysis. The results have been compared with data derived previously for halomethanes. Differences in the n-dependence of the M-Cl and M-Br bond lengths for M=C, Si, Ge are explained by a significant reduction in the closed-shell repulsion between the halide atoms. As M gets larger, a continuous decrease in the X-M-X bond angle is observed. Small bond angles (for n=2, 3) are favoured by the p-rich M orbitals in the M-X bonds. They are opposed, however, by the X⋯X repulsion. As M gets larger, the X⋯X separation for a given bond angle increases. A reduction in the X-M-X bond angle is therefore accomplished without overcompensation due to the X⋯X repulsion energy. The variation in the charge density at M as a function of n has been rationalized by differences in the electronegativity of the terminal atoms H and X. Dipole moments have been computed for the molecules in the series. As in the fluoromethanes, a maximum in the dipole moments at n=2 is explained by a combination of geometric and electronic properties unique to the fluoro-compounds. These are, an n-independent charge density at the F sites and a significant decrease in the M-F bond distance as n increases.