Study of Bond Angles and Bond Lengths in Disiloxane and Related Molecules in Terms of the Topology of the Electron Density and Its Laplacian.

We have calculated the electron density distributions for the series of molecules H(n)XOXH(n), X = Li to F and Na to Cl, and some related molecules. We have analyzed these distributions and their Laplacian to obtain atomic charges, electron densities at the bond critical point, and the charge concentrations revealed by the Laplacian. On the basis of this information and an analysis of the X-O bond lengths and angles, we have examined the factors that determine the lengths of the X-O bonds and the XOX bond angles. The XO bond length reaches a minimum value at boron in period 2 and at silicon in period 3 when the product of the charges on X and O reaches a maximum value, consistent with a predominately ionic model for the molecules X = Li, Be, B, Na, Mg, Al, and Si. In the remaining molecules of both series, the XO bonds have an increasing covalent character. The bond length and the bond angle in disiloxane are consistent with the ionic character of the molecule, and there is no evidence for the frequently quoted back-bonding model. In disiloxane and related molecules in which the ligand is considerably less electronegative than oxygen the electrons in the valence shell of oxygen are not well localized into pairs, so the bond angle is intermediate between the tetrahedral angle expected when the valence shell electrons of oxygen are strongly localized into four tetrahedral pairs and the 180 degrees bond angle expected on the basis of the electrostatic and/or steric repulsion between the positively charged X atoms. The effects on the bond lengths and angles of substituting oxygen by sulfur and hydrogen by fluorine are discussed.