The SiO bond and electron density distributions

 Electron density distributions, ϱ(r), have been calculated for the partially and fully geometry optimized molecules SiIO, SiIIO2, H2SiIIIO3, H4SiIVO4, H6SIV2O7, H8SiVIO6, H10SiVI2O10 and H12SiVIIIO8, using SCF Hartree-Fock wavefunctions calculated with a 6-311++G** basis set (the Roman numeral superscripts denote the coordination number of Si). The value of the electron density distribution, ϱ(rc), evaluated at (3,−1) critical (stationary) points, rc, along the SiO bonds of these molecules increases linearly with decreasing SiO bond length, R(SiO), from ϱ(rc)=0.50e/Å3 for R(SiVIIIO)=1.96Å to ϱ(rc)=1.36e/Å3 for R(SiIIO)= 1.48Å. A mapping of the Laplacian of the distribution, ∇2ϱ(r), together with a determination of the bonded radii, rb(O), calculated for the oxide ions indicate that the fractional ionic character of the bond increases with increasing coordination number from ∼0.40 for an SiIIO bond to ∼0.50 for an SiIVO bond to ∼0.60 for an SiVIO bond to ∼0.75 for a SiVIIIO bond. Unlike the ionic and crystal radii of the oxide ion, rb(O) is not constant for a given coordination number but increases linearly with R(SiO) from rb(O)=0.86Å for R(SiO)=1.48Å to rb(O)= 1.20Å for R(SiO)=1.96Å. Concomitant with this increase in the size of the oxide ion, the atomic charge on the O atom increases as the volume of its basin expands and the fractional ionic character, ƒi(SiO), of the SiO bond increases. As ƒi(SiO) increases, not only is there a decrease in the value of ϱ(r) at rc, but also the sharpness (curvature) of the maximum in ϱ(r) radiating perpendicular to the bond and the sharpness of the minimum in ϱ(r) along the bond are both diminished.