Bond Lengths and Bond Angles in Oxo, Hydroxo, and Alkoxo Molecules of Be, B, and C: A Close-Packed Nearly Ionic Model
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We have surveyed the experimental data for oxo, hydroxo, and alkoxo molecules of Be, B, and C and have shown that the intramolecular interligand distances for a given central atom are remarkably constant and independent of coordination number and of the presence of other ligands. Atomic charges obtained from the analysis of the calculated electron densities for a large selection of molecules of this type have shown that these molecules are predominately ionic. On the basis of these results we suggest that the bond lengths and geometries of these molecules can be best understood in terms of a model in which anion-like ligands are close-packed around a cation-like central atom. Values of the interligand radius of each ligand obtained from the intramolecular interligand contact distances are smaller than the crystal ionic radii and decrease as expected with decreasing ligand charge. This model provides a simple quantitative explanation of the decrease in the bond lengths in these molecules with decrease in the coordination number from four to three and of the changes in bond length caused by the presence of other ligands with different ligand radii. With decreasing bond length the electron density at the bond critical point increases correspondingly for Be-O, B-O, and C-O bonds. The nontetrahedral angles found in all A(OX)(4) molecule are explained on the basis of a noncylindrically symmetrical charge distribution around oxygen.
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