BOND LENGTHS AND BOND ANGLES IN OXO, HYDROXO, AND ALKOXO MOLECULES OFBE, B, AND C - A CLOSE-PACKED NEARLY IONIC MODEL

We have surveyed the experimental data for oxo, hydroxo, and alkoxo mo lecules of Be, B, and C and have shown that the intramolecular interli gand distances for a given central atom are remarkably constant and in dependent 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 t hat these molecules are predominately ionic. On the basis of these res ults we suggest that the bond lengths and geometries of these molecule s can be best understood in terms of a model in which anion-like ligan ds are close-packed around a cation-like central atom. Values of the i nterligand radius of each ligand obtained from the intramolecular inte rligand contact distances are smaller than the crystal ionic radii and decrease as expected with decreasing ligand charge. This model provid es a simple quantitative explanation of the decrease in the bond lengt hs in these molecules with decrease in the coordination number from fo ur to three and of the changes in bond length caused by the presence o f other ligands with different ligand radii. With decreasing bond leng th the electron density at the bond critical point increases correspon dingly for Be-O, B-O, and C-O bonds. The nontetrahedral angles found i n all A(OX)(4) molecule are explained on the basis of a noncylindrical ly symmetrical charge distribution around oxygen.