Ionic bonding radius ratio effects

Only BeO has CN4, yet it has a value of 7 that shows ionic bonding. This agrees with the calculation of the cohesive energy presented ealier. CN4 is forced upon BeO by the radius ratio effect. The value of 7 forBeO may be compared with the 7 for ZnO, which is just what is expected for covalent bonding. 

The co-ordination number in ionic compounds is determined by the radius ratio - a measure of the necessity to minimize cationic contacts. More subtle effects are the Jahn-Teller effect (distortions due to incomplete occupancy of degenerate orbitals) and metal-metal bonding. 

Most inorganic chemistry texts list cut-off values for ther+/r ratios corresponding to the various geometries of interstitial sites (Table 2.3). However, it should also be pointed out that deviations in these predictions are found for many crystals due to covalent bonding character. An example for such a deviation is observed for zinc sulfide (ZnS). The ionic radius ratio for this structure is 0.52, which indicates that the cations should occupy octahedral interstitial sites. However, due to partial covalent bonding character, the anions are closer together than would occur from purely electrostatic attraction. This results in an effective radius ratio that is decreased, and a cation preference for tetrahedral sites rather than octahedral. 

In the 1939 edition of his classic book The Nature of the Chemical Bond, Pauling listed an extensive table of ionic radii of main-group elements obtained by fixing the radius ratios between the four isoelectronic ion pairs and Na", CP and K", Br and Rb", and I and Cs" as the inverse ratio between the effective nuclear charges r+/r = [37]. The effective nuclear charge of an atom or ion is... 

---