Does covalency decrease with coordination number?

Abstract

It is traditionally believed that the degree of covalency of chemical bonds decreases with increasing coordination number, while the degree of ionicity increases. Using Bader charge analysis and our novel definition of atomic charges based on Wannier functions, we demonstrate that this expectation does not hold for zinc oxide (ZnO), carbon dioxide (CO2) and sodium chloride (NaCl). The low-pressure wurtzite phase of ZnO (with fourfold coordination of Zn and O) and the high-pressure rock salt phase (with sixfold coordination) exhibit nearly identical degrees of ionicity. For CO2, covalency slightly increases in the high-pressure β-cristobalite phase (where the carbon atom is fourfold coordinated) compared to its low-pressure molecular cubic phase (where it is twofold coordinated). Thus, contrary to the common belief, the degree of covalency of chemical bonds is governed primarily by the chemical nature of the atoms and their stoichiometric ratio, rather than by the coordination number.