Covalent bonding
Simply electrostatic forces among ions are nondirectional but with increasing covalent character the directional properties of valence orbitals become more significant. Compounds among nonmetallic elements have predominantly covalent bonding and the structures can frequently be rationalized from the supposed CN and bonding geometry of the atoms exists. So in SiC both elements have tetrahedral coordination; in SiO2 silicon also creates four tetrahedral bonds and oxygen two bonds with a nonlinear geometry.
Compounds of less electropositive metals also depict structural influences that can be attributed to partial covalent bonding. ZnO and CuCl have structures with tetrahedral coordination even though from radii the (octahedral) rock salt structure would seem more likely. Partial covalent bonding involves some electrons back's transfer from the anions, in the empty 4s and 4p orbitals on Cu+ and Zn2+. Tetrahedral coordination is the general bonding geometry when an entire set of s and p orbitals is employed in this way. Mercury creates an extreme instance of the lower coordination numbers frequently found with post-transition metals: HgII compounds are of usually low ionic character and two-coordination is common.
Covalent bonding influences sometimes dictate less regular structures than those displayed by Cu+ and Zn2+. Particular d electron effects operate in compounds like CuO and PdO, and some post-transition metal compounds like SnO and PbO apparently depict the structural affect of nonbonding electron pairs on the cation.
Covalent bonding interactions can also take place among atoms of similar element. Section D5 explains some structures that can occur in this way. Here it is worth noting that the NiAs structure, never supposed for purely ionic compounds because cations are closer together than in the rocksalt structure, is frequently found with transition metals in combination with less electronegative nonmetals like S, P and As. The compounds created are of low ionic
character and often show metallic conduction. The close contacts among metal atoms facilitate direct bonding interaction.