Definitions for "18-Electron rule"
The valence shells of a transition metal can accommodate 18 electrons: 2 in each of the five d orbitals (10 in total); 2 in each of the 3 p orbitals (6 in total); and 2 in the s orbital (see Electron counting). In practice, of course, these orbitals cannot directly accept electrons, otherwise one would encounter ions such as Fe10− and Pt8−. However, combination of these atomic orbitals with ligand orbitals gives rise to nine molecular orbitals which are either metal-ligand bonding or non-bonding.