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Why does the 18-electron rule apply more strongly to covalent complexes than to fluoro or aqua complexes?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

The 18-electron rule, which states that coordination compounds are most stable when their total valence electrons approach 18, applies strongly to covalent complexes—such as carbonyls, cyanides, and phosphines—because these species rely on specific electronic configurations to achieve stability. In these covalent systems, the metal-ligand bonding is highly dependent on the electronic structure, and the 18-electron count effectively mirrors the stable valence shell of noble gases.123

In contrast, the 18-electron rule is less applicable to more ionic, or "outer-orbital," complexes like fluoro or aqua complexes. In these compounds, electronic factors are less critical in determining coordination numbers, and the bonding is characterized by weaker interactions between the metal cation and the ligands. Consequently, these complexes frequently exhibit configurations that exceed 18 valence electrons; for example, several nickel(+2) complexes, including the hexaaqua and hexafluoro varieties, possess 20 valence electrons. While covalent complexes are governed by the need to fill specific bonding orbitals to reach a stable 18-electron state, ionic complexes are often more influenced by factors such as the relative sizes of the metal ion and ligands.14