eighteen-electron rule

Main-group atoms like to reach a filled outer shell of eight electrons — the octet rule. Transition metals have more room because they also have five d orbitals to fill. Counting one s, three p, and five d orbitals gives nine, and nine orbitals filled with pairs hold eighteen electrons. The eighteen-electron rule is the transition-metal cousin of the octet: many stable complexes are happiest when the metal is surrounded by a total of eighteen valence electrons.

To apply it, add up all the valence electrons around the metal: the metal's own d electrons plus the electrons donated by the ligands. In the common neutral (covalent) counting method, a CO or an amine donates two electrons, a halide one (as a neutral radical) or two (as an anion, in the ionic method — both methods reach the same total if used consistently). For example, in Ni(CO)4 nickel contributes ten valence electrons and each of the four carbonyls donates two, giving 10 + 8 = 18 — a very stable molecule. The rule works best for low oxidation states with strong pi-acceptor ligands like CO, which is why it is the backbone of organometallic chemistry and a quick guide to whether a proposed complex or reaction intermediate is reasonable.

Be honest about its limits: the 18-electron rule is a useful guideline, not a law, and there are whole classes of stable exceptions. Square-planar d8 complexes of Pt(II), Pd(II), Rh(I), and Ir(I) are famously stable at sixteen electrons, not eighteen. Early transition metals and complexes with mainly sigma-donor ligands (like many Werner-type ammine and aqua complexes) routinely sit below eighteen. The rule is most reliable in the middle-to-late d block with good pi-acceptors, and least reliable elsewhere — use it as a sanity check, not a commandment.