The 18-electron rule 

A huge majority of stable carbonyls follow the 18-electron rule (occasionally called the effective atomic number (EAN) rule). To make use of this rule one first counts the number of valence electrons in the neutral atom, equivalent to the group number (so both s and d electrons are involved), then adds two electrons for the lone-pair of every attached CO. For instance, in Fe(CO)₅, the group number of Fe is eight; five COs create 18. The EAN calculation begins with the actual atomic number (Fe=26). By Adding two electrons for every CO creates an EAN=36, that is the noble gas configuration of Kr. The only variation among the 18-electron and the EAN count is that the latter involves core electrons and so provides a dissimilar count for the three series: 36 (Kr core) for 3d, 54 (Xe core) for 4d and 86 (Ra core) for 5d.

All the mononuclear species apart from V(CO)₆ in Table 1 assure the 18-electron rule. The bi- nuclear and tri-nuclear species do also if (i) the two electrons within a metal-metal bond are counted like contributing to the valence shells of both metal atoms apprehensive, and (ii) a bridging CO gives one electron to each metal. Monomeric Mn and Co carbonyls would comprise an odd number of electrons and dimerize in result. V(CO)₆ is exceptional like a stable radical with 17 valenceshell electrons, most probably since it is sterically not possible for it to dimerize with no losing one CO ligand. Though it does, readily form the 18-electron anion [V(CO)₆]^-.

While other ligands are exist it is normal in 18-electron counting to suppose covalent rather than ionic bonding. In Mn(CO)₅X, in which X=H, Cl or CH₃, Mn and X thus contribute one electron each to the Mn-X bond, and X is considered as a one-electron ligand even if it is a halogen.

One can create a connection among the 18-electron rule and ligand field theory through noting that a d⁶ octahedral complex comprises 18 valence electrons. π-acceptor ligands give strong fields and therefore low-spin configurations so making the d⁶ octahedral combination very favorable. Generally, the 18-electron configuration along with π-acceptor ligands gives a large gap among the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO). With no the stabilization of the lower-energy set of d orbitals provided through a π-acceptor ligand the HOMO-LUMO gap is not very large, and the 18-electron rule does not normally be appropriate to complexes with weak-field ligands. Even with π-acceptor ligands it can break down under some conditions.

1. With elements early on in the transition series which contribute few electrons themselves it might be sterically not possible to coordinate sufficient ligands to get the 18-electron count. V(CO)₆ is an instance.
2. For later elements (group 9 onwards) there is a trend in the direction of lower electron counts.