Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive.
[1][2] The rule usefully predicts the formulas for low-spin complexes of the Cr, Mn, Fe, and Co triads.
In general, complexes that obey the rule are composed at least partly of π-acceptor ligands (also known as π-acids).
The relationship between oxidation state and the nature of the ligands is rationalized within the framework of π backbonding.
Computational findings suggest valence p-orbitals on the metal participate in metal-ligand bonding, albeit weakly.
Strong ligand fields lead to low-spin complexes which cause some exceptions to the 18-electron rule.
Examples include Monsanto acetic acid synthesis, hydrogenations, hydroformylations, olefin isomerizations, and some alkene polymerizations.
Examples: Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand.
Ligands where the coordinating atoms bearing nonbonding lone pairs often stabilize unsaturated complexes.
In the case of the metallocenes, the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal.
Somewhat satisfying are the two following observations: cobaltocene is a strong electron donor, readily forming the 18-electron cobaltocenium cation; and nickelocene tends to react with substrates to give 18-electron complexes, e.g. CpNiCl(PR3) and free CpH.
There is one occupied valence MO with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal AOs.
But the adducts TM(CO)8− (TM=Sc, Y) fulfill the 18-electron rule when one considers only those valence electrons, which occupy metal–ligand bonding orbitals.