The rule usefully predicts the formulas for low-spin complexes of the Cr, Mn, Fe, and Co triads.  Well-known examples include ferrocene, iron pentacarbonyl, chromium carbonyl, and nickel carbonyl. 
Ligands in a complex determine the applicability of the 18-electron rule. In general, complexes that obey the rule are composed at least partly of π-acceptor ligands (also known as π-acids). This kind of ligand exerts a very strong ligand field, which lowers the energies of the resultant molecular orbitals so that they are favorably occupied.  Typical ligands include olefins, phosphines, and CO. Complexes of π-acids typically feature metal in a low-oxidation state.  The relationship between oxidation state and the nature of the ligands is rationalized within the framework of π backbonding.