Compounds that obey the 18-electron rule are typically "exchange inert". Examples include [[Hexamminecobalt(III) chloride|[Co(NH)]Cl]], Mo(CO), and [[Ferrocyanide|[Fe(CN)]]].  In such cases, in general ligand exchange occurs via dissociative substitution mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand.  On the other hand, 18-electron compounds can be highly reactive toward electrophiles such as protons, and such reactions are associative in mechanism, being acid-base reactions.
Complexes with fewer than 18 valence electrons tend to show enhanced reactivity.  Thus, the 18-electron rule is often a recipe for non-reactivity in either a stoichiometric or a catalytic sense.