complex mechanism
This term is sometimes applied to an elementary reaction which proceeds via an intermediate species having a lifetime longer than a few rotational periods, in contrast to a direct reaction. However, in view of the danger of confusion with a reaction occurring in more than one step (a composite reaction ) this usage is not to be recommended; it is better to use the terms complex-mode mechanism or complex-mode reaction.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 160