Weakly bound complexes with a potential minimum that precedes the activation
barrier along the reaction path
. In contrast with the van der Waals complexes, which fall apart reversibly into their constituents, the pre-reactive complexes may undergo a vigorous reaction to form different products.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook