pre-reactive complexes

https://doi.org/10.1351/goldbook.PT07091
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.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1958 (https://doi.org/10.1351/pac199971101919)