Term: encounter complex
https://doi.org/10.1351/goldbook.E02087

Definition:
A complex of molecular entities produced at an encounter-controlled rate, and which occurs as an intermediate in a reaction mechanism. When the complex is formed from two molecular entities it is called an 'encounter pair'. A distinction between encounter pairs and (larger) encounter complexes may be relevant in some cases, e.g. for mechanisms involving pre-association.

Related Terms:
1) complex (http://doi.org/10.1351/goldbook.C01203).
2) molecular entities (http://doi.org/10.1351/goldbook.M03986).
3) encounter-controlled rate (http://doi.org/10.1351/goldbook.E02089).
4) pre-association (<em>of a reaction</em>) (http://doi.org/10.1351/goldbook.P04794).
5) reaction mechanism (<em>of a reaction</em>) (http://doi.org/10.1351/goldbook.M03804).
6) encounter pair (<em>of a reaction</em>) (http://doi.org/10.1351/goldbook.E02090).
7) encounter (<em>of a reaction</em>) (http://doi.org/10.1351/goldbook.E02086).

Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1112 (https://doi.org/10.1351/pac199466051077)

Citation: 'encounter complex' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.E02087

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