An adduct formed by electron-pair donation from a π-orbital into a σ-orbital, or from a σ-orbital into a π-orbital, or from a π-orbital into a π-orbital. For example:
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Such an adduct has commonly been known as a 'π-complex', but, as the bonding is not necessarily weak, it is better to avoid the term complex.
See also: coordination
Sources:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1154 (https://doi.org/10.1351/pac199466051077)
PAC, 1995, 67, 1307. 'Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)' on page 1312 (https://doi.org/10.1351/pac199567081307)