A @[email protected] by which one @[email protected] (the @[email protected]) is converted into another by the formation of one (and only one) two-centre @[email protected] between the substrate and another molecular entity and which involves no other changes in @[email protected] in the substrate. For example, the formation of an acyl @[email protected] by attachment of carbon monoxide to a @[email protected] (R⁺): The product of an attachment may also be the @[email protected] of the two reactants, but not all adducts can be represented as the products of an attachment. (For example, the Diels–Alder @[email protected]: results in an @[email protected] of buta-1,3-diene and ethene, but the reaction cannot be described as an attachment since bonds are formed between more than two centres.)