An object of the present invention is to provide a novel complex having at least one carbon-carbon double bond and/or carbon-carbon triple bond, and having at least one Diels-Alder reactive functional group such as tetrazine. The present invention provides a complex represented by a formula (1): [M4O(RCOO)6]p  (1) wherein in the formula (1), at least one of R is an alkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2 to 18 carbon atoms, and at least one of R is a monovalent group represented by a structural formula (2), wherein in the structural formula (2), A1 to A4 each independently represent a nitrogen atom or a carbon atom, X1 represents a single bond or a divalent organic group, and R1 represents a hydrogen atom or a monovalent organic group.