 Molecular complex

A molecular complex is an associated unit containing two or more component molecular entities, often held together by interactions weaker than ordinary covalent bonds. The components may be neutral or charged. The term needs context: coordination chemistry uses more specific descriptions for metal–ligand assemblies.^[ref]

A water dimer illustrates the idea: two water molecules associate through a hydrogen bond, while each retains its own ordinary O–H covalent bonds. Its overall composition is $({\mathrm{H}}_2\mathrm{O}{)}_2$. Distinguishing a bound pair from two independent molecules requires evidence about their association, rather than merely observing two water molecules in the same sample. Association and dissociation can coexist in an chemical equilibrium.