= 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.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[C01203, complex: component entities and contextual limitations]{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 $\mathrm{(H_2O)_2}$. 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>.
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