Molecular squares obtained from two olefin-bridged bis(NHC) ligands, NHC-Ar-C=C-Ar-NHC, and two Ag+ or Au+ ions undergo postsynthetic modifications via a UV-irradiation-initiated [2 + 2] cycloaddition reaction to yield the corresponding cyclobutane-bridged dinuclear tetrakis(NHC) complexes. The tetrakis(NHC) ligand can be liberated from the Ag-I complexes as the tetraimidazolium salt. For the Au-I complexes, the substituents at N3 and N3' of the dicarbene ligands determine the outcome of the reaction in the solid state.