Carbonyl adducts to CuMCl4 (M = Al and Ga) have been characterized by single-crystal and/or powder X-ray diffraction, IR and diffuse reflectance UV/vis spectroscopy. Up to two equivalents of carbon monoxide (similar to 200 cm(3)/g relative to stp) are sorbed at room temperature, with equilibrium binding pressures of below 0.5 atm of CO. The carbonyl bonding is shown to be nonclassical, implicating the dominance of sigma-bonding and absence of pi-back-bonding. Analysis of the crystalline structures of the parent and adduct phases provides an atomistic picture of the sorptive reconstruction reaction. Comparison of the reactivity of CO and ethylene with these CuMCl4 materials, as well as other copper(I) halide compounds that exhibit classical and nonclassical modes of bonding, demonstrates the ability to tune the reactivity of the crystalline frameworks with selectivity for carbon monoxide or olefins, respectively.