The GCMC simulation of CO 2 adsorption in MOF-5 is revisited, finding a surprising role of the BDC organic ligand in this process. A novel structure partition methodology is introduced to decompose the relative contributions of the metal oxoclusters and the organic ligands. The origin of the primary step in the CO 2 adsorption isotherm in MOF-5 is explained by a synergistic effect arising from specific short-range interactions between the sorbate and the MOF components with equal contributions from both the metal oxocluster and the organic ligand, together with an additional contribution due to the nano-confinement of the adsorbate.