The separation of O-2 from air over nickel-transition metal complexes has been studied using in situ infrared and Raman spectroscopy, thermogravimetric analysis, volumetric gas sorption, and quantum chemical simulation methods. Exposure of O-2 to the solid Ni-transition metal complexes produces a reactive oxygen species at ambient temperatures. The infrared transient responses, during the absorption process, indicate that the ligand groups interact with oxygen, producing both weakly bound and strongly bound oxygen species. The results indicate that the reactive oxygen interacts weakly with the cyanide ligand groups, which are easily removed during the pressure swing absorption/desorption process at 298 K and 689.5 kPa. Temperature-programmed desorption revealed that the oxygen absorbed at the Ni center was bound stronger than the ligand-bound oxygen, evidenced by its removal at 393 K and the disappearance of a hydrogen-bonded species. The results obtained for the absorption/desorption process suggest that the persistence of the activated oxygen and reactivity with the transition metal ligands are an important factor for improving the absorption capacity of the organometallic sorbent. The in situ infrared spectroscopy study reveals the chemical structure of the ligand groups acting as adsorption sites for the reversible 02 uptake of the Ni-transition metal complex; the ligand-O-2 interaction is an important factor for air separation sorbent development using organometallic complexes.