Then, we explored the adsorption of CO by 2 in the presence of IL-2 at rt; this adsorption should predominantly occur in the solvated state given the sufficient solubility of complexes 2 in IL-2 (Fig. ). Stirring 2a/2c and 350 mg of IL-2 under a CO/N2 (1 atm each) atmosphere afforded 3a/3c in 98% yields via the selective adsorption of CO with concomitant precipitation of fine crystals of 3 (3c is shown as an example in Fig. ). The reversible coordination of the Nphosphinoyl oxygen atom was again confirmed to be effective, as treatment of 2f with CO/N2 resulted in the formation of 3f in 47% after 30 minutes. In addition, CO was directly stored in 3c from gaseous mixtures of CO/CH4/N2 (1 atm each) and CO/H2/N2 (1 atm each) in excellent yields through adsorption by 2c. Thus, the present system could also be used for the purification of CH4 and H2 through the removal of the accompanying CO. Ni(0) dicarbonyl complexes 2g and 2h, which bear a bidentate carbene (1g) or phosphine (1h) ligand, respectively, did not react with excess CO even after being dissolved in THF-d8 (Fig. ). We also explored the preparation of Ni(0) dicarbonyl complexes that bear the bidentate ligands with a phosphine oxide group 1i and 1j. As a result, Ni(-C-1i)2(CO)2 (4i) and Ni(-P-1j)2(CO)2 (4j) were obtained even in the presence of excess CO (Fig. ). The results of these experiments show that our strategy based on the use of (S)PoxIm ligands and IL-based media is effective for the precise separation of CO from gaseous mixtures including N2, H2, and CH4. We then investigated the reusability of the present chemisorption system and found a significant acceleration of the rate of CO desorption between the first and second cycles (Fig. ). In fact, 2c was afforded in 87-88% yield within 2 h from 3c prepared under the optimized conditions via either the adsorption of CO on 2c or sequential CO desorption-adsorption reactions from crystalline 3c, whereas 2c was obtained in 65(3)% yield from crystalline 3c (run 5, Fig. ). This result can be explained by the increase in the total surface area of 3c exposed to the reduced pressure, as the crystals of 3c that were re-precipitated after CO adsorption were significantly smaller than those used in the first desorption process (Fig. ). Furthermore, CO was effectively desorbed from 3c even after five desorption-adsorption cycles. It should be noteworthy that 3c can be used for the purpose of CO-storage, as solid-state 3c and the mixture of 3c/IL-2 were stable for at least 7 days at rt and -30 °C, respectively (Fig. ). In the aforementioned five cycles, the mixture of 3c/IL-2 was stored at -30 °C for 14-16 h after each desorption/adsorption cycle completed. These results shed light on the key features of the Ni(0)-based reversible chemisorption of CO, i.e., this system can be reused without the removal/addition of the IL, and crystallization is not essential for the preparation of the adsorbents. To clarify the reason for the obvious difference in the CO desorption rates of 3a and 3c, density functional theory (DFT) calculations were carried out at the B97X-D/Def2-TZVPD//M06-L/Def2-SVPD (for Ni and O) and Def2-SVP for others//gas phase level of theory. First, we identified two plausible pathways that connect 3c and 2c; in the first, the C2≡O2 moiety at the distal position with respect to the N-phosphinoyl oxygen atom dissociates from 3c, while in the second, the C3≡O3 moiety at the proximal position dissociates (for atomic labels, see Fig. . For details of these two pathways, see Fig. ). A significant difference in the activation energy barriers (G ‡ ) of these pathways was observed (+13.3 kcal mol -1 for the former; +17.0 kcal mol -1 for the latter), indicating that the dissociation of CO from 3c should proceed via cleavage of the C2-Ni bond (Fig. ). In the optimized structure of TS1c shown in Fig. , the interatomic distance between Ni and O1 is shortened to 2.53 Å from the 3.14 Å found in the optimized structure of 3c, while the distance between Ni and C2 is elongated to 2.73 Å from 1.82 Å in 3c. Although these results are based on the structures optimized in the gas phase, the transformation of 3c to 2c should proceed via ligand substitution even under the applied experimental conditions. This ligand substitution results in the formation of the intermediate [2c•••CO] (G ° = +12.9 kcal mol -1 with respect to 3c). The 2c moiety in [2c•••CO] exhibits a geometry that is nearly identical to that of the optimized 2c, e.g., the Ni-O1 lengths are 2.31 Å in [2c•••CO] and 2.30 Å in 2c.