Herein, we present a method for the reversible pressure-swing chemisorption of CO on a Ni(0) complex at room temperature (rt, indicating a temperature of around 22-27 °C in this work) via ligand substitution (Fig. ). This mechanism for CO adsorption/desorption is based on using a multifunctional carbene ligand with a hemi-labile coordination site, whereas previously reported systems rely on the simple coordination/dissociation of CO on coordinatively unsaturated metal centers (6,9,10,13) (Fig. ). Furthermore, we demonstrate that the use of an ionic liquid (IL) as the reaction medium, i.e., as a dispersant and/or solvent for the adsorbents enhances the desorption effectively, which stands in sharp contrast to the typical use of ILs for the absorption of CO . For the design of the Ni(0)-based system, the choice of ancillary ligand is critical. This ligand should be equipped with a hemi-labile coordination moiety that can compete with the coordination of CO to the Ni(0) centers even in the solid (crystalline) state. We thus focused on the use of N-phosphine-oxide-substituted imidazolylidenes (PoxIms; 1a-b) and the corresponding imidazolinylidene (SPoxIm; 1c), as the N-phosphinoyl group can serve as a hemi-labile ligand to coordinate Ni(0) in addition to the diaminocarbene moiety (Fig. ) . To date, (S)PoxIms have demonstrated various coordination modes toward metals, including coordination by only the carbene atom (-C) , by only the N-phosphinoyl oxygen atom (-O) , and by both the carbene and oxygen atoms (-C,O) ; however, dynamic coordination exchange between the -C and -C,O modes remains unknown.