Under the optimal desorption conditions using IL-2 as the reaction medium, 3c was converted into 2c in 97% yield after 4 h at rt under reduced pressure (Fig. ). Interestingly, only 9% desorption of CO from 3a proceeded under identical conditions , even though the geometric and electronic features of 3a and 3c are almost identical (vide supra). This significant difference in the desorption rate could be interpreted in terms of the structural flexibility of the ancillary ligands. Complex 3b was also subjected to identical desorption conditions, but the resulting yield of 2b was only 19%. No reaction occurred for 3d or 3e. To evaluate the role of the N-phosphinoyl oxygen atom in 3c, we synthesized Ni(-C-1f)(CO)3 (3f), which underwent desorption of CO to generate Ni(-C,P-1f)(CO)2 (2f) in 39% yield, where 1f is a N-phosphanylsubstituted imidazolidin-2-ylidene. These results demonstrate that the hemi-labile behavior of the N-phosphinoyl moiety (3c vs 3e-f) and the structural flexibility derived from the ethylene moiety in the imidazolidin-2-ylidene ring (3c vs 3a) are both essential to achieve the efficient desorption of CO from the Ni(0) center under the applied conditions.