A comparison of the coordinates of the Ni, C5, C6, P, and O1 atoms between 3 and TS1 reveals that larger deviations are generated in these atoms during the transformation from 3c to TS1c than during that from 3a to TS1a, highlighting the enhanced flexibility of the skeleton of 1c (Fig. ). The C2, Ni, and O1 atoms in 3c can thus smoothly adopt a suitable orientation for the ligand substitution by reducing the Ni•••O1 distance by 0.61 Å to reach TS1c. In contrast, in the case of the formation of TS1a from 3a, the Ni•••O1 distance must be shortened by 0.69 Å under more structurally restricted conditions, resulting in a larger G ‡ to reach TS1a. The reported monodentate NHCs yielded either nickel dicarbonyl (e.g., 2k-l) or tricarbonyl (e.g., 3d-e and 3m-p) complexes, depending on their steric demand when a single molecule of NHC was treated with a Ni(0) species (Fig. ) . Interestingly, %Vbur values of around 39.5-40.0 seem to represent a plausible boundary that determines whether di-or tri-carbonyl complexes are generated as isolable species. In this context, (S)PoxIms 1a-c and N-phosphanylsubstituted 1f demonstrate unprecedented reactivity to afford both di-and tri-carbonyl complexes and realize their interconversion beyond the possible boundary of %Vbur by effectively scaling the spatial volume around the Ni center. from the SC-XRD analysis, reported in this work (2a-c, 2f, 3a-c and 3f) and the previous works (2k-l, 3d-e and 3m-p) .