In fact, the signal corresponding to the stretching vibration of the CO ligands in 3a was observed at 2019 cm -1 , i.e., at higher wavenumber than that of 2a (1967 cm -1 ). The AIM analysis clarifies the increase in the delocalization indices (Ni, CCO) and (Ni, OCO) in 3a compared to those in 2a, which represent a number of electron pairs delocalized between the Ni and carbonyl carbon atoms (CCO) and the Ni and carbonyl oxygen atoms (OCO), respectively. (Figures S7-S8. The mean values of (Ni, CCO) and (Ni, OCO) increase from 0.51 (2a) to 1.20 (3a) and from 0.063 (2a) to 0.23 (3a), respectively, indicating that electrons become effectively delocalized through the Ni, CCO, and OCO atoms in 3a. On the other hand, the values of (CCO, OCO) are nearly identical between 2a (1.70-1.71) and 3a (1.60-1.62). Thus, we attribute the aforementioned blue-shift to the increased polarization of the  CO bonding orbitals due to an increased cationic nature on the Ni in 3a (AIM atomic net charge = 0.50 e) compared to 2a (AIM atomic net charge = -0.1 e). Based on the aforementioned geometric parameters of 2a and 3a in the crystalline state, the impact of the N-phosphinoyl rotation on the spatial environment surrounding the Ni center was quantitatively evaluated on the basis of percent buried volume (%Vbur) (Figure ). Topographic steric maps that visualize the %Vbur values in the north-western (NW), north-eastern (NE), south western (SW), and south-eastern (SE) quadrants, as well as their average (Av), are also depicted; these maps were produced using the program SambVca 2.1. The Av(%Vbur) values in 2a and 3a were calculated to be 43.9 and 51.2, respectively, clearly demonstrating that the space around the Ni center is significantly altered via rotation of the N-phosphinoyl moiety.