In the case of 2a, the transition from the Ni 1s orbital to the LUMO+2 significantly contributes to the pre-edge peak, where the LUMO+2 mainly consists of the Ni 3d (6.7%), Ni 4p (7.8%), and CO 2p (30.7%) orbitals (Figure ). On the other hand, in the case of 3a, the increased intensity of the corresponding pre-edge peak can be rationalized predominantly by the transition from the Ni 1s orbital to the LUMO. The LUMO of 3a features increased contribution from the Ni 4p orbitals (17.1%) and CO ligands (52.3%) compared to their contributions to the LUMO+2 in 2a (Figure ), while the Ni 3d orbitals exhibit a nearly identical degree of contribution to the respective MOs in 2a and 3a. We therefore attributed the increase in the pre-edge peak intensity of 3a induced by the Al(C6F5)3-mediated geometric change to the increased efficiency of the Ni 3d-4p mixing and contribution of CO ligands (e' symmetry) in the trigonal planar geometry.