To gain further insight, the Ni K-edge XAS spectra of 2a and 3a were simulated using the time-dependent DFT (TDDFT) calculation at the B3LYP level with the zeroth-order regular approximation (ZORA) to take the relativistic effect into consideration. The CP(PPP) basis set was used for Ni, while the ZORA-def2-TZVP(-f) basis set was used for all other elements.