Axial ligation mechanisms of a metalloporphyrin, nickel(ii) tetramesitylporphyrin (NiTMP), were investigated by static and transient X-ray absorption spectroscopy at Ni K-edge (8.333 keV). A surprisingly broad (i.e. ∼1.4 eV) linewidth for the 1s → 3dx2-y2 transition in the ground state was attributed to strong geometry dependent 3d molecular orbital (MO) energies due to coexisting conformers in solution. The broad distribution of 3d MO energy levels enables transient degeneracy of the 3dz2 and 3dx2-y2 MOs to produce a temporary vacancy in the 3dz2 MO which favors axial ligation. Photoexcitation also induces the vacancy in the 3dz2 MO, leading to a more than two-fold enhancement in the axial ligated species. Therefore, a unified axial ligation mechanism for both the ground and excited state is proposed based on the elucidation of the excited state structural dynamics, which will have a broad impact in understanding and controlling axial ligation in enzymatic reactions and molecular catalysis involving transient axial ligation.
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