Revealing the Evolution of Hybridized Electronic States with the Coordination Number in Surface-Supported Metal–Organic Frameworks

In metal–organic complexes, the coordination number defines the number of σ-bonds between ligands and the central metal atom and thus plays a vital role in determining the electronic, magnetic, optical, and catalytic properties of metal–organic complexes. Here, by a joint study of low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we have investigated the coordination interaction between Fe atoms and pyridyl ligands with increasing coordination number from 2 to 4 in Fe-4,4′-di(4-pyridyl)biphenyl (Fe-DPBP) coordination networks on the Au(111) substrate. The hybridized electronic state located at the central Fe atom and the surrounding pyridyl ligands shifts from 1.04 eV in 2-fold to 1.24 eV in 3-fold and 1.41 eV in 4-fold coordination motifs. The shifting rate, 0.19 eV per pyridyl, gives an experimental estimation of the induced energy shift because of the repulsive potential applied by a pair of ligand electrons in the coordination interaction. Based on DFT calculations, we further reveal the Fe 3d orbitals and N 2p orbitals that participate in the coordination interaction in each motif. Our work provides insights into the correlation between the coordination geometry and electronic coupling at an atomic level.