Understanding the Fe-CO bond through the electronic structure of Fe(CO)L, m = 2, 3, n = 0-3, L = Cl, Br, HO or NH.

Carbon monoxide (CO) exerts various protective effects on the body. Drugs known as CORMs (CO-releasing molecules) can continuously release small doses of CO into diseased tissues and cells. Transition metals interact strongly with the carbonyl group, and coordination compounds bearing carbonyl groups are a promising class of CORMs. This study investigates the octahedral coordination of Fe and Fe compounds with carbonyl groups (to give Fe[CO]) and subsequent substitutions with Cl, Br, NH, and HO, to understand how these ligands interfere in the M-CO bond. The geometry optimization calculations were performed with the methods BP86 and B3LYP and the atomic basis set def2-TZVP. The molecular orbitals and the properties derived from the electronic density based on QTAIM were analyzed. Coordination with ligands increased the influence of the metal atomic basin on the Fe-C bond, especially for the Fe compounds, and the Cl and Br ligands led to lower local ionization energies at the Fe-C bonds. Trans effects were also observed in the QTAIM real functions: Fe-C bond distances were shorter when C was in trans position to a ligand.