Understanding the Fe-CO bond through the electronic structure of Fem+(CO)6-nLn, m = 2, 3,n = 0-3, L = Cl-, Br-, H2O or NH3.

Abstract

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 Fe2+ and Fe3+ compounds with carbonyl groups (to give Fen+[CO]6) and subsequent substitutions with Cl-, Br-, NH3, and H2O, 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 Fe2+ 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.