Iron dihalides: structures and thermodynamic properties from computation and an electron diffraction study of iron diiodide

The molecular structures, vibrational frequencies, and thermodynamic properties of all four monomeric and dimeric iron dihalide molecules, FeF2, FeCl2, FeBr2, and FeI2, were determined by quantum chemical calculations and the structure of iron diiodide also by gas-phase electron diffraction (ED). The earlier ED study of iron dibromide was also repeated. All iron dihalides are stable molecules in contrast to the iron trihalides, for which FeBr3 and FeI3 are unstable and easily decompose to the corresponding dihalides. The structures of the trimers and tetramers of FeCl2 were also calculated and compared to the crystal structure.