The bonding in FeN2, FeCO, and Fe2N2: Model systems for side-on bonding of CO and N2

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Abstract
CO and N2 side‐on bonding to one Fe atom are found to be very similar in many respects. In both cases the bonding leads to an increase in the CO or N2 bond length and a decrease in the vibrational frequency. The CO or N2 stretching modes lead to a large dipole derivative along the metal–ligand bond axis. The populations show an almost identical, large donation from the Fe 3d orbitals into the CO or N2 π*. For FeN2, the shift in the observed N2 frequency is smaller than observed for the α state of N2/Fe(111). The shift in the N2 vibrational frequency is found to increase when the N2 interacts with two Fe atoms, either at the Fe–Fe nearest neighbor distance or at the first layer Fe–Fe distance, when the side‐on N2 axis is oriented perpendicular to an Fe–Fe bond. The very large shifts in the N2 ωe are similar to those observed in experiment. When the N2 and Fe2 bond axes are parallel, the N2 vibrational frequency is similar to that found for the interaction with one Fe atom. The Fe2N2 calculations yield two different low‐lying Fe occupations; in one, the two Fe atoms have occupation 3d74s1 while in the second, 4s promotion to 3d mixes in the occupation 3d8 and a larger charge transfer from Fe to N2 is observed. This might be of importance for breaking the N–N bond on an Fe surface.