Fe(N2)n (n=1-5):: Structure, bonding, and vibrations from density functional theory

The Fe(N-2)(n) (n = 1-5) complexes have been studied with the LCGTO-KS-DF method. The structures containing end-on and side-on N2 ligands have been fully optimized and the dissociation energies estimated. The ground states are predicted to be end-on complexes with the exception of n = 2. The vibrational analysis of all predicted ground states is reported. The effect of N-15 isotopic substitution on the vibrational frequencies has been estimated. Comparisons are made with the isoelectronic species Fe(CO)(n). The Fe-N-2 bonding has been discussed in terms of sigma donation and pi back-donation and the Mulliken population analysis. The predicted harmonic frequencies show that the infrared spectra of Fe(N-2)(4) and Fe(N-2)(5) are similar, and the two complexes could not be distinguished in nitrogen matrix experiments using infrared spectroscopy.