Fe(N-2)(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 m ethod. The structures containing end-on and side-on N2 ligands have been fu lly optimized and the dissociation energies estimated. The ground states ar e predicted to be end-on complexes with the exception of n = 2. The vibrati onal analysis of all predicted ground states is reported. The effect of N-1 5 isotopic substitution on the vibrational frequencies has been estimated. Comparisons are made with the isoelectronic species Fe(CO)(n). The Fe-N-2 b onding has been discussed in terms of sigma donation and pi back-donation a nd the Mulliken population analysis. The predicted harmonic frequencies sho w that the infrared spectra of Fe(N-2)(4) and Fe(N-2)(5) are similar, and t he two complexes could not be distinguished in nitrogen matrix experiments using infrared spectroscopy.