CO Adsorption on FeN (N = 1-4) Transition Metal Clusters: a Density Functional Theory Study

The structures and stabilities of the Fe_N(CO)m (N = 1-4, m = 1-5) clusters have been theoretically studied at the density functional theory (DFT) and ab initio levels of theory. In particular, the energetic, structural and vibrational frequencies of carbon monoxide (CO) chemisorptions on iron clusters are studied and compared with those of nitric oxide (NO) and ammonia (NH₃) adsorption. While the C-O and N-O bond lengths and vibrational frequencies strongly depend on cluster size and ligand population, the N-H bond length and vibrational frequency are independent on both cluster size and ligand population. For a particular Fe_N cluster, the vibrational frequency increases as the ligand population increases. On the other hand, keeping the number of ligands fixed, the vibrational frequency decreases significantly with the Fe_N cluster size. NO and CO being strong field ligands in the spectrochemical series, we have seen red shift of the vibrational frequencies with both cluster size and ligand population. As NH3 is a weak field ligand, we do not observe any variation. We have also observed that the HOMO-LUMO gap of both Fe_N(CO)_m and Fe_N(NO)_m clusters strongly depends on N and m. For Fe_N(NH₃)_m cluster, HOMO-LUMO gap decreases as more NH₃ molecules are adsorbed on the metal centre.