Molecular structures and vibrational frequencies of iron carbonyls:: Fe(CO)5, Fe2(CO)9, and Fe3(CO)12

Hybrid Hartree-Fock/density functional theory (HF/DFT) explicitly including all electrons has been employed to study the three iron carbonyls Fe(CO)(5), Fe-2(CO)(9), and Fe-3(CO)(12). The geometries are fully optimized by the BP86 and B3LYP methods in conjunction with two basis sets, including as many as 507 contracted Gaussian functions for Fe-3(CO)(12). Our theoretical geometries and harmonic vibrational frequencies are carefully compared with available experimental results. For Fe-3(CO)(12) most of the fundamental vibrational frequencies have never been assigned experimenally. Several of the experimental assignments for Fe-2(CO)(9) and Fe-3(CO)(12) are dubious.