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.