Bonding in homoleptic iron carbonyl cluster cations: a combined infrared photodissociation spectroscopic and theoretical study

Abstract

Infrared spectra of mass-selected homoleptic iron carbonyl cluster cations including mononuclear Fe(CO)₅⁺ and Fe(CO)₆⁺, dinuclear Fe₂(CO)₈⁺ and Fe₂(CO)₉⁺, and trinuclear Fe₃(CO)₁₂⁺ are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. Only one IR band is observed for the Fe(CO)₅⁺ cation, which is predicted to have a C_4v structure. The Fe(CO)₆⁺ cation is determined to be a weakly bound complex involving a Fe(CO)₅⁺ core ion. In contrast to neutral clusters which have symmetric structures with two and three bridging carbonyl ligands, the dinuclear Fe₂(CO)₈⁺ and Fe₂(CO)₉⁺ cations are characterized to have unbridged asymmetric (OC)₅Fe–Fe(CO)_n⁺ (n = 3 and 4) structures. The trinuclear Fe₃(CO)₁₂⁺ cluster cation is determined to have an open chain like (OC)₅Fe–Fe(CO)₂–Fe(CO)₅ structure instead of the triangular structure with two bridging CO groups for the Fe₃(CO)₁₂ neutral. The di- and trinuclear cluster cations all involve a square pyramid like Fe(CO)₅ building block that satisfies the 18-electron configuration of this iron center. The Fe(CO)₅ building block is isolobal to the CH₃ fragment in hydrocarbon chemistry, the Fe₂(CO)₉⁺ and Fe₃(CO)₁₂⁺ cluster cations may be considered through isolobality to be metal carbonyl analogues of the ethyl and isopropyl cations.