Spectroscopic characterization of heteronuclear iron–chromium carbonyl cluster anions

Abstract

Infrared photodissociation spectroscopy has been used to investigate CrFe(CO)_n⁻ (n = 4–9) clusters in the gas phase. Comparison of the observed spectra in the carbonyl stretching frequency region with those predicted for low-lying isomers by DFT calculations showed that the observed CrFe(CO)_n⁻ (n = 4–8) clusters could be characterized to have Cr–Fe bonded (OC)₄Fe–Cr(CO)_n−4 structures. The coexistence of isomers with the (OC)Fe–Cr(CO)₅ and (OC)₃Fe–Cr(CO)₄ structures was also observed for CrFe(CO)₆⁻ and CrFe(CO)₇⁻ anions, respectively. The CrFe(CO)_n⁻ (n = 4–8) complexes were strongly bonded systems. The CrFe(CO)₈⁻ complex was a coordination-saturated cluster, and the CrFe(CO)₉⁻ anion was characterized to contain a CrFe(CO)₈⁻ core tagged by one CO molecule. Bonding analysis revealed that the Cr–Fe bonds in the CrFe(CO)_n⁻ (n = 4–8) clusters were predominantly σ-type single bonds. The iron center in the Fe(CO)₄ moiety and the chromium center in the Cr(CO)₅ moiety fulfilled the 18-electron configuration for the CrFe(CO)_n⁻ (n = 4–6) clusters. As in the CrFe(CO)_n⁻ (n = 7, 8) complexes, the iron center in the Fe(CO)₄ moiety exhibited a 17-electron configuration, while the chromium center in the Cr(CO)₄ moiety exhibited a 16-electron configuration. These findings provide valuable insights into the structure and bonding mechanism of heterometallic carbonyl clusters.