Spectroscopic characterization of heteronuclear iron-chromium carbonyl cluster anions.

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