Infrared Photodissociation Spectroscopy of Mononuclear Iron Carbonyl Anions

Abstract

The infrared photodissociation spectroscopy of mass-selected mononuclear iron carbonyl anions Fe(CO)_n^– (n = 2–8) were studied in the carbonyl stretching frequency region. The FeCO^– anion does not fragment when excited with infrared light. Only a single IR active band was observed for the Fe(CO)₂^– and Fe(CO)₃^– anions, consistent with theoretical predictions that these complexes have linear D_∞h and planar D_3h symmetry, respectively. The Fe(CO)₄^– anion is the most intense peak in the mass spectra and was characterized to have a completed coordination sphere with high stability. Anion clusters larger than n = 4 were determined to involve a Fe(CO)₄^– core anion that is progressively solvated by external CO molecules. Three CO stretching vibrational fundamentals were observed for the Fe(CO)₄^– core anion, indicating that the Fe(CO)₄^– anion has a C_3v structure. All the carbonyl stretching frequencies of the Fe(CO)_n^– anion complexes are red-shifted with respect to those of the corresponding neutrals.