The three known iron carbonyls Fe(C0)5, Fe~(C0)9, and Fe3- (CO)12 were discovered by Mond2 and Dewar3 at the beginning of this century. Though Dewar noted the disproportionation reactions of Fez(C0)9 and Fe3(C0)12 induced by pyridine and alumina, the true complexity of these reactions was only recently indicated. Electron spin resonance studies have revealed several odd-electron iron species intermediate in these reactions, sug- gesting that single-electron-transfer processes play an important part in the overall mechanisms."~~ Reactions are frequently both solvent and atmosphere dependent. In the case of solutions of Fe2(C0)9 in THF, the presence of a CO atmosphere leads to stabilization of the species in solution (presumably (THF)Fe- (CO)4),6 whereas, under vacuum or another inert atmosphere, such as N2 or Ar, these solutions undergo disproportionation to give red solutions which have been characterized as containing HFe3(C0)11- anion^.^ Recently the radical anion Fe3(C0)1 I*- was found to decompose in THF to give the iron carbonylate anion HFe3(CO)l I-.' The characterization of these reaction mixtures has, in general, been incomplete. In most cases, the HF~~(CO)II- anion was extracted from the mixture using a suitable counterion before being identified; in other cases the presence of the anion was inferred from UV or IR spectra of the complex mixture. Such procedures leave the exact nature of the counterion undetermined and the source of the hydrogen atom ambiguous. Prompted by our results with other organic molecules,* we have reinvestigated the reaction of diiron nonacarbonyl with THF. We report here the structure of a minor product of this reaction: F~(THF)~[HF~~(CO)I 112 (l), the first reported transition metal complex of the HF~~(CO)II- anion. We also note that the major product of this reaction does not contain the HFej(C0)ll- anion or the related radical anion Fe3(C0)11*- though unfortunately the reactive and noncrystalline nature of this material prevented further characterization.