Hydridocarbonylferrates are weak acids of the general formula
[HFen(CO)[n(11−n)/2]−1]− (n=1–4). Their conjugated bases, the dianonic
[Fen(CO)[n(11−n)/2]−1]2− complexes are also known [3]. The most popular of these
dianionic carbonylferrates is Na2Fe(CO)4, commercially available as the dioxane
complex (‘‘Collman reagent’’). Its use for the stoichiometric carbonylation of reactive
alkyl halides has met with large success [8] and evidence exists for the involvement
of [Fe(CO)4]2− in the catalytic carbonylation of benzyl halides [9].