Disodium tetracarbonylferrate

Disodium tetracarbonylferrate is the chemical compound with the formula Na2[Fe(CO)4]. This oxygen-sensitive colourless solid is employed in organic synthesis", mainly to synthesise aldehydes. It is commonly used with dioxane complexed to the sodium cation, this dioxane solvate being known as Collman's reagent.  The tetracarbonylferrate dianion is tetrahedral.

Synthesis
The reagent was reported by Cooke in 1970. The current synthesis entails the reduction of a solution of iron pentacarbonyl in tetrahydrofuran by sodium-naphthenide. The efficiency of the synthesis depends on the quality of the iron pentacarbonyl.
 * Fe(CO)5 + 2 Na →  Na2[Fe(CO)4]  +  CO

Reactions
The reagent was originally described for the conversion of primary alkyl bromides, RBr, to the corresponding aldehydes in a two-step, "one-pot" reaction:
 * Na2[Fe(CO)4] +  RBr  →  Na[RFe(CO)4]  +  NaBr

This solution is then treated sequentially with PPh3 and then acetic acid to give the aldehyde, RCHO.

Disodium tetracarbonylferrate can be used to convert acid chlorides to aldehydes. As for Cooke’s early discovery, an iron acyl complex undergoes protonolysis to give the aldehyde.
 * Na2[Fe(CO)4] +  RCOCl  →  Na[RC(O)Fe(CO)4]  +  NaCl
 * Na[RC(O)Fe(CO)4] +  HCl  →  RCHO  +  "Fe(CO)4"  +  NaCl

Disodium tetracarbonylferrate reacts with alkyl halides (RX) to produce alkyl complexes:
 * Na2[Fe(CO)4] +  RX  →  Na[RFe(CO)4]  +  NaX

Such iron alkyls can be converted to the corresponding carboxylic acid and acid halides:
 * Na[RFe(CO)4] +  O2, H+  →→  RCO2H  + Fe...
 * Na[RFe(CO)4] +  X2  →  RC(O)X  +  FeX2  +  3 CO  +  NaCl

One attraction of these methods is the low cost of the iron carbonyl as well as the fact that the procedures are relatively “green” because the side product is iron-based.