Triiron dodecacarbonyl

Iron dodecarbonyl is the chemical compound Fe3(CO)12. This species was one of the first metal carbonyl clusters prepared. It is a more reactive form of Fe(0) than Fe(CO)5.

General properties
Fe3(CO)12 is a dark green solid, which vacuum-sublimes with significant decomposition at elevated temperatures. It is soluble in nonpolar organic solvents to give intensely green solutions. Most low nuclearity clusters are pale yellow or orange. Heating solutions of Fe3(CO)12 affords iron mirrors, which can be pyrophoric in air. The solid decomposes slowly in air, and thus samples are typically stored cold under an inert atmosphere.

Synthesis
It was occasionally obtained from the thermolysis of Fe(CO)5:
 * 3 Fe(CO)5 →  Fe3(CO)12  +  3 CO

Traces of the compound are easily detected because of its characteristically deep green color. UV-photolysis of Fe(CO)5 produces Fe2(CO)9, not Fe3(CO)12.

An efficient synthesis of Fe3(CO)12 proceeds via the reaction of Fe(CO)5 with base:
 * 3 Fe(CO)5 +  [(C2H5)3N]  +  H2O  →  [(C2H5)3NH][HFe3(CO)11]  +  3 CO  +  CO2

followed by oxidation of the resulting hydride with acid:
 * [(C2H5)3NH][HFe3(CO)11]  +  HCl  +  CO  →  Fe3(CO)12  +   H2  +  [(C2H5)3NH]Cl

The original synthesis, by Walter Hieber, entailed the reaction of H2Fe(CO)4 with MnO2. The cluster was formulated merely as "Fe(CO)4".

Structure
Fe3(CO)12 features a triangle of Fe atoms surrounded by 12 CO ligands. Ten of the CO ligands are terminal and two span an Fe---Fe edge, resulting in an overall C2v point group symmetry. In contrast, Ru3(CO)12 and Os3(CO)12 adopt D3h-symmetric structures, wherein all 12 CO ligands are terminally bound to the metals. Overall, it can be appreciated that these three clusters formally arise from condensation of three 16-electron M(CO)4 fragments, akin to the condensation of CH2 into cyclopropane.

Elucidation of the structure of Fe3(CO)12 proved to be challenging because the CO ligands are disordered in the crystals. Early evidence for its distinctive C2v structure came from Mößbauer spectroscopic measurements that revealed two quadrupole doublets with similar isomer shifts but different (1.13 and 0.13 mm/s) quadrupole coupling constants.

The anion [HFe3(CO)11]- is structurally related to Fe3(CO)12, with the hydride replacing one bridging CO ligand. The bonding in the Fe-H-Fe subunit is described using concepts developed for diborane.

Reactions
Like most metal carbonyl, Fe3(CO)12 undergoes substitution reactions, making, for example, Fe3(CO)11(P(C6H5)3.

Heating Fe3(CO)12 gives a low-yield of the carbido cluster Fe5(CO)15C. Such reactions proceed via disproportionation of CO to give CO2 and carbon.

Fe3(CO)12 reacts with 1,3-propanedithiol to air-stable µ-(1,3-Propanedithiolato)-hexacarbonyldiiron in which both thiol sulfur atoms form a bridge between two iron atoms. This compound is a model compound for certain all-iron dehydrogenases.

Safety
Fe3(CO)12, like all metal carbonyls is hazardous as a source of volatile iron and as a source of carbon monoxide. Solid samples, especially when finely divided, and residues from reactions can be pyrophoric, which can ignite the organic solvents used for such reactions.