# Triiron dodecacarbonyl

Triiron dodecacarbonyl is the organoiron compound with the formula Fe$_3$(CO)$_{12}$. It is a dark green solid that sublimes under vacuum. It is soluble in nonpolar organic solvents to give intensely green solutions. Most low-nuclearity clusters are pale yellow or orange. Hot solutions of Fe$_3$(CO)$_{12}$ decompose to an iron mirror, which can be pyrophoric in air. The solid decomposes slowly in air, and thus samples are typically stored cold under an inert atmosphere. It is a more reactive source of iron(0) than iron pentacarbonyl.

## Synthesis

It was one of the first metal carbonyl clusters synthesized. It was occasionally obtained from the thermolysis of Fe(CO)$_5$:
    3 Fe(CO)$_5$ → Fe$_3$(CO)$_{12}$ + 3 CO
Traces of the compound are easily detected because of its characteristic deep green colour. UV-photolysis of Fe(CO)$_5$ produces Fe$_2$(CO)$_9$, not Fe$_3$(CO)$_{12}$.
The usual synthesis of Fe$_3$(CO)$_{12}$ starts with the reaction of Fe(CO)$_5$ with base:
    3 Fe(CO)$_5$ + (C$_2$H$_5$)$_3$N + H$_2$O → [(C$_2$H$_5$)$_3$NH][HFe$_3$(CO)$_{11}$] + 3 CO + CO$_2$
followed by oxidation of the resulting hydrido cluster with acid:
    [(C$_2$H$_5$)$_3$NH][HFe$_3$(CO)$_{11}$] + HCl + CO → Fe$_3$(CO)$_{12}$ + H$_2$ + [(C$_2$H$_5$)$_3$NH]Cl
The original synthesis by Walter Hieber et al. entailed the oxidation of H$_2$Fe(CO)$_4$ with manganese dioxide. The cluster was originally formulated incorrectly as "Fe(CO)$_4$".

## Structure

Elucidation of the structure of Fe$_3$(CO)$_{12}$ proved to be challenging because the CO ligands are disordered in the crystals. Early evidence for its distinctive C$_{2v}$ structure came from Mössbauer spectroscopic measurements that revealed two quadrupole doublets with similar isomer shifts but different (1.13 and 0.13 mm s$^{−1}$) quadrupolar coupling constants.
Fe$_3$(CO)$_{12}$ consists of a triangle of iron atoms surrounded by 12 CO ligands. Ten of the CO ligands are terminal and two span an Fe---Fe edge, resulting in C$_{2v}$ point group symmetry. By contrast, Ru$_3$(CO)$_{12}$ and Os$_3$(CO)$_{12}$ adopt D$_{3h}$-symmetric structures, wherein all 12 CO ligands are terminally bound to the metals. In solution Fe$_3$(CO)$_{12}$ is fluxional, resulting in equivalencing all 12 CO groups on the $^{13}$C NMR timescale.
The anion [HFe$_3$(CO)$_{11}$]$^−$ is structurally related to Fe$_3$(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

Solutions of Fe$_3$(CO)$_{12}$ reacts with triphenylphosphine to give (triphenylphosphine)iron tetracarbonyl (and some bis(triphenylphosphine)iron tricarbonyl).
    Fe$_3$(CO)$_{12}$ + 3 P(C$_6$H$_5$)$_3$ → 3 Fe(CO)$_4$(P(C$_6$H$_5$)$_3$)
Heating Fe$_3$(CO)$_{12}$ gives a low yield of the carbido cluster Fe$_5$(CO)$_{15}$C. Such reactions proceed via disproportionation of CO to give CO$_2$ and carbon.
Fe$_3$(CO)$_{12}$ forms "ferroles" upon reaction with heterocycles such as thiophenes.
Fe$_3$(CO)$_{12}$ reacts with thiols and disulfides to give thiolate-bridged complexes, such as methylthioirontricarbonyl dimer:
    2 Fe$_3$(CO)$_{12}$ + 3 (CH$_3$)$_2$S$_2$ → 3 [Fe(CO)$_3$SCH$_3$]$_2$ + 6 CO. These complexes are studied as hydrogenase mimics.

## Safety

Fe$_3$(CO)$_{12}$ is hazardous 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.