Triiron dodecacarbonyl


Triiron dodecarbonyl is the organoiron compound with the formula Fe312. 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 Fe312 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 than iron pentacarbonyl.

Synthesis

It was one of the first metal carbonyl clusters synthesized. It was occasionally obtained from the thermolysis of Fe5:
Traces of the compound are easily detected because of its characteristic deep green colour. UV-photolysis of Fe5 produces Fe29, not Fe312.
The usual synthesis of Fe312 starts with the reaction of Fe5 with base:
followed by oxidation of the resulting hydrido cluster with acid:
The original synthesis by Walter Hieber et al. entailed the oxidation of H2Fe4 with MnO2. The cluster was originally formulated incorrectly as "Fe4".

Structure

Elucidation of the structure of Fe312 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 quadrupolar coupling constants.
Fe312 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 C2v point group symmetry. By contrast, Ru312 and Os312 adopt D3h-symmetric structures, wherein all 12 CO ligands are terminally bound to the metals. Spectroscopic evidence indicates that the two carbonyl groups may be unsymmetrical, in which case the idealized C2v symmetry is reduced to C2. In solution Fe312 is fluxional, resulting in equivalencing all 12 CO groups. Overall, it can be appreciated that these three clusters formally arise from condensation of three 16-electron M4 fragments, akin to the theoretical condensation of three methylene molecules into cyclopropane.
The anion − is structurally related to Fe312, 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 carbonyls, Fe312 undergoes substitution reactions, making, for example, Fe311 upon reaction with triphenylphosphine.
Heating Fe312 gives a low yield of the carbido cluster Fe515C. Such reactions proceed via disproportionation of CO to give CO2 and carbon.
Fe312 form "ferroles" upon reaction with heterocycles such as thiophenes.
Fe312 reacts with thiols and disulfides to give thiolate-bridged complexes, such as methylthioirontricarbonyl dimer:

Safety

Fe312 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.