Triiron Dodecacarbonyl

Triiron dodecarbonyl is the organoiron compound with the formula Fe₃(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 Fe₃(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 → Fe₃(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 Fe₂(CO)₉, not Fe₃(CO)₁₂.

The usual synthesis of Fe₃(CO)₁₂ starts with the reaction of Fe(CO)₅ with base:
:3 Fe(CO)₅ + (C₂H₅)₃N + H₂O → C₂H₅)₃NHHFe₃(CO)₁₁] + 3 CO + CO₂
followed by oxidation of the resulting hydride, hydrido cluster with acid:
: C₂H₅)₃NHHFe₃(CO)₁₁] + HCl + CO → Fe₃(CO)₁₂ + H₂ + C₂H₅)₃NHl

The original synthesis by Walter Hieber et al. entailed the oxidation of H₂Fe(CO)₄ with manganese dioxide. The cluster was originally formulated incorrectly as "Fe(CO)₄".

Structure

left, 140px, Saturated solution of Fe₃(CO)₁₂ in mesitylene.
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 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.

Fe₃(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, Ru₃(CO)₁₂ and Os₃(CO)₁₂ adopt D_3h-symmetric structures, wherein all 12 CO ligands are terminally bound to the metals. In solution Fe₃(CO)₁₂ is fluxional, resulting in equivalencing all 12 CO groups on the ¹³C NMR timescale. Overall, it can be appreciated that these three clusters formally arise from condensation of three 16-electron M(CO)₄ fragments, akin to the theoretical condensation of three methylene (:CH₂) molecules into cyclopropane.

The anion Fe₃(CO)₁₁sup>− 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 carbonyls, Fe₃(CO)₁₂ undergoes substitution reactions, making, for example, Fe₃(CO)₁₁ upon reaction with triphenylphosphine.

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 forms "ferroles" upon reaction with heterocycles such as thiophenes.

Fe₃(CO)₁₂ reacts with thiols and disulfides to give thiolate-bridged complexes, such as methylthioirontricarbonyl dimer:
:2 Fe₃(CO)₁₂ + 3 (CH₃)₂S₂ → 3 e(CO)₃SCH₃sub>2 + 6 CO. These complexes are studied as hydrogenase mimics.''Synthesis, Purification, and Characterization of a µ-(1,3-Propanedithiolato)-hexacarbonyldiiron Laboratory Experiment or Mini-Project for Inorganic Chemistry or Integrated Laboratory'' Carmen F. Works 836 Journal of Chemical Education Vol. 84 No. 5 May 2007''
Abstract
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Safety

Fe₃(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.

References

{{Iron compounds

Carbonyl complexes
Iron complexes
Organoiron compounds
Chemical compounds containing metal–metal bonds
Iron(0) compounds