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Concept
Triiron dodecacarbonyl
Summary
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.
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)3NH][HFe3(CO)11] + 3 CO + CO2
followed by oxidation of the resulting hydrido cluster with acid:
[(C2H5)3NH][HFe3(CO)11] + HCl + CO → Fe3(CO)12 + H2 + [(C2H5)3NH]Cl
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".
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−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 C2v point group symmetry. By contrast, Ru3(CO)12 and Os3(CO)12 adopt D3h-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.
Official source
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