Triiron dodecacarbonyl is the organoiron compound with the formula Fe<sub>3</sub>(CO)<sub>12</sub>. 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<sub>3</sub>(CO)<sub>12</sub> 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)<sub>5</sub>:
Traces of the compound are easily detected because of its characteristic deep green colour. UV-photolysis of Fe(CO)<sub>5</sub> produces Fe<sub>2</sub>(CO)<sub>9</sub>, not Fe<sub>3</sub>(CO)<sub>12</sub>.
The usual synthesis of Fe<sub>3</sub>(CO)<sub>12</sub> starts with the reaction of Fe(CO)<sub>5</sub> with base:
followed by protonation of the resulting hydrido cluster with acid:
The original synthesis by Walter Hieber et al. entailed the oxidation of H<sub>2</sub>Fe(CO)<sub>4</sub> with manganese dioxide. The cluster was originally formulated incorrectly as "Fe(CO)<sub>4</sub>".
Elucidation of the structure of Fe<sub>3</sub>(CO)<sub>12</sub> proved to be challenging because the CO ligands are disordered in the crystals. Early evidence for its distinctive C<sub>2v</sub> structure came from Mössbauer spectroscopic measurements that revealed two quadrupole doublets with similar isomer shifts but different (1.13 and quadrupolar coupling constants.
Fe<sub>3</sub>(CO)<sub>12</sub> 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<sub>2v</sub> point group symmetry. By contrast, Ru<sub>3</sub>(CO)<sub>12</sub> and Os<sub>3</sub>(CO)<sub>12</sub> adopt D<sub>3h</sub>-symmetric structures, wherein all 12 CO ligands are terminally bound to the metals. In solution Fe<sub>3</sub>(CO)<sub>12</sub> is fluxional, resulting in equivalencing all 12 CO groups on the <sup>13</sup>C NMR timescale.
The anion [HFe<sub>3</sub>(CO)<sub>11</sub>]<sup>âÂÂ</sup> is structurally related to Fe<sub>3</sub>(CO)<sub>12</sub>, with the hydride replacing one bridging CO ligand. The bonding in the Fe-H-Fe subunit is described using concepts developed for diborane.
Solutions of Fe<sub>3</sub>(CO)<sub>12</sub> reacts with triphenylphosphine to give (triphenylphosphine)iron tetracarbonyl (and some bis(triphenylphosphine)iron tricarbonyl).
Heating Fe<sub>3</sub>(CO)<sub>12</sub> gives a low yield of the carbido cluster Fe<sub>5</sub>(CO)<sub>15</sub>C. Such reactions proceed via disproportionation of CO to give CO<sub>2</sub> and carbon.
Fe<sub>3</sub>(CO)<sub>12</sub> forms "ferroles" upon reaction with heterocycles such as thiophenes.
Fe<sub>3</sub>(CO)<sub>12</sub> reacts with thiols and disulfides to give thiolate-bridged complexes, such as methylthioirontricarbonyl dimer:
Fe<sub>3</sub>(CO)<sub>12</sub> 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.