Formula FeC4H2O4 Boiling point -20 °C | Molar mass 169.9 g/mol Appearance Liquid (at -20 °C) | |
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Iron tetracarbonyl hydride is the organometallic compound with the formula H2Fe(CO)4. Also known as tetracarbonyldihydridoiron, tetracarbonyldihydroiron, or iron tetracarbonyl dihydride, this compound was the first metal hydride discovered. The complex is only stable at low temperatures and decomposes rapidly at temperatures above –20 °C.
Contents
Preparation
Iron tetracarbonyl hydride was originally produced by Hieber and Leutert, who developed a two-step process starting from iron pentacarbonyl:
Fe(CO)5 + 2OH− → HFe(CO)−4 + HCO−
3 HFe(CO)−
4 + H+ → H2Fe(CO)4
Current procedures consist of treatment of iron pentacarbonyl with potassium hydroxide and barium hydroxide to yield an orange solution. From this point in the reaction, ideal conditions consist of a cold dark environment, thus dubbing the method the "polar night synthesis". This dark, cold environment stabilizes the dianion species Fe(CO)2−
4, which is light and temperature sensitive. The orange solution is then treated with sulfuric acid to protonate the anionic intermediate, giving the neutral product.
Structure and properties
In iron tetracarbonyl hydride the Fe(CO)4 group has C2v molecular symmetry with a geometry intermediate between octahedral and tetrahedral. Viewed as an octahedral complex, the hydride ligands are cis. Viewed as a tetrahedral Fe(CO)4 complex, the hydrides occupy adjacent faces of the tetrahedron. Although the structure of tetracarbonyliron with the hydrogen atoms bound as a single H2 ligand has been proposed as an intermediate in some rearrangement reactions, the stable state for the compound has the two atoms as independent ligands.
Reactions
H2Fe(CO)4 undergoes rapid ligand substitutions. Upon warming, the complex liberates H2, giving tetracarbonyliron derivatives.
H2Fe(CO)4 + PPh3 → H2Fe(CO)3PPh3 H2Fe(CO)3PPh3 → trans-Fe(CO)3PPh3 + H2Another way the complex has been used is for cooperative bimetallic activation of CO2.
Cp2MoCO2 + H2Fe(CO)4 → Cp2MoHCO+ HFe3(CO)−11 + H2O
[HFe(CO)4]−
H2Fe(CO)4 has pK1 of 6.8 and pK2 of 15. The monoanion [HFe(CO)4]− has more extensive reaction chemistry because it is more stable than the dihydride. The monoanion is an intermediate in the homogeneous water-gas shift reaction (WGSR). The slow step in the iron carbonyl-catalyzed WGSR is the proton transfer from water to the iron hydride anion.
HFe(CO)−4 + H2O → H2Fe(CO)4 + OH−