Iron tetracarbonyl hydride

Iron tetracarbonyl hydride

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Names
Preferred IUPAC name Tetracarbonyldihydroiron
Other names Iron tetracarbonyl dihydride, tetracarbonyldihydroiron
Identifiers
CAS Number	12002-28-7 Y
ChemSpider	452380 N
Jmol 3D model	Interactive image
PubChem	518470
InChI InChI=1S/4CO.Fe.2H/c4*1-2;;; N Key: WZSHZFPIRKSPBD-UHFFFAOYSA-N N
SMILES [O+]#[C-].[O+]#[C-].[O+]#[C-].[O+]#[C-].[Fe]
Properties
Chemical formula	FeC 4H 2O 4
Molar mass	169.901 g mol−1
Appearance	Liquid (at -20 °C)
Melting point	−70 °C (−94 °F; 203 K)
Boiling point	−20 °C (−4 °F; 253 K) (decomposes)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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.

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⁻ + HCO₃⁻

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)₄²⁻, 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 C_2v 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.

Chemistry

Iron tetracarbonyl hydride undergoes rapid ligand substitutions. Upon warming, the complex liberates H₂, giving trans tetracarbonyliron.

H₂Fe(CO)₄ + PPh₃ → H₂Fe(CO)₃PPh₃

H₂Fe(CO)₃PPh3 → trans-Fe(CO)₃PPh₃ + H₂

The hydrides in tetracarbonyldihydroiron have a pK₁ of 6.8 and pK₂ of 15. The monoanion itself has been reviewed extensively. The monoanion is an important intermediate in the 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⁻

Another way the complex has been used is for cooperative bimetallic activation of CO₂.

Cp₂MoCO₂ + H₂Fe(CO)₄ → Cp₂MoHCO⁺ HFe₃(CO)₁₁⁻ + H₂O