Iron tetracarbonyl hydride is the organometallic compound with the formula H₂Fe(CO)₄. Also known as tetracarbonyldihydridoiron, or iron tetracarbonyldihydride, this compound was the first metal hydride

Transition metal hydride

Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions...

 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 as outlined below.

Fe(CO)₅ + 2OH^- → HFe(CO)₄^- + HCO₃^-

HFe(CO)₄^- + H⁺ → H₂Fe(CO)₄

Current procedures consist of treatment of iron pentacarbonyl

Iron pentacarbonyl

Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula 5. Under standard conditions Fe5 is a free-flowing, straw-colored liquid with a pungent odour. This compound is a common precursor to diverse iron compounds, including many that are useful in organic synthesis. Fe5 is...

 with potassium hydroxide

Potassium hydroxide

Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

 and barium hydroxide

Barium hydroxide

Barium hydroxide is the chemical compound with the formula Ba2. Also known as baryta, it is one of the principal compounds of barium. The white granular monohydrate is the usual commercial form.-Preparation:...

 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)₄^2-, which is light and temperature sensitive. The orange solution was then treated with sulfuric acid.

Structure and properties

In iron tetracarbonyl hydride the Fe(CO)₄ group has C_2v molecular symmetry

Molecular symmetry

Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

 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 tetrahedral.

Main reactions

Iron tetracarbonyl hydride undergoes rapid ligand substitutions. Liberation of H₂ upon warming occurs to give the trans product.

H₂Fe(CO)₄ + PPh₃ → H₂Fe(CO)₃PPh₃

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

Acidity

The hydrides in tetracarbonyldihydroiron have a pK₁ of 6.8 and pK₂ of 15. The monoanion is noteworthy and is 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^-

As described and outlined above, iron tetracarbonyl hydride undergoes rapid ligand substitutions due to its thermal instability, with the liberation of H₂. Another way the complex has been used is for cooperative bimetallic activation of CO₂.

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