Iron tetracarbonyl hydride

Iron tetracarbonyl hydride
Preferred IUPAC name Tetracarbonyldihydroiron
Other names Iron tetracarbonyldihydride
Identifiers
CAS number	12002-28-7^Y
PubChem	518470
ChemSpider	452380^N
Jmol-3D images	Image 1 Image 2
SMILES [H][#26&a;v8]([H])(:c=O)(:c=O)(:c=O):c=O O=C=[FeH2](=C=O)(=C=O)=C=O
InChI InChI=1S/4CO.Fe.2H/c4*1-2;;;^N Key: WZSHZFPIRKSPBD-UHFFFAOYSA-N^N
Properties
Molecular formula	FeC₄H₂O₄
Molar mass	169.901 g mol^-1
Exact mass	169.930250685 g mol^-1
Appearance	Liquid (at -20 °C)
Melting point	-70 °C, 203 K, -94 °F
Boiling point	-20 °C, 253 K, -4 °F (decomposes)
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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 discovered. The complex is only stable at low temperatures and decomposes rapidly at temperatures above -20 °C.

1 Preparation
2 Structure and properties
3 Main reactions
4 Acidity
5 References

Preparation

Iron tetracarbonyl hydride was originally produced by Hieber and Leutert as outlined below.^[1]^[2]

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”.^[3] 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 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.^[4]

Main reactions

Iron tetracarbonyl hydride undergoes rapid ligand substitutions. Liberation of H₂ upon warming occurs to give the trans product.^[5]

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.^[6] The monoanion is noteworthy and is reviewed extensively.^[7] 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.^[8]

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.^[5] Another way the complex has been used is for cooperative bimetallic activation of CO₂.^[9]

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

References

^ W. Hieber, F. Leutert (1931). "Zur Kenntnis des koordinative gebundene Kohlenoxyds: Bildung von Eisencarbonylwasserstoff". Naturwissenschaften 19 (17): 360. doi:10.1007/BF01522286.
^ Rittmeyer, P.; Wietelmann, U. (2005), "Hydrides", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a13_199
^ Vancea L. and Graham W.A.G. (1977). "Stereochemically Nonrigid Six-Coordinate Metal Carbonyl Complexes". J. Organomet. Chem. 134 (2): 219. doi:10.1016/S0022-328X(00)81421-7.
^ McNeill, E. A.; Scholer, F. R. (1977). "Molecular structure of the gaseous metal carbonyl hydrides of manganese, iron, and cobalt". J. Am. Chem. Soc. 99 (19): 6243. doi:10.1021/ja00461a011.
^ ^a ^b Peason R. G., H.W. Walker, H. Mauermann, P.C. Ford (1981). "Hydrogen migration mechanism for ligand substitution reactions in metal carbonyl hydrides". Inorg. Chem. 20 (8): 2741. doi:10.1021/ic50222a078.
^ Shapely J.R., G. F. Stuntz (1979). "Rates of Deprotonation and pKa Values of Transition Metal Carbonyl Hydrides". J. Am. Chem. Soc. 101 (24): 7428. doi:10.1021/ja00518a061.
^ J. Brunet (1990). "Tetracarbonylhydridoferrates, MHFe(CO)₄: Versatile tools in Organic Synthesis and Catalysis". Chem. Rev. 90 (6): 1041. doi:10.1021/cr00104a006.
^ Crabtree R.H.; Mingos D.M.P. 2007. Comprehensive Organometallic Chemistry III From Fundamentals to Applications. Elsevier Ltd.
^ Tsai J., M.A. Khan, K.M. Nicholas (1991). "Reduction of Coordinated Carbon Dioxide by Transition-Metal Hydrides". Organometallics 10: 29. doi:10.1021/om00047a016.