Zeise's salt

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The anion of Zeise's salt
The anion of Zeise's salt
[PtCl3(η2-C2H4)]−
[PtCl3(η2-C2H4)]−

Zeise's salt is an air-stable, yellow, crystalline compound of platinum and ethylene, with the chemical formula K[PtCl3(η2-C2H4)].H2O. It is commonly made from K2[PtCl4] and ethylene with some SnCl2 as a catalyst.

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[edit] History

Zeise's salt was one of the first organometallic compounds to be reported.[1] Its inventor W. C. Zeise, a professor at the University of Copenhagen, prepared this compound in 1820's while investiging the reaction of PtCl4 with boiling ethanol. He proposed that the resulting compound contained ethylene. Justus von Liebig, an influential chemist of that era, often criticised Zeise's proposal, but Zeise's theories were decisively supported in 1868 when Birnbaum prepared the complex using ethylene.[2][3]

Zeise's salt received a great deal of attention during the second half of the 19th century because chemists could not properly explain the molecular structure of the salt. This question remained unanswered until the advent of x-ray diffraction in the 20th century. [4]

Zeise's salt stimulated much scientific research in the organometallic field, and would be key in defining new concepts in chemistry such as "Hapticity".[1]

[edit] Dewar-Chatt-Duncanson model

The metal-alkene bonding is a classic example of the Dewar-Chatt-Duncanson model of synergic bonding between a ligand and a metal. The alkene donates electron density into a metal d-orbital from carbon-carbon π-symmetry bonding orbital, and the metal donates it back from (a different) d-orbital into the carbon-carbon π* orbital. Both of these effects tend to reduce the carbon-carbon bond order from 2 to 1, making this bond longer and lowering its vibrational frequency. They also move the hybridisation at the carbon atoms from sp2 towards sp3, which has the effect of making the protons on the ethylene bend back away from the metal.[5]

[edit] Related compounds

  • Zeise's dimer [{(η2-C2H4)PtCl2}2], derived from Zeise's salt by elimination of KCl followed by dimerisation.
  • "COD-platinum dichloride," (cyclooctadiene)PtCl2, derived from platinum(II) chloride and 1,5-cyclooctadiene, is a commonly platinum(II) alkene complex.

Many other ethylene complexes are of course been prepared. For example, ethylenebis(triphenylphosphine)platinum(0), [(C6H5)3P]2Pt(H2C=CH2), wherein the platinum is three-coordinated and zero-valent (Zeise's salt is a derivative of platinum(II)).

[edit] Structure

In Zeise's anion and related compounds, the alkene rotates about the metal-alkene bond with a modest activation energy. Analysis of the barrier heights indicates that the π-bonding between most metals and the alkene is weaker than the σ-bonding. In Zeize's anion, this rotational barrier cannot be assessed by NMR spectroscopy because all four protons are equivalent. Lower symmetry complexes of ethylene, e.g. CpRh(C2H4)2, are, however, suitable for analysis of the rotational barriers associated with the metal-etheylene bond.[6]

[edit] External links

[edit] References

  1. ^ Zeise, W. C. “Von der Wirkung zwischen PIatinchlorid und Alkohol, und von den dabei entstehenden neuen Substanzen ; “Annalen der Physik und Chemie 1831, Volume 97, pages 497-541.
  2. ^ R. A. Love, T. F. Koetzle, G. J. B. Williams, L. C. Andrews, R. Bau (1975). "Neutron diffraction study of the structure of Zeise's salt, KPtCl3•C2H4•H2O". Inorganic Chemistry 14 (11): 2653 - 2657. DOI:10.1021/ic50153a012. 
  3. ^ L. B. Hunt (1984). "The First Organometallic Compounds: WILLIAM CHRISTOPHER ZEISE AND HIS PLATINUM COMPLEXES". Platinum Metals Review 28 (2): 76-83. 
  4. ^ M. Black, R. H. B. Mais and P. G. Owston (1969). "The crystal and molecular structure of Zeise's salt, KPtCl3•C2H4•H2O". Acta Crystallographica Section B B25: 1753-1759. DOI:10.1107/S0567740869004699. 
  5. ^ ^ Miessler, Gary L.; Donald A. Tarr (2004). Inorganic Chemistry. Upper Saddle River, New Jersey: Pearson Education, Inc. Pearson Prentice Hall. ISBN 0-13-035471-6. .
  6. ^ Elschenbroich, C. ”Organometallics : A Concise Introduction” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6
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