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Zeise's salt is the chemical compound with the formula K[PtCl3(C2H4)].H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The complex is commonly prepared from K2[PtCl4] and ethylene in the presence of a catalytic amount of SnCl2. The anion features a platinum atom with a square planar geometry.
Zeise's salt was one of the first organometallic compounds to be reported. Its inventor W. C. Zeise, a professor at the University of Copenhagen, prepared this compound in 1820s 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.
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. 
Zeise's salt stimulated much scientific research in the field of organometallic chemistry, and would be key in defining new concepts in chemistry such as "Hapticity". The Dewar-Chatt-Duncanson model explains how the metal is coordinated to the double bond.
Many other ethylene complexes have been prepared. For example, ethylenebis(triphenylphosphine)platinum(0), [(C6H5)3P]2Pt(H2C=CH2), wherein the platinum is three-coordinate and zero-valent (Zeise's salt is a derivative of platinum(II)).
In Zeise's salt 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 Zeise'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-ethylene bond.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Zeise's_salt". A list of authors is available in Wikipedia.|