Zeise's salt

Zeise's salt is the chemical compound with the formula K [ PtCl3(C2H4)].H2O. This air-stable, yellow, coordination complex contains an η2-ethylene ligand. The complex, which is primarily of historical interest, is commonly prepared from K2[PtCl4] and ethylene in the presence of a catalytic amount of SnCl2. The anion features square planar platinum(II).

History
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 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.

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.

Related compounds
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)).
 * 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.

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 &pi;-bonding between most metals and the alkene is weaker than the &sigma;-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.