Zeise's salt

    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.

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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 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.^[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 field of organometallic chemistry, and would be key in defining new concepts in chemistry such as "Hapticity".^[1] The Dewar-Chatt-Duncanson model explains how the metal is coordinated to the double bond.

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 common platinum(II) alkene complex.

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)).

Structure

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.^[5]

References