"Zeise's salt, potassium trichloro(ethene)platinate(II), 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 anion features a platinum atom with a square planargeometry. The salt is of historical importance in the area of organometallic chemistry as one of the first examples of an alkene complex."
"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."