Zeise's salt is a platinum-ethylene complex with the formula K[PtCl₃(η²-C₂H₄)].H₂O. It's an air-stable, yellow, crystalline compound, which was one of the first organometallic compounds ever prepared, synthesised for the first time in 1827 when Zeise boiled PtCl₄ in ethanol. Nowadays, it's normally made from K₂[PtCl₄] and free ethylene with a little bit of SnCl₂ as a catalyst. There's also Zeise's dimer, which is essentially Zeise's salt that has lost KCl: [{(η²-C₂H₄)PtCl₂}₂].

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.

In both of these compounds the alkene is free to rotate about the metal-alkene bond at higher temperatures. However, at lower temperatures the alkene is static, and is perpendicular to the square plane about the platinum atom.

Category:Organometallic chemistry