Claim: The Dewar–Chatt–Duncanson model has an elongated C−C distance especially in Zeise's salt.
Evidence: The π-acid alkene donates electron density into a metal d-orbital from a π-symmetry bonding orbital between the carbon atoms. The metal donates electrons back from a (different) filled d-orbital into the empty π antibonding orbital. Both of these effects tend to reduce the carbon-carbon bond order, leading to an elongated C−C distance and a lowering of its vibrational frequency. In Zeise's salt KPtCl3(C2H4)]H2O the C−C bond length has increased to 134 picometres from 133 pm for ethylene.