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