In order to be able to describe the ethylene molecule bonded to an active site of a Titanium- or Nickel fluoride crystal, we have used the Hartree-Fock-Slater model, implemented by a Discrete Variational Method, as introduced by Ellis and Painter. The study of the ethylene molecule at a crystal surface then gives a clear, easily interpreted picture of the electronic structure. The π-back donation from metal to olefin is found to be extremely important, both in the Ti- and in the Ni-complex. This back donation is caused by a strong interaction of a d orbital of the central ion with a σ* as well as the π* molecular orbital of ethylene. As a result of these interactions, the C-C bond of ethylene is weakened considerably. A comparison is made between the Ti-ethylene and the Ni-ethylene systems.
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