The
alkene donates
electron density into a π-acid 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.
The interaction also causes carbon atoms to "rehybridise" from
sp2 towards
sp3, which is indicated by the bending of the hydrogen atoms on the ethylene back away from the metal.[4]In silico calculations show that 75% of the binding energy is derived from the forward donation and 25% from backdonation.[5] This model is a specific manifestation of the more general
π backbonding model.
^"Olefin Co-ordination Compounds. Part III. Infra-Red Spectra and Structure: Attempted Preparation of Acetylene Complexes" J. Chatt and L. A. Duncanson, Journal of the Chemical Society, 1953, 2939
doi:
10.1039/JR9530002939
^Directing effects in inorganic substitution reactions. Part I. A hypothesis to explain the trans-effect J. Chatt, L. A. Duncanson, L. M. Venanzi, Journal of the Chemical Society, 1955, 4456-4460
doi:
10.1039/JR9550004456
^Miessler, Gary L.; Donald A. Tarr (2004). Inorganic Chemistry. Upper Saddle River, New Jersey: Pearson Education, Inc. Pearson Prentice Hall.
ISBN0-13-035471-6..
^Herrmann/Brauer: Synthetic Methods of Organometallic and Inorganic Chemistry Georg Thieme, Stuttgart, 1996