Orbital interactions in a metal-ethylene complex. On the left, a filled pi-orbital on C2H4 overlaps with an empty d-orbital on the metal. On the right, an empty pi-antibonding orbital on C2H4 overlaps with a filled d-orbital on the metal
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.
Main group elements can also form π-complexes with alkenes and alkynes. The β-diketiminato aluminum(I) complex Al{HC(CMeNAr)2} (Ar = 2,6-diisopropylphenyl), which bears an Al-based spx lone pair, reacts with alkenes and alkynes to give alumina(III)cyclopropanes and alumina(III)cyclopropenes in a process analogous to the formation of π-complexes by transition metals.[6][7] However, in most cases, the backbonding interaction is absent in these complexes due to the lack of energetically accessible filled orbitals for backdonation, resulting in π-complexes that dissociate readily and are therefore more challenging to observe or isolate.[8][9]
^"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
