Editing Pi backbonding (section)

==Metal–alkene and metal–alkyne complexes==
[[File:Alkene-M Sigma Bond.png|thumb|σ bonding from electrons in alkene's HOMO to metal center d-orbital.]]
[[File:Alkene-M Pi Backbond.png|thumb|π backbonding from electrons in metal center d-orbital to alkene's LUMO.]]
As in metal–carbonyls, electrons are partially transferred from a d-orbital of the metal to antibonding molecular orbitals of the alkenes and alkynes.<ref name=":02">{{Cite book |last1=Elias |first1=Anil J. |title=Basic Organometallic Chemistry: Concepts, Syntheses and Applications |last2=Gupta |first2=B D |date=January 1, 2013 |publisher=Universities Press |isbn=978-8173718748 |edition=2nd}}</ref><ref name=":12">{{Cite book |last=Hartwig |first=John Frederick |title=Organotransition metal chemistry: from bonding to catalysis |date=2010 |publisher=University science books |isbn=978-1-891389-53-5 |location=Sausalito (Calif.)}}</ref> This electron transfer strengthens the metal–ligand bond and weakens the C–C bonds within the ligand.<ref>{{Cite book |last1=Elschenbroich |first1=Christoph |title=Organometallics |last2=Elschenbroich |first2=Christoph |date=2011 |publisher=WILEY-VCH |isbn=978-3-527-29390-2 |edition=3., compl. rev. and extended |location=Weinheim}}</ref> In the case of metal-alkenes and alkynes, the strengthening of the M–C<sub>2</sub>R<sub>4</sub> and M–C<sub>2</sub>R<sub>2</sub> bond is reflected in bending of the C–C–R angles which assume greater sp<sup>3</sup> and sp<sup>2</sup> character, respectively.<ref>{{Cite journal |last1=Zhao |first1=Haitao |last2=Ariafard |first2=Alireza |last3=Lin |first3=Zhenyang |date=2006-08-01 |title=In-depth insight into metal–alkene bonding interactions |url=https://www.sciencedirect.com/science/article/pii/S0020169305006961 |journal=Inorganica Chimica Acta |series=Protagonists in Chemistry: Professor D.M.P. Mingos |volume=359 |issue=11 |pages=3527–3534 |doi=10.1016/j.ica.2005.12.013 |issn=0020-1693|url-access=subscription }}</ref><ref name=":12"/> Thus strong π backbonding causes a metal-alkene [[Coordination complex|complex]] to assume the character of a metallacyclopropane.<ref name=":02"/> Alkenes and alkynes with electronegative substituents exhibit greater π backbonding.<ref name=":12" /> Some strong π backbonding ligands are [[tetrafluoroethylene]], [[tetracyanoethylene]], and [[hexafluoro-2-butyne]].