User:Firebirdrebirth/Pi backbonding

In chemistry, π backbonding is a π-bonding interaction between a filled (or half filled) orbital of a transition metal atom and a vacant orbital on an adjacent ion or molecule.^[1]^[2] In this type of interaction, electrons from the metal are used to bond to the ligand, which dissipates excess negative charge and stabilizes the metal. It is common in transition metals with low oxidation states that have ligands such as carbon monoxide, olefins, or phosphines. The ligands involved in π backbonding can be broken into three groups: carbonyls and nitrogen analogs, alkenes and alkynes, and phosphines. Compounds where π backbonding is prominent include Ni(CO)₄, Zeise's salt, and molybdenym and iron dinitrogen complexes.

Metal carbonyls, nitrosyls, and isocyanides[edit]

σ bonding from electrons in CO's HOMO to metal center d-orbital.

π backbonding from electrons in metal center d-orbital to CO's LUMO.
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Metal–alkene and metal–alkyne complexes[edit]

σ bonding from electrons in alkene's HOMO to metal center d-orbital.

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.^[3] ^[4] This electron transfer strengthens the metal–ligand bond and weakens the C–C bonds within the ligand.^[5] In the case of metal-alkenes and alkynes, the strengthening of the M–C₂R₄ and M–C₂R₂ bond is reflected in bending of the C–C–R angles which assume greater sp³ and sp² character, respectively.^[6] ^[4]Thus strong π backbonding causes a metal-alkene complex to assume the character of a metallacyclopropane.^[3] Alkenes and alkynes with electronegative substituents exhibit greater π backbonding.^[4] Thus, strong π backbonding ligands are tetrafluoroethylene, tetracyanoethylene, and hexafluoro-2-butyne.

References[edit]

^ Miessler, Gary L.; Tarr, Donald A. (1999). Inorganic chemistry (2nd ed.). Upper Saddle River, N.J: Prentice Hall. p. 338. ISBN 978-0-13-841891-5.
^ Cotton, Frank Albert; Wilkinson, Geoffrey; Murillo, Carlos A., eds. (1999). Advanced inorganic chemistry (6th ed.). New York: Wiley. ISBN 978-0-471-19957-1.
^ ^a ^b Elias, Anil J.; Gupta, B D (January 1, 2013). Basic Organometallic Chemistry: Concepts, Syntheses and Applications (2nd ed.). Universities Press. ISBN 978-8173718748.
^ ^a ^b ^c Hartwig, John Frederick (2010). Organotransition metal chemistry: from bonding to catalysis. Sausalito (Calif.): University science books. ISBN 978-1-891389-53-5.
^ Elschenbroich, Christoph; Elschenbroich, Christoph (2011). Organometallics (3., compl. rev. and extended ed.). Weinheim: WILEY-VCH. ISBN 978-3-527-29390-2.
^ Zhao, Haitao; Ariafard, Alireza; Lin, Zhenyang (2006-08-01). "In-depth insight into metal–alkene bonding interactions". Inorganica Chimica Acta. Protagonists in Chemistry: Professor D.M.P. Mingos. 359 (11): 3527–3534. doi:10.1016/j.ica.2005.12.013. ISSN 0020-1693.

[1] Miessler, Gary L.; Tarr, Donald A. (1999). Inorganic chemistry (2nd ed.). Upper Saddle River, N.J: Prentice Hall. p. 338. ISBN 978-0-13-841891-5.

[2] Cotton, Frank Albert; Wilkinson, Geoffrey; Murillo, Carlos A., eds. (1999). Advanced inorganic chemistry (6th ed.). New York: Wiley. ISBN 978-0-471-19957-1.

[:0-3] Elias, Anil J.; Gupta, B D (January 1, 2013). Basic Organometallic Chemistry: Concepts, Syntheses and Applications (2nd ed.). Universities Press. ISBN 978-8173718748.

[:1-4] Hartwig, John Frederick (2010). Organotransition metal chemistry: from bonding to catalysis. Sausalito (Calif.): University science books. ISBN 978-1-891389-53-5.

[5] Elschenbroich, Christoph; Elschenbroich, Christoph (2011). Organometallics (3., compl. rev. and extended ed.). Weinheim: WILEY-VCH. ISBN 978-3-527-29390-2.

[6] Zhao, Haitao; Ariafard, Alireza; Lin, Zhenyang (2006-08-01). "In-depth insight into metal–alkene bonding interactions". Inorganica Chimica Acta. Protagonists in Chemistry: Professor D.M.P. Mingos. 359 (11): 3527–3534. doi:10.1016/j.ica.2005.12.013. ISSN 0020-1693.

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