Editing Pi backbonding (section)

==Metal-phosphine complexes==
[[File:Connelly-Orpen-R3P-M-sigma-bonding.png|thumb|right|150px|R<sub>3</sub>P–M σ bonding]]
[[File:Connelly-Orpen-R3P-M-pi-backbonding.png|thumb|right|150px|R<sub>3</sub>P–M π backbonding]]
Phosphines accept electron density from metal p or d orbitals into combinations of P–C σ* antibonding orbitals that have π symmetry.<ref>{{cite journal | title = Structural systematics: the role of P–A σ* orbitals in metal–phosphorus π-bonding in redox-related pairs of M–PA<sub>3</sub> complexes (A = R, Ar, OR; R = alkyl) | first1 = A. G. | last1 = Orpen | first2 = N. G. | last2 = Connelly | journal = [[Organometallics]] | year = 1990 | volume = 9 | issue = 4 | pages = 1206–1210 | doi = 10.1021/om00118a048 }}</ref> When phosphines bond to electron-rich metal atoms, backbonding would be expected to lengthen P–C bonds as P–C σ* orbitals become populated by electrons. The expected lengthening of the P–C distance is often hidden by an opposing effect: as the phosphorus lone pair is donated to the metal, P(lone pair)–R(bonding pair) repulsions decrease, which acts to shorten the P–C bond. The two effects have been deconvoluted by comparing the structures of pairs of metal-phosphine complexes that differ only by one electron.<ref>{{cite book | title = The Organometallic Chemistry of the Transition Metals | last = Crabtree | first = Robert H. | author-link = Robert H. Crabtree | year = 2009 | edition = 5th | publisher = Wiley | isbn = 978-0-470-25762-3 | pages = 99–100 }}</ref> Oxidation of R<sub>3</sub>P–M complexes results in longer M–P bonds and shorter P–C bonds, consistent with π-backbonding.<ref>{{Cite journal | last1 = Dunne | first1 = B. J. | last2 = Morris | first2 = R. B. | last3 = Orpen | first3 = A. G. | title = Structural systematics. Part 3. Geometry deformations in triphenylphosphine fragments: A test of bonding theories in phosphine complexes | doi = 10.1039/dt9910000653 | journal = Journal of the Chemical Society, Dalton Transactions | pages = 653 | year = 1991 }}</ref>  In early work, phosphine ligands were thought to utilize 3d orbitals to form M–P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding as they are too high in energy.<ref>{{cite journal | title = No d Orbitals but Walsh Diagrams and Maybe Banana Bonds: Chemical Bonding in Phosphines, Phosphine Oxides, and Phosphonium Ylides | first1 = D. G. | last1 = Gilheany | journal =[[Chemical Reviews|Chem. Rev.]] | year = 1994 | volume = 94 | issue = 5 | pages = 1339–1374 | doi = 10.1021/cr00029a008 | pmid = 27704785 }}</ref><ref>{{cite journal | first1 = N. | last1 = Fey | first2 = A. G. | last2 = Orpen | first3 = J. N. | last3 = Harvey | title = Building ligand knowledge bases for organometallic chemistry: Computational description of phosphorus(III)-donor ligands and the metal–phosphorus bonds | journal = [[Coordination Chemistry Reviews|Coord. Chem. Rev.]] | volume = 253 | issue = 5–6 | pages = 704–722 | year = 2009 | doi = 10.1016/j.ccr.2008.04.017 }}</ref>