Editing Pi backbonding

{{Short description|Form of interaction between two atoms}}

In [[chemistry]], '''pi backbonding''' or '''π backbonding''' is a [[Pi bond|π-bonding]] interaction between a filled (or half filled) [[Atomic orbital|orbital]] of a transition metal atom and a vacant [[Atomic orbital|orbital]] on an adjacent ion or molecule.<ref>{{Cite book |last1=Miessler |first1=Gary L. |title=Inorganic chemistry |last2=Tarr |first2=Donald A. |date=1999 |publisher=Prentice Hall |isbn=978-0-13-841891-5 |edition=2nd |location=Upper Saddle River, N.J |pages=338}}</ref><ref>{{Cite book |title=Advanced inorganic chemistry |date=1999 |publisher=Wiley |isbn=978-0-471-19957-1 |editor-last=Cotton |editor-first=Frank Albert |edition=6th |location=New York |editor-last2=Wilkinson |editor-first2=Geoffrey |editor-last3=Murillo |editor-first3=Carlos A.}}</ref> In this type of interaction, electrons from the metal are used to bond to the [[ligand]], which dissipates excess negative [[Electric charge|charge]] and stabilizes the metal. It is common in [[transition metal]]s with low oxidation states that have ligands such as [[Metal carbonyl|carbon monoxide]], [[olefins]], or [[phosphine]]s. The [[ligand]]s involved in π backbonding can be broken into three groups: [[Metal carbonyl|carbonyls]] and nitrogen analogs, [[alkene]]s and [[alkyne]]s, and [[phosphines]]. Compounds where π backbonding is prominent include [[Nickel carbonyl|Ni(CO)<sub>4</sub>]], [[Zeise's salt]], and [[Abiological nitrogen fixation using homogeneous catalysts|molybdenum and iron dinitrogen complexes]].

==Metal carbonyls, nitrosyls, and isocyanides==
[[File:CO-M σ bonding.png|thumb|σ bonding from electrons in CO's HOMO to metal center d-orbital.]]
[[File:CO-M Pi Backbond.png|thumb|π backbonding from electrons in metal center d-orbital to CO's LUMO.]]
The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogs). This electron-transfer strengthens the metal–C bond and weakens the C–O bond. The strengthening of the M–CO bond is reflected in increases of the vibrational frequencies for the M–C bond (often outside of the range for the usual IR spectrophotometers). Furthermore, the M–CO bond length is shortened. The weakening of the C–O bond is indicated by a decrease in the wavenumber of the ''ν''<sub>CO</sub> band(s) from that for free CO (2143&nbsp;cm<sup>−1</sup>), for example to 2060&nbsp;cm<sup>−1</sup> in Ni(CO)<sub>4</sub> and 1981&nbsp;cm<sup>−1</sup> in Cr(CO)<sub>6</sub>, and 1790&nbsp;cm<sup>−1</sup> in the anion [Fe(CO)<sub>4</sub>]<sup>2−</sup>.<ref>{{cite book|last1=Housecroft |first1=C. E. |last2=Sharpe |first2=A. G. |title=Inorganic Chemistry |edition=2nd |publisher=Pearson Prentice-Hall |date=2005 |page=702 |isbn=978-0-130-39913-7}}</ref> For this reason, [[IR spectroscopy]] is an important diagnostic technique in [[metal carbonyl|metal–carbonyl chemistry]]. The article [[infrared spectroscopy of metal carbonyls]] discusses this in detail.

Many ligands other than CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than CO and ν<sub>NO</sub> is a diagnostic tool in [[Metal nitrosyl complex|metal–nitrosyl chemistry]]. [[Isocyanide]]s, RNC, are another class of ligands that are capable of π-backbonding. In contrast with CO, the σ-donor lone pair on the C atom of isocyanides is antibonding in nature and upon complexation the CN bond is strengthened and the ν<sub>CN</sub> increased. At the same time, π-backbonding lowers the ''ν''<sub>CN</sub>. Depending on the balance of σ-bonding versus π-backbonding, the ν<sub>CN</sub> can either be raised (for example, upon complexation with weak π-donor metals, such as Pt(II)) or lowered (for example, upon complexation with strong π-donor metals, such as Ni(0)).<ref>{{cite book|first=Robert H. |last=Crabtree |title=The Organometallic Chemistry of the Transition Metals |edition=6th |publisher=Wiley |date=2014 |pages=105–106 |isbn=978-1-11813807-6}}</ref> For the isocyanides, an additional parameter is the MC=N–C angle, which deviates from 180° in highly electron-rich systems. Other ligands have weak π-backbonding abilities, which creates a labilization effect of CO, which is described by the [[cis effect|''cis'' effect]].

==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]].

==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>

==IUPAC definition of Back Donation==
The full [[IUPAC]] definition of back donation is as follows:
<blockquote>A description of the bonding of π-conjugated ligands to a transition metal which involves a [[synergy|synergic]] process with donation of electrons from the filled π-orbital or lone electron pair orbital of the ligand into an empty orbital of the metal (donor–acceptor bond), together with release (back donation) of electrons from an ''n''d orbital of the metal (which is of π-symmetry with respect to the metal–ligand axis) into the empty π*-[[antibonding]] orbital of the ligand.<ref>{{cite book
|title=IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book")
|last1 = McNaught
|first1= A. D.
|first2=A. |last2=Wilkinson
|year= 2006
|publisher=Blackwell Scientific Publications
|location =Oxford
|isbn= 978-0-9678550-9-7
|doi= 10.1351/goldbook }}</ref></blockquote>

== See also ==
* [[Bridging carbonyl]]
* [[Dewar–Chatt–Duncanson model]]
* [[18-electron rule]]
* [[Ligand field theory]]
* [[Metal–ligand multiple bond#Pi-donor ligands|Pi-donor ligands]]

== References ==
{{reflist}}
{{chemical bonds}}
{{organometallics}}

{{DEFAULTSORT:Pi backbonding}}
[[Category:Chemical bonding]]
[[Category:Coordination chemistry]]
[[Category:Organometallic chemistry]]