Pi backbonding

Inchemistry,π backbondingis aπ-bondinginteraction between a filled (or half filled)orbitalof a transition metal atom and a vacantorbitalon an adjacent ion or molecule.^[1]^[2]In this type of interaction, electrons from the metal are used to bond to theligand,which dissipates excess negativechargeand stabilizes the metal. It is common intransition metalswith low oxidation states that have ligands such ascarbon monoxide,olefins,orphosphines.Theligandsinvolved in π backbonding can be broken into three groups:carbonylsand nitrogen analogs,alkenesandalkynes,andphosphines.Compounds where π backbonding is prominent includeNi(CO)₄,Zeise's salt,andmolybdenum and iron dinitrogen complexes.

Metal carbonyls, nitrosyls, and isocyanides

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 theν_COband(s) from that for free CO (2143 cm⁻¹), for example to 2060 cm⁻¹in Ni(CO)₄and 1981 cm⁻¹in Cr(CO)₆,and 1790 cm⁻¹in the anion [Fe(CO)₄]²⁻.^[3]For this reason,IR spectroscopyis an important diagnostic technique inmetal–carbonyl chemistry.The articleinfrared spectroscopy of metal carbonylsdiscusses this in detail.

Many ligands other than CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than CO and ν_NOis a diagnostic tool inmetal–nitrosyl chemistry.Isocyanides,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_CNincreased. At the same time, π-backbonding lowers theν_CN.Depending on the balance of σ-bonding versus π-backbonding, the ν_CNcan 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)).^[4]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 theciseffect.

Metal–alkene and metal–alkyne complexes

As in metal–carbonyls, electrons are partially transferred from a d-orbital of the metal to antibonding molecular orbitals of the alkenes and alkynes.^[5]^[6]This electron transfer strengthens the metal–ligand bond and weakens the C–C bonds within the ligand.^[7]In the case of metal-alkenes and alkynes, the strengthening of the M–C₂R₄and M–C₂R₂bond is reflected in bending of the C–C–R angles which assume greater sp³and sp²character, respectively.^[8]^[9]Thus strong π backbonding causes a metal-alkenecomplexto assume the character of a metallacyclopropane.^[10]Alkenes and alkynes with electronegative substituents exhibit greater π backbonding.^[9]Some strong π backbonding ligands aretetrafluoroethylene,tetracyanoethylene,andhexafluoro-2-butyne.

Metal-phosphine complexes

Phosphines accept electron density from metal p or d orbitals into combinations of P–C σ* antibonding orbitals that have π symmetry.^[11]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.^[12]Oxidation of R₃P–M complexes results in longer M–P bonds and shorter P–C bonds, consistent with π-backbonding.^[13]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.^[14]^[15]

IUPAC definition of Back Donation

The fullIUPACdefinition of back donation is as follows:

A description of the bonding of π-conjugated ligands to a transition metal which involves asynergicprocess 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 annd orbital of the metal (which is of π-symmetry with respect to the metal–ligand axis) into the empty π*-antibondingorbital of the ligand.^[16]

References

^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.
^Housecroft, C. E.; Sharpe, A. G. (2005).Inorganic Chemistry(2nd ed.). Pearson Prentice-Hall. p. 702.ISBN 978-0-130-39913-7.
^Crabtree, Robert H. (2014).The Organometallic Chemistry of the Transition Metals(6th ed.). Wiley. pp. 105–106.ISBN 978-1-11813807-6.
^Elias, Anil J.; Gupta, B D (January 1, 2013).Basic Organometallic Chemistry: Concepts, Syntheses and Applications(2nd ed.). Universities Press.ISBN 978-8173718748.
^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.
^^a ^bHartwig, John Frederick (2010).Organotransition metal chemistry: from bonding to catalysis.Sausalito (Calif.): University science books.ISBN 978-1-891389-53-5.
^Elias, Anil J.; Gupta, B D (January 1, 2013).Basic Organometallic Chemistry: Concepts, Syntheses and Applications(2nd ed.). Universities Press.ISBN 978-8173718748.
^Orpen, A. G.; Connelly, N. G. (1990). "Structural systematics: the role of P–A σ* orbitals in metal–phosphorus π-bonding in redox-related pairs of M–PA₃complexes (A = R, Ar, OR; R = alkyl) ".Organometallics.9(4): 1206–1210.doi:10.1021/om00118a048.
^Crabtree, Robert H.(2009).The Organometallic Chemistry of the Transition Metals(5th ed.). Wiley. pp. 99–100.ISBN 978-0-470-25762-3.
^Dunne, B. J.; Morris, R. B.; Orpen, A. G. (1991). "Structural systematics. Part 3. Geometry deformations in triphenylphosphine fragments: A test of bonding theories in phosphine complexes".Journal of the Chemical Society, Dalton Transactions:653.doi:10.1039/dt9910000653.
^Gilheany, D. G. (1994). "No d Orbitals but Walsh Diagrams and Maybe Banana Bonds: Chemical Bonding in Phosphines, Phosphine Oxides, and Phosphonium Ylides".Chem. Rev.94(5): 1339–1374.doi:10.1021/cr00029a008.PMID 27704785.
^Fey, N.; Orpen, A. G.; Harvey, J. N. (2009). "Building ligand knowledge bases for organometallic chemistry: Computational description of phosphorus(III)-donor ligands and the metal–phosphorus bonds".Coord. Chem. Rev.253(5–6): 704–722.doi:10.1016/j.ccr.2008.04.017.
^McNaught, A. D.; Wilkinson, A. (2006).IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book" ).Oxford: Blackwell Scientific Publications.doi:10.1351/goldbook.ISBN 978-0-9678550-9-7.

[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.

[3] Housecroft, C. E.; Sharpe, A. G. (2005).Inorganic Chemistry(2nd ed.). Pearson Prentice-Hall. p. 702.ISBN 978-0-130-39913-7.

[4] Crabtree, Robert H. (2014).The Organometallic Chemistry of the Transition Metals(6th ed.). Wiley. pp. 105–106.ISBN 978-1-11813807-6.

[:0-5] Elias, Anil J.; Gupta, B D (January 1, 2013).Basic Organometallic Chemistry: Concepts, Syntheses and Applications(2nd ed.). Universities Press.ISBN 978-8173718748.

[:1-6] Hartwig, John Frederick (2010).Organotransition metal chemistry: from bonding to catalysis.Sausalito (Calif.): University science books.ISBN 978-1-891389-53-5.

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

[8] 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.

[:12-9] Hartwig, John Frederick (2010).Organotransition metal chemistry: from bonding to catalysis.Sausalito (Calif.): University science books.ISBN 978-1-891389-53-5.

[:02-10] Elias, Anil J.; Gupta, B D (January 1, 2013).Basic Organometallic Chemistry: Concepts, Syntheses and Applications(2nd ed.). Universities Press.ISBN 978-8173718748.

[11] Orpen, A. G.; Connelly, N. G. (1990). "Structural systematics: the role of P–A σ* orbitals in metal–phosphorus π-bonding in redox-related pairs of M–PA₃complexes (A = R, Ar, OR; R = alkyl) ".Organometallics.9(4): 1206–1210.doi:10.1021/om00118a048.

[12] Crabtree, Robert H.(2009).The Organometallic Chemistry of the Transition Metals(5th ed.). Wiley. pp. 99–100.ISBN 978-0-470-25762-3.

[13] Dunne, B. J.; Morris, R. B.; Orpen, A. G. (1991). "Structural systematics. Part 3. Geometry deformations in triphenylphosphine fragments: A test of bonding theories in phosphine complexes".Journal of the Chemical Society, Dalton Transactions:653.doi:10.1039/dt9910000653.

[14] Gilheany, D. G. (1994). "No d Orbitals but Walsh Diagrams and Maybe Banana Bonds: Chemical Bonding in Phosphines, Phosphine Oxides, and Phosphonium Ylides".Chem. Rev.94(5): 1339–1374.doi:10.1021/cr00029a008.PMID 27704785.

[15] Fey, N.; Orpen, A. G.; Harvey, J. N. (2009). "Building ligand knowledge bases for organometallic chemistry: Computational description of phosphorus(III)-donor ligands and the metal–phosphorus bonds".Coord. Chem. Rev.253(5–6): 704–722.doi:10.1016/j.ccr.2008.04.017.

[16] McNaught, A. D.; Wilkinson, A. (2006).IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book" ).Oxford: Blackwell Scientific Publications.doi:10.1351/goldbook.ISBN 978-0-9678550-9-7.

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v t e Organometallic chemistry
Principles	Crystal field theory Ligand field theory 18-electron rule d electron count Polyhedral skeletal electron pair theory Isolobal principle π backbonding Dewar–Chatt–Duncanson model Hapticity spin states Agostic interaction Metal–ligand multiple bond
Reactions	Oxidative addition / reductive elimination Migratory insertion β-hydride elimination Transmetalation Carbometalation
Types of compounds	Gilman reagents Grignard reagents Cyclopentadienyl complexes Transition metal indenyl complexes Transition metal fullerene complexes Metallocenes Metal tetranorbornyls Sandwich compounds Half sandwich compounds Transition metal acyl complexes Transition metal carbene complexes Transition metal carbyne complexes Transition metal alkene complexes Transition metal alkyne complexes Transition-metal allyl complexes Transition metal carbides Arene complexes of univalent gallium, indium, and thallium
Applications	Carbonylation Cativa process Grignard reaction Monsanto process Olefin metathesis Shell higher olefin process Ziegler–Natta process
Related branches ofchemistry	Organic chemistry Inorganic chemistry Bioinorganic chemistry

Metal carbonyls, nitrosyls, and isocyanides

Metal–alkene and metal–alkyne complexes

Metal-phosphine complexes

IUPAC definition of Back Donation

See also

References