View source for Chemistry:Transition metal complexes of aldehydes and ketones

[[File:OsP2(CO)2(CH2O).svg|thumb|140px|Structure of an η<sup>2</sup>-formaldehyde complex.]]
'''Transition metal complexes of aldehydes and ketones''' describes [[Chemistry:Coordination complex|coordination complex]]es with [[Chemistry:Aldehyde|aldehyde]] (RCHO) and [[Chemistry:Ketone|ketone]] {{chem2|(R2CO)}} [[Chemistry:Ligand|ligand]]s.  Because aldehydes and ketones are common, the area is of fundamental interest.  Some reactions that are useful in [[Chemistry:Organic chemistry|organic chemistry]] involve such complexes.

==Structure and bonding==
[[File:ZnBr3(acetone) anion.svg|thumb|140px|Structure of {{chem2|[ZnBr3(\h{1}acetone)](-)}}.]]
In monometallic complexes, aldehydes and ketones can bind to metals in either of two modes, η<sup>1</sup>-O-bonded and η<sup>2</sup>-C,O-bonded.  These bonding modes are sometimes referred to sigma- and pi-bonded. These forms may sometimes interconvert.

The sigma bonding mode is more common for higher valence, Lewis-acidic metal centers (e.g., Zn<sup>2+</sup>).<ref>{{cite journal |doi=10.1134/S1070363217070210|title=Coordination of Zinc Tetraphenylporphyrin with Pyridine Derivatives in Chloroform Solution and in the Solid Phase|year=2017|last1=Andreev|first1=V. P.|last2=Sobolev|first2=P. S.|last3=Tafeenko|first3=V. A.|journal=Russian Journal of General Chemistry|volume=87|issue=7|pages=1572–1579|s2cid=102539549}}</ref> The pi-bonded mode is observed for low valence, electron-rich metal centers (e.g., Fe(0) and Os(0)).<ref>{{cite journal |doi=10.1016/S0022-328X(00)90020-2|title=Struktur und Reaktivität eines Formaldehydeisen-Komplexes|year=1981|last1=Berke|first1=Heinz|last2 =Huttner|first2=Gottfried|last3=Weiler|first3=Gertrud|last4= Zsolnai|first4=Laszlo|journal=Journal of Organometallic Chemistry|volume=219|issue=3|pages=353–362}}</ref>

For the purpose of electron-counting, O-bonded ligands count as [[Chemistry:Covalent bond classification method|2-electron "L ligands"]]: they are Lewis bases.  η<sup>2</sup>-C,O ligands are described as analogues of alkene ligands, i.e. the Dewar-Chatt-Duncanson model.<ref>{{cite journal |doi=10.1021/ed065p298|title=Aldehyde and Ketone Ligands in organometallic complexes and catalysis|year=1988|last1=Huang|first1=Yo Hsin|last2=Gladysz|first2=J. A.|journal=Journal of Chemical Education|volume=65|issue=4|page=298|bibcode=1988JChEd..65..298H|doi-access=free}}</ref>

η<sup>2</sup>-C,O ketones and aldehydes can function as bridging ligands, utilizing a lone pair of electrons on oxygen. One such complex is {{chem2|[(C5H5)2Zr(CH2O)]3}}, which features a {{chem2|Zr3O3}} ring.<ref>{{cite journal |doi=10.1021/ja00348a075|title=Fischer-Tropsch intermediates: Tris&#91;(.eta.2-formaldehyde)zirconocene&#93; from the carbonylation of a zirconium hydride|year=1983|last1=Kropp|first1=Kurt|last2=Skibbe|first2=Volker|last3=Erker|first3=Gerhard|last4=Krueger|first4=Carl|journal=Journal of the American Chemical Society|volume=105|issue=10|pages=3353–3354}}</ref>

==Related ligands==
Related to η<sup>1</sup>-O-bonded complexes of aldehydes and ketones are [[Chemistry:Metal acetylacetonates|metal acetylacetonates]] and related species, which can be viewed as a combination of ketone and [[Chemistry:Enolate|enolate]] ligands.

==Reactions==
[[File:(benzylideneacetone)iron-tricarbonyl-2D-skeletal.png |thumb|left|150px|[[Chemistry:(Benzylideneacetone)iron tricarbonyl|(Benzylideneacetone)iron tricarbonyl]] is an organoiron compound with an η{{sup|2}} ketone ligand.]]

Some η<sup>2</sup>-aldehyde complexes insert alkenes to give five-membered [[Chemistry:Metallacycle|metallacycle]]s.<ref>{{cite journal |doi=10.1021/acs.accounts.5b00061|title=Catalytic Transformation of Aldehydes with Nickel Complexes through η<sup>2</sup>-Coordination and Oxidative Cyclization|year=2015|last1=Hoshimoto|first1=Yoichi|last2=Ohashi|first2=Masato|last3=Ogoshi|first3=Sensuke|journal=Accounts of Chemical Research|volume=48|issue=6|pages=1746–1755|pmid=25955708}}</ref>

η<sup>1</sup>-Complexes of [[Chemistry:Alpha-beta Unsaturated carbonyl compounds|alpha-beta unsaturated carbonyls]] exhibit enhanced reactivity toward dienes.  This interaction is the basis of [[Chemistry:Lewis acid catalysis|Lewis-acid catalyzed]] Diels-Alder reactions.

==References==
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{{Coordination complexes}}

[[Category:Organometallic chemistry]]
[[Category:Transition metals]]
[[Category:Coordination chemistry]]

{{Sourceattribution|Transition metal complexes of aldehydes and ketones}}