View source for Group 2 organometallic chemistry

[[image:GOHRUQ.png|thumb|right|Magnesium anthracenide with three thf ligands.<ref>{{cite journal|author=Borislav Bogdanovic|title=Magnesium Anthracene Systems and their Application in Synthesis and Catalysis|journal=Accounts of Chemical Research |volume=21|year=1988|issue=7 |pages=261–267|doi=10.1021/ar00151a002}}</ref>]] '''Group 2 organometallic chemistry''' refers to the [[organic compound|organic derivatives]]s of any [[group 2 element]].  It is a subtheme to [[main group organometallic chemistry]].<ref>''Comprehensive Organometallic Chemistry'' by Mike Mingos, Robert Crabtree '''2007''' {{ISBN|978-0-08-044590-8}}</ref><ref>C. Elschenbroich, A. Salzer ''Organometallics : A Concise Introduction'' (2nd Ed) ('''1992''') from Wiley-VCH: Weinheim. {{ISBN|3-527-28165-7}}</ref> By far the most common group 2 organometallic compounds are the magnesium-containing [[Grignard reagent]]s which are widely used in [[organic chemistry]]. Other organometallic group 2 compounds are typically limited to academic interests.

==Characteristics==
As the [[group 2 elements]] (also referred to as the alkaline earth metals) contain two [[valence electron]]s, their chemistries have similarities [[group 12 element|group 12]] organometallic compounds. Both readily assume a +2 [[oxidation state]]s with higher and lower states being rare, and are less electronegative than carbon. However, as the group two elements (with the exception of beryllium) have considerably low [[electronegativity]] the resulting C-M bonds are more highly polarized and [[ionic bond|ionic]]-like, if not entirely ionic for the heavier barium compounds. The lighter [[organoberyllium]] and [[organomagnesium]] compounds are often considered [[covalent bond|covalent]], but with some ionic bond characteristics owing to the attached carbon bearing a negative [[molecular dipole moment|dipole moment]]. This higher ionic character and bond polarization tends to produce high [[coordination number]]s and many compounds (particularly dialklys) are polymeric in solid or liquid states with highly complex structures in solution, though in the gaseous state they are often monomeric.

[[Metallocene]] compounds with group 2 elements are rare, but some do exist. Bis(cyclopentadienyl)beryllium or beryllocene (Cp<sub>2</sub>Be), with a [[molecular dipole moment]] of 2.2 [[Debye|D]], is so-called slipped <sup>5</sup>η/<sup>1</sup>η sandwich. While magnesocene (Cp<sub>2</sub>Mg) is a regular metallocene, bis(pentamethylcyclopentadienyl)calcium (Cp<sup>*</sup>)<sub>2</sub>Ca is bent with an angle of 147°.

[[image: Me2Mg.png|right|thumb|322 px|[[Dimethylmagnesium]] is a polymer built up from 3-center, 2-electron bonded bridging methyl groups.<ref>{{cite journal | last1 = Weiss | first1 = E. | title = Die Kristallstruktur des Dimethylmagnesiums | journal = [[J. Organomet. Chem.]] | year = 1964 | volume = 2 | issue = 4 | pages = 314–321 | doi = 10.1016/S0022-328X(00)82217-2 }}</ref>  [[Dimethylberyllium]] adopts the same structure.<ref>{{cite journal | last1 = Snow | first1 = A.I.|last2 = Rundle| first2=R.E.|title = Structure of Dimethylberyllium
|journal=Acta Crystallographica|year=1951|volume=4| issue = 4|pages=348–52|doi=10.1107/S0365110X51001100|hdl=2027/mdp.39015095081207|hdl-access=free}}</ref>]]

==Synthesis==
Mixed alkyl/aryl-halide compounds, which contain a single C-M bond and a C-X bond, are typically prepared by oxidative addition. Magnesium-containing compounds of this configuration are known as the [[Grignard reagent]]s, though some calcium Grignard's are known and more reactive and sensitive to decomposition. Calcium grignard's must be pre-activated prior to synthesis.<ref>{{cite journal|title=Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol|author=Reuben D. Rieke |author2=Tse-Chong Wu |author3=Loretta I. Rieke |journal=Org. Synth.|year=1995|volume=72|page=147|doi=10.15227/orgsyn.072.0147}}</ref>

There are three key reaction pathways for dialkyl and diaryl group 2 metal compounds.
*[[Salt metathesis reaction|metathesis]]:
:MX<sub>2</sub> + R-Y &rarr; MR<sub>2</sub> + Y-X'

*[[transmetallation]]:
:M'R<sub>2</sub> + M &rarr; MR<sub>2</sub> + M'

*via the [[Schlenk equilibrium]]:
:2 RMX &rarr; MR<sub>2</sub> + MX<sub>2</sub>

==Compounds==
Although organomagnesium compounds are widespread in the form of [[Grignard reagent]]s, the other organo-group 2 compound are almost exclusively of academic interest. Organoberyllium chemistry is limited due to the cost and toxicity of beryllium. [[Calcium]] is nontoxic and cheap but organocalcium compounds are difficult to prepare, [[strontium]] and [[barium]] compounds even more so. One use for these type of compounds is in [[chemical vapor deposition]].

===Organoberyllium===
{{Main|Organoberyllium chemistry}}
Beryllium derivatives and reagents are often prepared by alkylation of [[beryllium chloride]].<ref name=naglav>''Off the Beaten Track—A Hitchhiker's Guide to Beryllium Chemistry'' D. Naglav, M. R. Buchner, G. Bendt, F. Kraus, S. Schulz, Angew. Chem. Int. Ed. 2016, 55, 10562. {{doi|10.1002/anie.201601809}}</ref> Examples of known organoberyllium compounds are dineopentylberyllium,<ref>{{cite journal|doi=10.1039/J19710001308|title=Preparation of base-free beryllium alkyls from trialkylboranes. Dineopentylberyllium, bis(trimethylsilylmethyl)beryllium, and an ethylberyllium hydride|year=1971|last1=Coates|first1=G. E.|last2=Francis|first2=B. R.|journal=Journal of the Chemical Society A: Inorganic, Physical, Theoretical|page=1308}}</ref> beryllocene (Cp<sub>2</sub>Be),<ref>{{cite journal|doi=10.1002/cber.19590920233|title=Über Aromatenkomplexe von Metallen, XXV. Di-cyclopentadienyl-beryllium|year=1959|last1=Fischer|first1=Ernst Otto|last2=Hofmann|first2=Hermann P.|journal=Chemische Berichte|volume=92|page=482|issue=2}}</ref><ref>{{cite journal|doi=10.1071/CH9841601|title=A precise low-temperature crystal structure of Bis(cyclopentadienyl)beryllium|year=1984|last1=Nugent|first1=KW|last2=Beattie|first2=JK|last3=Hambley|first3=TW|last4=Snow|first4=MR|journal=Australian Journal of Chemistry|volume=37|page=1601|issue=8}}</ref><ref>{{cite journal|doi=10.1016/S0022-328X(00)92065-5|title=The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction|year=1979|last1=Almenningen|first1=A|journal=Journal of Organometallic Chemistry|volume=170|page=271|issue=3|last2=Haaland|first2=Arne|last3=Lusztyk|first3=Janusz}}</ref><ref>{{cite journal|doi=10.1107/S0567740872004820|title=Crystal structure of bis(cyclopentadienyl)beryllium at −120&nbsp;°C|year=1972|last1=Wong|first1=C. H.|last2=Lee|first2=T. Y.|last3=Chao|first3=K. J.|last4=Lee|first4=S.|journal=Acta Crystallographica Section B|volume=28|page=1662|issue=6|doi-access=free}}</ref> ''diallylberyllium'' (by exchange reaction of diethyl beryllium with triallyl boron),<ref>{{cite journal|doi=10.1002/zaac.19744050111|title=Ein Beitrag zur Existenz von Allylberyllium- und Allylaluminiumverbindungen|year=1974|last1=Wiegand|first1=G.|last2=Thiele|first2=K.-H.|journal=Zeitschrift für anorganische und allgemeine Chemie|volume=405|pages=101–108}}</ref> bis(1,3-trimethylsilylallyl)beryllium<ref>{{cite journal|doi=10.1002/anie.201001866|pmid=20575128|title=Bis(1,3-trimethylsilylallyl)beryllium|year=2010|last1=Chmely|first1=Stephen C.|last2=Hanusa|first2=Timothy P.|last3=Brennessel|first3=William W.|journal=Angewandte Chemie International Edition|volume=49|issue=34|pages=5870–4}}</ref> and Be(mes)2.<ref name=naglav/><ref>''Synthesis and structural characterization of the beryllium compounds [Be(2,4,6-Me3C6H2)2(OEt2)], [Be{O(2,4,6-tert-Bu3C6H2)}2(OEt2)], and [Be{S(2,4,6-tert-Bu3C6H2)}2(THF)].cntdot.PhMe and determination of the structure of [BeCl2(OEt2)2]'' Karin Ruhlandt-Senge, Ruth A. Bartlett, Marilyn M. Olmstead, and Philip P. Power Inorganic Chemistry 1993 32 (9), 1724-1728 {{doi|10.1021/ic00061a031}}</ref> Ligands can also be aryls<ref>{{cite journal|doi=10.1021/ic00061a031|title=Synthesis and structural characterization of the beryllium compounds [Be(2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)<sub>2</sub>(OEt<sub>2</sub>)], [Be{O(2,4,6-tert-Bu<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)}<sub>2</sub>(OEt<sub>2</sub>)], and [Be{S(2,4,6-tert-Bu<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)}<sub>2</sub>(THF)].cntdot.PhMe and determination of the structure of [BeCl<sub>2</sub>(OEt<sub>2</sub>)<sub>2</sub>]|year=1993|last1=Ruhlandt-Senge|first1=Karin|last2=Bartlett|first2=Ruth A.|last3=Olmstead|first3=Marilyn M.|last4=Power|first4=Philip P.|journal=Inorganic Chemistry|volume=32|page=1724}}</ref> and alkynyls.<ref>{{cite journal|doi=10.1016/S0022-328X(00)87485-9|title=The crystal structure of dimeric methyl-1-propynyl- beryllium-trimethylamine|year=1971|last1=Morosin|first1=B|journal=Journal of Organometallic Chemistry|volume=29|page=7|last2=Howatson|first2=J.}}</ref>

===Organomagnesium===
{{Also|Grignard reagent}}
The distinctive feature of the Grignard reagents is their formation from the organic halide and magnesium metal.  Most other group II organic compounds are generated by [[salt metathesis]], which limits their accessibility.  The formation of the Grignard reagents has received intense scrutiny. It proceeds by a [[single electron transfer|SET]] process.  For less reactive organic halides, activated forms of magnesium have been produced in the form of [[Rieke magnesium]]. Examples of Grignard reagents are [[phenylmagnesium bromide]] and [[ethylmagnesium bromide]]. These simplified formulas are deceptive: Grignard reagents generally exist as dietherates, RMgX(ether)2.  As such they obey the [[octet rule]].

Grignard reagents participate in the Schlenk equilibrium.  Exploiting this reaction is a way to generate [[dimethylmagnesium]].  Beyond Grignard reagents, another organomagnesium compound is [[magnesium anthracene]]. This orange solid is used as a source of highly active magnesium.  [[Butadiene]]-magnesium serves as a source for the butadiene dianion.  [[Ate complex]]es of magnesium are also well known, e.g LiMgBu<sub>3</sub>.<ref>{{cite journal |doi=10.15227/orgsyn.089.0460|first1=Juan D.|last1=Arredondo|first2=Hongmei|last2=Li|first3=Jaume|last3=Balsells|
title=Preparation of t-Butyl-3-Bromo-5-Formylbenzoate Through Selective Metal-Halogen Exchange Reactions|journal=Organic Syntheses|year=2012|volume=89|page=460|doi-access=free}}</ref>

===Organocalcium===
{{Main|Organocalcium chemistry}}

Dimethylcalcium is obtained by [[Salt metathesis reaction|metathesis]] reaction of [[Metal bis(trimethylsilyl)amides|calcium bis(trimethylsilyl)amide]] and [[methyllithium]] in [[diethyl ether]]:<ref>"Dimethylcalcium" Benjamin M. Wolf, Christoph Stuhl, Cäcilia Maichle-Mössmer, and Reiner Anwander ''J. Am. Chem. Soc.'' '''2018''', Volume 140, Issue 6, Pages 2373–2383 {{doi|10.1021/jacs.7b12984}}</ref>
:<math>\mathrm{Ca[N\{Si(CH_3 )_3\}_2]_2 + 2\ LiCH_3 \longrightarrow Ca(CH_3)_2 + 2\ Li[N\{Si(CH_3)_3\}_2]}</math>

A well known organocalcium compound is ([[cyclopentadienyl|Cp]])calcium(I).{{cn|date=January 2021}} Bis(allyl)calcium was described in 2009.<ref>"Bis(allyl)calcium" Phillip Jochmann, Thomas S. Dols, Thomas P. Spaniol, Lionel Perrin, Laurent Maron, Jun Okuda ''Angewandte Chemie International Edition'' Volume 48 Issue 31, Pages 5715–5719 2009 {{doi|10.1002/anie.200901743}}</ref> It forms in a metathesis reaction of [[allylpotassium]] and [[calcium iodide]] as a stable non-[[pyrophoric]] off-white powder:
:<chem>\overset{allylpotassium}{2KC3H5} + \overset{calcium\ iodide}{CaI2} ->[\ce{THF}][25^\circ \ce C] {(C3H5)2Ca} + 2KI</chem>
The bonding mode is [[hapticity|η]]<sup>3</sup>. This compound is also reported to give access to an [[hapticity|η]]<sup>1</sup> polymeric (CaCH<sub>2</sub>CHCH<sub>2</sub>)<sub>''n''</sub> compound.<ref>Lichtenberg, C., Jochmann, P., Spaniol, T. P. and Okuda, J. (2011), "The Allylcalcium Monocation: A Bridging Allyl Ligand with a Non-Bent Coordination Geometry". ''Angewandte Chemie International Edition'', 50: 5753–5756. {{doi|10.1002/anie.201100073}}</ref>

The compound [(thf)<sub>3</sub>Ca{μ-C<sub>6</sub>H<sub>3</sub>-1,3,5-Ph<sub>3</sub>}Ca(thf)<sub>3</sub>] also described in 2009<ref>"Stable 'Inverse' Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(thf)2Mg(Br)-C6H2-2,4,6-Ph3] and [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3]" Sven Krieck, Helmar Görls, Lian Yu, Markus Reiher and Matthias Westerhausen ''[[J. Am. Chem. Soc.]]'', '''2009''', 131 (8), pp 2977–2985 {{doi|10.1021/ja808524y}}</ref><ref>"Organometallic Compounds of the Heavier s-Block Elements—What Next?" J. David Smith ''[[Angew. Chem. Int. Ed.]]'' '''2009''', 48, 6597–6599 {{doi|10.1002/anie.200901506}}</ref> is an inverse [[sandwich compound]] with two calcium atoms at either side of an arene.

[[Olefin]]s tethered to [[cyclopentadienyl ligand]]s have been shown to coordinate to calcium(II), strontium(II), and barium(II):<ref name=schumann1>{{cite journal | author1 = H. Schumann | author2 = S. Schutte | author3 = H.-J. Kroth | author4= D. Lentz | title = Butenyl-Substituted Alkaline-Earth Metallocenes: A First Step towards Olefin Complexes of the Alkaline-Earth Metals | journal = Angew. Chem. Int. Ed. | year = 2004 | volume = 43 | issue = 45 | pages = 6208–6211 | doi = 10.1002/anie.200460927| pmid = 15549740 | doi-access = free }}</ref>

:[[File:Olefine coordination at calcium strontium barium.png|400px|Olefin complexes of calcium, strontium and barium<ref name=schumann1/>]]

Organocalcium compounds have been investigated as catalysts.<ref>{{cite journal|doi=10.1021/om101063m|title=Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination|year=2011|last1=Arrowsmith|first1=Merle|last2=Crimmin|first2=Mark R.|last3=Barrett|first3=Anthony G. M.|last4=Hill|first4=Michael S.|last5=Kociok-KöHn|first5=Gabriele|last6=Procopiou|first6=Panayiotis A.|journal=Organometallics|volume=30|issue=6|pages=1493–1506}}</ref>

===Organostrontium===
'''Organostrontium''' compounds have been reported as intermediates in [[Barbier reaction|Barbier-type]] reactions.<ref>{{Cite journal| doi = 10.1246/bcsj.77.341| title = The Barbier-Type Alkylation of Aldehydes with Alkyl Halides in the Presence of Metallic Strontium| year = 2004| last1 = Miyoshi | first1 = N.| last2 = Kamiura | first2 = K.| last3 = Oka | first3 = H.| last4 = Kita | first4 = A.| last5 = Kuwata | first5 = R.| last6 = Ikehara | first6 = D.| last7 = Wada | first7 = M.| journal = Bulletin of the Chemical Society of Japan| volume = 77| issue = 2| page = 341 }}</ref><ref>{{Cite journal| doi = 10.1246/cl.2005.760| title = The Chemistry of Alkylstrontium Halide Analogues: Barbier-type Alkylation of Imines with Alkyl Halides| year = 2005| last1 = Miyoshi | first1 = N.| last2 = Ikehara | first2 = D.| last3 = Kohno | first3 = T.| last4 = Matsui | first4 = A.| last5 = Wada | first5 = M.| journal = Chemistry Letters| volume = 34| issue = 6| page = 760 }}</ref><ref>{{Cite journal| doi = 10.1002/ejoc.200500484| title = The Chemistry of Alkylstrontium Halide Analogues, Part 2: Barbier-Type Dialkylation of Esters with Alkyl Halides| year = 2005| last1 = Miyoshi | first1 = N.| last2 = Matsuo | first2 = T.| last3 = Wada | first3 = M.| journal = European Journal of Organic Chemistry| volume = 2005| issue = 20| page = 4253 }}</ref>

[[File:LOMBOF.png|thumb|upright=1.1|Structure of Ba(CH(tms)<sub>2</sub>)<sub>2</sub>(thf)<sub>3</sub> (tms = Si(CH<sub>3</sub>)<sub>3</sub>), with H atoms omitted. Even with bulky alkyl substituents, Ba coordinates to three THF ligands.]]
===Organobarium===
'''Organobarium''' compounds<ref>''Comprehensive organic functional group transformations'' [[Alan R. Katritzky]], Otto Meth-Cohn, [[Charles Rees|Charles Wayne Rees]]</ref> of the type (allyl)BaCl can be prepared by reaction of activated barium (Rieke method [[redox|reduction]] of [[barium iodide]] with lithium biphenylide) with allyl halides.<ref>{{Cite journal| doi = 10.1021/ja00023a058| title = Allylbarium in organic synthesis: unprecedented .alpha.-selective and stereospecific allylation of carbonyl compounds| year = 1991| last1 = Yanagisawa | first1 = A.| last2 = Habaue | first2 = S.| last3 = Yamamoto | first3 = H.| journal = Journal of the American Chemical Society| volume = 113| issue = 23| page = 8955 }}</ref><ref>{{Cite journal| doi = 10.1021/ja00093a010| title = Allylbarium Reagents: Unprecedented Regio- and Stereoselective Allylation Reactions of Carbonyl Compounds| year = 1994| last1 = Yanagisawa | first1 = A.| last2 = Habaue | first2 = S.| last3 = Yasue | first3 = K.| last4 = Yamamoto | first4 = H.| journal = Journal of the American Chemical Society| volume = 116| issue = 14| page = 6130 }}</ref> These allylbarium compounds react with carbonyl compounds.  Such reagents are more alpha-selective and more stereoselective than the related Grignards or organocalcium compounds. The [[metallocene]] ([[Cp*]])<sub>2</sub>Ba has also been reported.<ref>{{Cite journal | first3 = J. C.| last3 = Huffman| journal = Journal of the Chemical Society, Chemical Communications| issue = 15| page = 1045 | first2 = T. P.| last2 = Hanusa| title = Solid state structure of bis(pentamethylcyclopentadienyl)barium, (Me5C5)2Ba; the first X-ray crystal structure of an organobarium complex| year = 1988| last1 = Williams | first1 = R. A.| doi = 10.1039/C39880001045 }}</ref>

===Organoradium===
The only known '''organoradium''' compound is the gas-phase [[acetylide]].{{Citation needed|date=January 2025}}

==See also==
* [[Organometallic chemistry]]
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==References==
{{reflist|30em}}

{{ChemicalBondsToCarbon}}

{{DEFAULTSORT:Group 2 Organometallic Chemistry}}
[[Category:Organometallic chemistry]]