Magnesium anthracenide with three thf ligands. Group 2 organometallic chemistry refers to the
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
of compounds containing carbon bonded to any
group 2 element
A group is a number of persons or things that are located, gathered, or classed together.
Groups of people
* Cultural group, a group whose members share the same cultural identity
* Ethnic group, a group whose members share the same ethnic ide ...
. By far the most common group 2 organometallic compounds are the magnesium-containing
Grignard reagent
A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...
s which are widely used in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
. Other organmetallic group 2 compounds are rare and are typically limited to academic interests.
Characteristics
As the
group 2 elements
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...
(also referred to as the alkaline earth metals) contain two
valence electron
In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms ...
s, their chemistries have similarities
group 12 organometallic compounds. Both readily assume a +2
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
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
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
the resulting C-M bonds are more highly polarized and
ionic-like, if not entirely ionic for the heavier barium compounds. The lighter
organoberyllium
Organoberyllium chemistry involves the synthesis and properties of organometallic compounds featuring the group 2 alkaline earth metal beryllium (Be). Beryllium is best known to have a +2 oxidation state and one of the smallest atoms and it is un ...
and
organomagnesium Magnesium anthracenide with three thf ligands. Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. By far the most common group 2 organometallic compounds are the magnesium-contain ...
compounds are often considered
covalent
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
, but with some ionic bond characteristics owning to the attached carbon bearing a negative
dipole moment. This higher ionic character and bond polarization tends to produce high
coordination number
In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central i ...
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
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...
compounds with group 2 elements are rare, but some do exist. Bis(cyclopentadienyl)beryllium or beryllocene (Cp
2Be), with a
molecular dipole moment
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
of 2.2
D, is so-called slipped
5η/
1η sandwich. While magnesocene (Cp
2Mg) is a regular metallocene, bis(pentamethylcyclopentadienyl)calcium (Cp
*)
2Ca is bent with an angle of 147°.
322 px, Dimethylmagnesium is a polymer built up from 3-center, 2-electron bonded bridging methyl groups. Dimethylberylium adopts the same structure.
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
A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...
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.
There are three key reaction pathways for dialkyl and diaryl group 2 metal compounds.
*
metathesis:
:MX
2 + R-Y → MR
2 + Y-X'
*
transmetallation Transmetalation (alt. spelling: transmetallation) is a type of organometallic reaction that involves the transfer of ligands from one metal to another. It has the general form:
:M1–R + M2–R′ → M1–R′ + M2–R
where R and R′ can be, but ...
:
:M'R
2 + M → MR
2 + M'
*via the
Schlenk equilibrium Schlenk can refer to:
People
* Wilhelm Schlenk (1879-1943), German chemist
In chemistry
* Schlenk flask
* Schlenk line. or Schlenk apparatus
* Schlenk equilibrium
Other
* Mepco Schlenk Engineering College MEPCO may refer to:
* Mepco Schlenk En ...
:
:2 RMX → MR
2 + MX
2
Compounds
Although organomagnesium compounds are widespread in the form of
Grignard reagent
A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...
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
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
is nontoxic and cheap but organocalcium compounds are difficult to prepare,
strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
and
barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
compounds even more so. One use for these type of compounds is in
chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
.
Organoberyllium
Beryllium derivatives and reagents are often prepared by alkylation of
beryllium chloride
Beryllium chloride is an inorganic compound with the formula BeCl2. It is a colourless, hygroscopic solid that dissolves well in many polar solvents. Its properties are similar to those of aluminium chloride, due to beryllium's diagonal relations ...
.
[''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. ] Examples of known organoberyllium compounds are dineopentylberyllium, beryllocene (Cp
2Be), ''diallylberyllium'' (by exchange reaction of diethyl beryllium with triallyl boron), bis(1,3-trimethylsilylallyl)beryllium and Be(mes)2.
[ Ligands can also be aryls and alkynyls.
]
Organomagnesium
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
A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...
, which limits their accessibility. The formation of the Grignard reagents has received intense scrutiny. It proceeds by a SET
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
*Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electro ...
process. For less reactive organic halides, activated forms of magnesium have been produced in the form of Rieke magnesium
A Rieke metal is a highly reactive metal powder generated by reduction of a metal salt with an alkali metal. These materials are named after Reuben D. Rieke, who first described the recipes for their preparation. Among the many metals that have bee ...
. Examples of Grignard reagents are phenylmagnesium bromide
Phenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is o ...
and ethylmagnesium bromide
Ethylmagnesium bromide is a Grignard reagent with formula C2H5MgBr. It is widely used in the laboratory synthesis of organic compounds.
Reactions
Apart from acting as the synthetic equivalent of an ethyl anion synthon for nucleophilic addition ...
. These simplified formulas are deceptive: Grignard reagents generally exist as dietherates, RMgX(ether)2. As such they obey the octet rule
The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule i ...
.
Grignard reagents participate in the Schlenk equilibrium. Exploiting this reaction is a way to generate dimethylmagnesium
Dimethylmagnesium is an organomagnesium compound. It is a white pyrophoric solid. Dimethylmagnesium is used in the synthesis of organometallic compounds.
Preparation
Like other dialkylmagnesium compounds, dimethylmagnesium is prepared by adding d ...
. Beyond Grignard reagents, another organomagnesium compound is magnesium anthracene
Magnesium anthracene is an organomagnesium compound that is almost invariably isolated as its adduct with three tetrahydrofuran (thf) ligands. With the formula Mg(C14H10)(thf)3, this air- and water-sensitive orange solid is obtained by heating a ...
. This orange solid is used as a source of highly active magnesium. Butadiene
1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two viny ...
-magnesium serves as a source for the butadiene dianion. Ate complex
In chemistry, an ate complex is a salt formed by the reaction of a Lewis acid with a Lewis base whereby the central atom (from the Lewis acid) increases its valence and gains a negative formal charge. (In this definition, the meaning of valen ...
es of magnesium are also well known, e.g LiMgBu3.
Organocalcium
Dimethylcalcium is obtained by metathesis reaction of calcium bis(trimethylsilyl)amide and methyllithium
Methyllithium is the simplest organolithium reagent with the empirical formula CH3Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used in so ...
in diethyl ether
Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable liq ...
:
:
A well known organocalcium compound is ( Cp)calcium(I). Bis(allyl)calcium was described in 2009. It forms in a metathesis reaction of allylpotassium and calcium iodide
Calcium iodide (chemical formula CaI2) is the ionic compound of calcium and iodine. This colourless deliquescent solid is a salt that is highly soluble in water. Its properties are similar to those for related salts, such as calcium chloride. It ...
as a stable non-pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
off-white powder:
:\overset + \overset -> ce25^\circ \ce C] + 2KI
The bonding mode is hapticity, η3. This compound is also reported to give access to an hapticity, η1 polymeric (CaCH2CHCH2)''n'' compound.
The compound 3CaCa(thf)3">thf)3CaCa(thf)3also described in 2009 is an inverse sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ) and heterocyclic deriv ...
with two calcium atoms at either side of an arene.
Olefin
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s tethered to cyclopentadienyl ligand
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (''η''5-) bonding mode. The metal–cyclopentadien ...
s have been shown to coordinate to calcium(II), strontium(II), and barium(II):
:
Organocalcium compounds have been investigated as catalysts.
Organostrontium
Organostrontium compounds have been reported as intermediates in Barbier-type reactions.
![LOMBOF](https://upload.wikimedia.org/wikipedia/commons/1/10/LOMBOF.png)
Organobarium
Organobarium compounds of the type (allyl)BaCl can be prepared by reaction of activated barium (Rieke method reduction of barium iodide
Barium iodide is an inorganic compound with the chemical formula, formula BaI2. The compound exists as an anhydrous and a hydrate (BaI2(H2O)2), both of which are white solids. When heated, hydrated barium iodide converts to the anhydrous salt. Th ...
with lithium biphenylide) with allyl halides. 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
A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...
( Cp*)2Ba has also been reported.
Organoradium
The only known organoradium compound is the gas-phase acetylide
In organometallic chemistry, acetylide refers to chemical compounds with the chemical formulas and , where M is a metal. The term is used loosely and can refer to substituted acetylides having the general structure (where R is an organic side c ...
.
See also
* Organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
References
{{DEFAULTSORT:Group 2 Organometallic Chemistry
Organometallic chemistry