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Organometallic Compounds

Structure of Organometallic Compounds

In the synthesis of Grignard compounds (organomagnesium halides, RMgX), the solvent, which is usually diehtyl ether or tetrahydrofuran (THF), plays a most crucial role. The magnesium atom of a Grignard compound lacks four electrons to achieve an electron octet.

Diethyl ether

If the synthesis is carried out in diethyl ether or THF, the magnesium atom is additionally complexed by two solvent molecules. This is possible due to the lone electron pairs of the solvent's oxygen atom. As a result, the magnesium atom achieves an electron octet and the Grignard compound is thus stabilized. In etherial solution Grignard compounds mainly form dimers in which two magnesium atoms are connected through two halogen atom bridges. In addition, each magnesium is complexed by one alkyl and one solvent substituent. These Grignard compound dimers are in equilibrium with the corresponding magnesium dihalide and the dialkylmagnesium compound. Such an equilibrium is also known as Schenk equilibrium.

Grignard compound's formation and Schenk equilibrium.
Apparatus for the synthesis of a Grignard compound.

In organometallic compounds such as organolithium and Grignard compounds, the carbon is considerably negatively polarized. Thus, due to the large electronegativity difference between the carbon and the metal, the carbon is nucleophilic. The carbon-metal bond is strongly polarized. As a result, the chemical properties of such an organometallic compound resemble that of separated magnesium cations and alkyl anions even though the magnesium atom and the alkyl group are not actually separated into ions.

Properties of organometallic compounds.
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In the mechanism of Grignard compound formation, a radical intermediate evolves. Grignard compounds hydrolyze rapidly when they are treated with water. The hydrolysis yields the corresponding alkane and magnesium hydroxide. Therefore, the synthesis of Grignard compounds and the subsequent nucleophilic attack of the Grignard compound on an electrophile, such as a carbonyl compound, must be performed out of contact with water.

Mechanism of Grignard compounds' fromation (X = Cl, Br or I).

Exercise: Structure of organometallic compounds

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