What are organometallic compounds?

Organometallic compounds have a minimum of one bond between a metal compound and a carbon atom. Metalloid elements such as silicon, tin, and boron form organometallic compounds which can be utilized in a few commercial chemical reactions.

In general, the bond between the metallic atom and the carbon of an organic compound is covalent. When metals with high electropositivity such as sodium and lithium form these compounds, a carbanionic nature is exhibited using the carbon which is certain to the central metallic atom. A few examples are Grignard reagents, tetracarbonyl nickel, and dimethyl magnesium.

Synthesis methods

Most organometallic compounds may be prepared by the reactions of the metallic compound with organic halides, metathesis, and hydrometallation.

Prepartion of organometallic compound by reaction with metal and transmetallation

The overall reaction of halogen-substituted hydrocarbon and an electropositive metallic compound is,

$2M+RX⟶MR+MX$

If there is transfer of ligands from one metal to another., it is known as transmetallation. It happens only when the metal atom which is going to displace the location of another is greater in the electrochemical series than the displaced metal.

The transmetallation reaction is,

$M+M\prime R⟶M\prime +MR$

Preparation by means of metathesis

An extensively used artificial method in organometallic chemistry is the metathesis of an organometallic compound $MR$ and a binary halide $EX$. This reaction is often be expected from electronegativity or hard & soft acid-base considerations.

$MR+EX⟶ER+MX$

Hydrocarbons tend to form a bond with the highly electronegative element. The alkyl & aryl compounds tend to migrate from the less electronegative element to the element with high electronegativity. If the electronegativity of the elements is similar to one another, the results can be expected to be mixture of the softer element with the organic compounds and the harder element with fluoride or chloride.

Example:

$SnP{h}_4 \left(THF\right) +HgB{r}_2 \left(THF\right) ⟶HgPhBr\left(s\right) +PhSnBr \left(THF\right)$

Metathesis reactions related to the equal crucial detail are frequently called redistribution reactions.

$$\begin{gathered} 3SiC{l}_4+5SiM{e}_4⟶4M{e}_{3}SiCl+4M{e}_{2}SiC{l}_2 \\ 3GeC{l}_4+2AlM{e}_6⟶3GeM{e}_4+4AlC{l}_3 \end{gathered}$$

$Al$ is higher electropositive than $Ge$, this reaction happens as it is thermodynamically favorable.

Preparation by means of hydrometallation

The final results of the addition of a metallic hydride to an alkene are an alkyl metal compound.

$EH+H_{2}C=C{H}_2⟶E-C{H}_2-C{H}_3$

The reaction is pushed with the aid of using the excessive energy of the $E-C$ bond relative to that of maximum $E-H$ bonds and happens with a various compounds that contain $E-H$ bonds.

Organometallic compounds of alkali metals

Alkali metal organometallics are extremely reactive and should be handled in air- and moisture-free environments; $NaMe$, for example, burns explosively in the air. Lithium alkyls are chemical compounds each in solution and within the solid-state.

Organolithium compounds

$nBuCl+2Li\stackrel{Hydrocarbon solvent}{\to } nBuLi+LiCl$

Organolithium compounds are important among organometallics. Several organometallic compounds are commercially on the market as solutions in organic compound solvents. Solvent decisions for reactions involving organometallics of the alkali metals are critical. For example, $nBuLi$ is decomposed by $E{t}_{2}O$to form $nBuH$, $C_2{H}_4$, and $LiOEt$.

The structures of ${\left(tBuLi\right)}_4$ and ${\left(MeLi\right)}_4$are similar. $nBuLi$ once mixed with $TMEDA$ offers a chemical compound chain. TMEDA links cubane units along through the formation of bond between lithium and nitrogen.

Alkyl lithium compounds are soluble in organic solvents whereas Na and K salts are insoluble, however are solubilized by the chelating matter $TMEDA$. The addition of $TMEDA$ might break down the aggregates of lithium alkyls to give lower nuclearity complexes. For example ${[nBuLi.TMEDA]}_2$. However, careful studies have disclosed that the system is way from simple, and it is attainable to isolate crystals of either ${[nBULi.TMEDA]}_2$ or ${[{\left(nBuLi\right)}_4.TMEDA]}_{\infty }$. Within the case of ${\left(MeLi\right)}_4$, the addition of $TMEDA$doesn't cause cluster breakdown, and the X-ray structure confirms the composition ${\left(MeLi\right)}_4.2TMEDA$, the presence of each tetramer, and the amino alkanoic molecules in the crystal lattice.

Lithium alkyls and aryls are helpful reagents in organic synthesis and also in creating corresponding carbon compounds of main cluster elements. Lithium alkyls are necessary catalysts in the rubber business for the stereospecific chemical process of alkenes.

Organometallic compounds of alkaline earth metals

Beryllium

$$\begin{gathered} HgM{e}_2 + Be⟶M{e}_{2}Be + Hg \left(at383K\right) \\ 2PhLi + BeC{l}_2⟶P{h}_{2}Be + 2LiCl \end{gathered}$$

In the vapor phase, the dimethyl group is monomeric. The solid structure is polymeric and resembles that of beryllium chloride.

$2Na{C}_p + BeC{l}_2⟶C{p}_{2}Be + 2NaCl$

X-ray diffraction indicates $\left[\right({\eta }^1{C}_p) ({\eta }^5{C}_p) Be]$. However, the 1H nuclear magnetic resonance spectrum shows that each proton environments are equal even at one sixty-three kelvin. Also, the solid-state structure shows the beryllium atom is disordered over two equivalent sites and NMR information may be taken in terms of the fluxional method within which the beryllium atom moves between these two sites. However, $C_p*2Be$ possesses a sandwich structure with each of the rings are coplanar.

Magnesium

Magnesium halides of aryl & alkyl compounds are otherwise called Grignard reagents. $RMgX$is known for its use in synthetic chemistry. Transmetallation is a useful means of making pure Grignard reagents.

$$\begin{gathered} Mg + RHgBr⟶Hg + RMgBr \\ Mg + R_{2}Hg⟶Hg + R_{2}Mg \end{gathered}$$

Double coordination in Mg is ascertained in R₂Mg if the R groups are sufficiently bulky, e.g. The $Mg{C{\left(SiM{e}_3\right)}_3}_2.RMgX$ are usually solvated and the center of Magnesium is typically tetrahedral.  has a stepped sandwich structure. Solutions of Grignard chemical agent might contain many species, for example, $RMgX, R_{2}Mg, Mg{X}_2, RMg{(\mu -X)}_{2}MgR$, which are any difficult by solvation.

Organosilicon Compounds

Organosilicon compounds have extensively economic applications as lubricants, water repellents & sealants. several oxo-bridged organosilicon compounds will be synthesized. For example, ${\left(C{H}_3\right)}_{3}Si—O—Si{\left(C{H}_3\right)}_3$ is proof against wetness and air.

The lone pairs present in the oxygen atom are delocalized to vacant σ *orbitals of silicon, as a consequence the radial asymmetry of the silicon and oxygen bond is reduced creating the structure a lot of flexibility. This flexibility allows siloxane elastomers to stay rubber-like right down to terribly low temperatures. Delocalization additionally accounts for the lower basicity of oxygen atoms connected to silicon because the number of electrons needed for an oxygen atom to act as a base is removed. The planarity is additionally explained by the delocalization of electrons on nitrogen that makes it debile basic.

$$\begin{gathered} nMeCl+Si/C{u}^{-}\to MenSiC{l_4}^{-} \\ nSiC{l}_4+4RLi\to R_{4}Si \\ SiC{l}_4+RLi\to RSiC{l}_3 \\ SiC{l}_4+2RMgCl\to R_{2}SiC{l}_2+2MgC{l}_2 \\ M{e}_{2}SiC{l}_2+BuLit\to BuM{e}_{2}tSiCl+LiCl \end{gathered}$$

The bonds between silicon and carbon are comparatively strong. The bond energy of hydrogen atom is and tetra alkyl silicon derivatives have high thermal stabilities. Tetraethyl silicon on chlorination provides${\left(ClC{H}_{2}C{H}_2\right)}_{4}Si$. The reactions of $M{e}_{2}SiC{l}_2$ manufacture elements are,

$M{e}_{3}SiC{l}_2+Na{C}_p\to ({\eta }^1-C_p)SiHM{e}_3$

Context and Applications

This topic is significant for both undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.

Practice Problems

Question 1: Which is not there in the Grignard reagent?

a) –CH₃ group

b) Mg

c) Halogen

d) Carboxylic group

Answer: Option d is correct.

Explanation: The formula of Grignard reagent is where X refers to halogen, and R refers to alkyl or aryl group.

Question 2: Alkyl halides are changed into Grignard reagents by _______________.

a) Boiling alkyl halides with magnesium ribbon in alcoholic solution

b) Warming alkyl halides with magnesium powder in dry ether

c) Refluxing alkyl halides with a magnesium chloride solution

d) Warming alkyl halides with magnesium chloride

Answer: Option b is correct.

Explanation: The Grignard reagent is synthesized from alkyl halides by heating them with Mg powder in dry ether.

Question 3: Find the compounds which do not give tertiary alcohol upon reaction with CH₃MgBr.

a) 3-methylpentanal

b) Ethyl benzoate

c) 4,4-dimethylcyclohexanone

d) 4-heptanone

Answer: Option d is correct.

Explanation: The addition of an organomagnesium halide to a ketone gives secondary alcohol thus 4-heptanone does not give tertiary alcohol.

Question 4: Among the following compounds, which are used to prepare Grignard reagents?

a) Methylamine

b) Diethyl ether

c) Ethyl iodide

d) Ethyl alcohol

Answer: Option c is correct.

Explanation: Grignard’s reagent is prepared by reacting Mg with CH₃I.

Question 5: Find the compound which gives secondary alcohol upon reaction with methylmagnesium bromide.

a) Butyl formate

b) 3- pentanone

c) Pentanal

d) Methyl butanoate

Answer: Option c is correct.

Explanation: Pentanal gives secondary alcohol upon reaction with methylmagnesium bromide.