Organometallic Compounds

Organometallic compounds are organic molecules that contain carbon-metal linkages. Alkali metals and alkaline earth metals are among the metals. Metalloids, such as boron, silicon, and selenium, are also known to produce organometallic compounds, which are used in industrial chemical reactions. Those compounds which have at least one metal-carbon bond, are called organometallic compounds. The most common example of an organometallic compound is Grignard reagent – RMgX. Few other examples of organometallic compounds are given below –

• Gilman reagent – R2CuLi 

• Dimethylmagnesium – Me2Mg

• Triethylborane – Et3B

• Ferrocene 

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• Cobaltocene 

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Wilkinson Catalyst - [Rh(PPh3)3Cl] is also an organometallic compound. Although it does not possess a direct metal-carbon bond. But it forms a metal carbon bond during hydrogenation. 

Exceptions - Cyanides such as NaCN and carbides such as CaC2 are not organometallic compounds. We count them as inorganic compounds. While carbonyl compounds such as Ni(CO)4 are counted as organometallic compounds. 

Thus, in organometallic compounds, metal-carbon bonds can either be direct carbon to metal bond means sigma bond or a metal complex bond means pi bond. The branch of chemistry which includes a study of organometallic compounds is called organometallic chemistry. It is also called organometallics. 

Presently organometallic compounds are a huge subject of research due to their various pharmaceutical applications. Many journals are published on the subject such as the American Chemical Society publishes biweekly journals on organometallic compounds called organometallics. 

Organometallic Compounds' Stability and Reactivity

The nature of these compounds affects the stability and reactivity of organometallic complexes. The thermal stability of an organometallic compound declines from the lightest to the heaviest element in each of the main groups of the periodic table (groups 1, 2, and 13–15). 

For example, methyl lithium (LiCH₃) is far more stable in group-1 metal compounds than methyl potassium (KCH₃), and tetramethyl silicon, Si(CH₃)₄, is stable at 500 °C (932°F) in the absence of air, whereas tetramethyl lead, Pb(CH₃)₄, rapidly decomposes at the same temperature. The d-block components (groups 3–12), which reject this pattern by increasing MC bond strengths and stability as you progress down a group, defy this trend.

Structure and Properties of Organometallic Compound

Metal carbonyl organometallic compounds generally follow the 18-electron rule which is helpful in predicting the stability of metal carbonyls. Although other organometallic compounds do not follow the 18-electron rule. In this rule, it is assumed that metal atoms gain electrons from the ligands and attain the nearest noble gas configuration. The total of d-electrons (outermost electrons of transition elements) and the number of electrons supplied by ligands should be 18. It is assumed that the valence shell of the metal will contain 18 electrons.

Generally, organosodium and organopotassium compounds are ionic in nature while most of the other organometallic compounds form polar covalent bonds. 

Properties of Organometallic Compounds  

• They have relatively low melting points. 

• They are insoluble in water.

• They are soluble in ether.

• They are highly reactive. That is why they are kept in organic solvents. 

• In organometallic compounds, carbon has an electronegativity of 2.5 while most metals have electronegativities less than 2.0. 

• The majority of organometallic compounds, especially those containing aromatic or ring-structured hydrocarbon groups, are solid.

• The metal-carbon atom link is usually covalent in character.

• These compounds, especially those produced by highly electropositive metals, have the ability to reduce.

• Highly electropositive metals, such as sodium or lithium, are highly volatile and can spontaneously fire.

• In many cases, organometallic compounds have been shown to be harmful to people.

Classification of Organometallic Compounds 

Organometallic compounds can be classified into the following three types –

• Main group organometallic compounds 

• Transition metal organometallic compounds 

• Lanthanide and actinide organometallic compounds

Main Group Organometallic Compounds – These organometallic compounds have s or p – block elements (metals) in them. The most common example of a main group organometallic compound is Grignard reagent – RMgX. Cacodyl oxide [(CH3)2As]2O was the first main group organometallic compound which was isolated by Louis Claude Cadet de Gassicourt in 1760. Other examples include organoborane, AlEt3, etc.

Structures of a Few of Them are Given Below –

Cacodyl Oxide

Triethylaluminium (AlEt3)

Transition Metal Organometallic Compounds – In these organometallic compounds d-block metals are present. Following are the main examples of transition metal organometallic compounds –

• Gillmann’s Reagent – R2CuLi 

• Collmann’s Reagent – [Fe(CO)4]2-

• Wilkinson’s Catalyst - [Rh(PPh3)3Cl]

• Palladium Catalyst – Pd(PPh3)4 for coupling reaction is also an example of a transition metal organometallic compound. 

• Vaska’s Complex – [Ir(PPh3)(CO)Cl]

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Lanthanide and Actinide Organometallic Compounds - In these organometallic compounds f-block metal/s are present. Following are the main examples of lanthanide and actinide metal organometallic compounds –

• Uranocene

• Cyclopentadienides (C5H5-) Compound

Organometallic Compounds and Their Uses

The importance of organometallic compounds cannot be overstated. Progress in this area has led to the creation of new synthesis reagents and catalysts. Organometallic compounds have a variety of uses, including the following:

1. Homogeneous Catalysis: Organometallic compounds or intermediates produced from transition metal complexes catalyse many processes in solution.

2. Metal Purification: Impure metals are first transformed to carbonyls, which are then decomposed to generate pure metal.

3. Organic Synthesis: They are often utilized in the synthesis of many types of organic compounds, such as organolithium and organomagnesium compounds.

4. Heterogeneous catalysis such as trialkyl aluminum combined with a transition metal halide, such as titanium trichloride or tetrachloride, can be utilized as a heterogeneous catalyst for the polymerization of alkanes at low temperatures.

5. Agriculture: To avoid infection of immature plants, seeds are treated with organometallics such as ethyl mercury chloride.

6. Medicine: The principal treatment for syphilis is a variety of organoarsenic chemicals. Silicone rubbers are employed as body spare parts in modern surgery.

Applications of Organometallic Compounds 

Organometallic compounds are very useful in various fields. A few of them are listed below –

• Organometallic compounds are used as reagents. 

• Wilkinson’s catalyst is used in the hydrogenation of alkenes.

• Ziegler – Natta catalyst [(C2H5)3AlTiCl4] is used for the polymerization of alkenes. 

• Organoarsenic compounds are used for the treatment of syphilis. 

• Palladium catalysts are used in coupling reactions. 

• The Grignard reagent is used in the synthesis of many compounds such as secondary alcohols, aldehydes, etc. 

• Organometallic compounds have a wide range of industrial applications. Such an organolithium is highly basic and so useful in many polymerization reactions stoichiometrically.

• Cp2TiCl2 (Cp is cyclopentadienyl anion) organometallic compound is used as a drug.

• Cis-Platin is used as an anticancer drug. 

• Organometallic compounds are used as additives such as TEL (Tetraethyl lead) is used as an anti-knocking agent in fuels. 

Conclusion

Organometallic compounds are a vast topic of chemistry. This was brief on organometallic compounds. Focus on the concept and learn how these compounds vary from organic and inorganic compounds. Understand their features, structures, and uses to determine how these compounds are classified.