Organometallic Compounds: Structure, Properties, and Applications

Organometallic compounds are compounds having a metal-carbon linkage in which the carbon belongs to an organic molecule. The metal-carbon bond is covalent in nature. Metalloid elements such as tin, silicon, and boron are also known to form organometallic compounds that are used in a wide variety of industrial chemical reactions. A few examples of organometallic compounds are tetracarbonyl nickel, Grignard reagent, and dimethyl magnesium. The study of these organometallic substances is known as Organometallic Chemistry. 

Key Terms: Organometallic Compounds, Carbon, Metal, Covalent Bond, 18-Electron Rule, Transition Metals, Stability, Reactivity

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What are Organometallic Compounds?

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Organometallic compounds are compounds having at least one chemical link between a carbon atom from an organic molecule and a metal. The metal link can include alkali and transition metals, as well as metalloids. Organometallic compounds are also called organyl compounds and are prefixed with “organo” before their names. 

• The distinguishing characteristic of organometallic compounds is the metal-carbon bond that is highly covalent. 
• The majority of these compounds are solid at normal temperature, while others, such as methylcyclopentadienyl manganese tricarbonyl, are liquid.
•  Owing to the covalent nature of the bond, when metals having high electropositivity (such as lithium and sodium) form organometallic compounds, a carbanionic nature is demonstrated by the carbon that is bound to the central metal atom.
• Organometallic Compunds helps in the catalysis of reactions in which the target molecules are polymers or pharmaceuticals. It, increases the rate of the reactions.

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Structure of Organometallic Compounds

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Metal carbonyl organometallic compounds are based on the 18-electron rule. This rule is efficient in predicting the stability of metal carbonyls. However, this is not the case with some other organometallic compounds. They do not follow the 18-electron rule. Let us briefly understand the 18-electron rule.

18-Electron Rule

In a compound, the central atom makes bonds by adding electrons to its orbitals. Thus, the central atom is capable of accommodating electrons in its s, p, and d orbitals. When the central metal atom adds electrons from Lewis base/ligands, it adds a maximum of :

• 2 electrons in the s orbital
• 6 electrons in the p orbital
• 10 electrons in the d orbital

Therefore,

s(2) + p(6) + d(10) = 18 electrons

This is the 18-Electron Rule also known as, the Effective Atomic Number (EAN). 

The 18-electron rule is followed in order to attain the nearest noble gas configuration. The total of d-electrons, i.e., the outermost electrons of transition elements, and the number of electrons supplied by ligands hence must be18. This ensures that the valence shell of the metal will contain 18 electrons.

18 Electron Rule

Read more: Valence Electrons

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Properties of Organometallic Compounds

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Some of the key properties exhibited by organometallic compounds are:

• State: Exist in liquid and mainly in solid form, especially with aromatic or ring-structured hydrocarbon groups. 
• Solubility: Organometallic compounds are insoluble in water and soluble in organic solvents such as ether.
• Metal-Carbon Bond: This bond is highly covalent.
• Reactivity: Extremely reactive in nature and hence stored in organic solvents only.
• Electronegativity: The carbon in an organometallic compound has an electronegativity of 2.5. However, in the case of most metals, the electronegativity is less than 2.
• Reducers: Organometallic compounds produced by electropositive metals act as reducing agents
• Spontaneous Combustion: The compounds formed from highly electropositive metals such as lithium and sodium are extremely volatile in nature and can spontaneously catch fire.
• Toxicity: Organometallic compounds, especially volatile compounds have been reported to be toxic to humans.

Also Read: Octet Rule

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Stability and Reactivity of Organometallic Compounds

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The stability and reactivity of organometallic compounds can be described as below – 

Stability

The stability of organometallic compounds is dependent on the nature of the metal attached to the organic ligand.

• The thermal stability is seen to decrease from the lightest to the heaviest elements in the periodic table for 1,2 and 13-15 groups.
• This decrease in thermal stability is due to the decrease in the metal-carbon bond strength as we go down the groups in the periodic table.
• In case of d-block elements (groups 3-12), the metal-carbon bond strength and its stability are seen to increase as we go down the elements of this group in the periodic table. 

Reactivity

Organometallic compounds react vividly with water or air.

• Main-group metals are highly reactive and sensitive organometallic compounds when exposed to air or water. These include elements like  Li, Na, Mg, Al, etc. 
• A classic example of this Al₂(CH₃)₆. This organometallic compound rapidly undergoes a violent reaction with water resulting in the release of methane gas. This gas when comes in contact with air, bursts into a flame immediately.
• Elements of groups 14 and 15 are not reactive with water or air. For example, tetramethylsilicon is inert at room temperature. 

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Classification of Organometallic Compounds

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Organometallic compounds are broadly classified as:

1. Main Group - consists of metals of s or p-block in them. The most common example is the Grignard reagent, R-Mg-X. 
2. Transition Metal – consists of d block metals. Examples are Pd(PPh₃)₄, R₂CuLi (Gillman’s Reagent), etc.
3. Lanthanide and Actinide -The f-block metals are present in these organometallic compounds. An example of lanthanide and actinide organometallic compound include Uranocene.

Based on the type of the bond, organometallic compounds can be classified into the following three types -

Sigma Bonded

The metal atom and carbon atom of the ligand are joined together by a sigma bond. For example, Grignard reagents, R – Mg – X where R is an alkyl or aryl group and X is a halogen and zinc compounds of the formula R₂Zn such as (C₂H₅)₂Zn (isolated by Frankland).

Pi-Bonded

These are compounds of metals with alkenes, alkynes, benzene, and other ring compounds. In these complexes, the metal and ligand form a bond that involves the π-electrons of the ligand. The common examples are Zeise’s salt, ferrocene, and di benzene chromium. The number of carbon atoms bonded to the metal in these compounds is indicated by the Greek letter η(eta) with a number. The prefixes η2, η5, and η6 indicate that 2, 5 and 6 carbon atoms are the metal in the compound.

Pi Bonded Organometallic Compounds

Sigma and Pi Bonded

Metal carbonyl compounds formed between metal and carbon monoxide, belong to this class. These compounds possess both σ-and π- bonding. Generally, the oxidation state of metal atoms in these compounds is zero. Carbonyls may be mononuclear, bridged, or polynuclear. For example – Ni(CO)₄Fe(CO)₅Cr(CO)₆Fe(CO)₉

Also Read: Sigma and Pi Bonds

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Uses of Organometallic Compounds

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The uses of organometallic compounds are – 

1. Organometallic compounds or intermediates that are produced from transition metal complexes are used as homogeneous catalysts for many processes.
2. They are used in the field of medicine. For instance, silicone rubbers are used as spare parts of body in modern surgery.
3. Organometallic compounds are also used to purify metals.
4. They are used in the synthesis of many organic compounds, such as organomagnesium and organolithium compounds.
5. They are also used in agriculture to treat seeds to prevent any infection of immature plants.

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Applications of Organometallic Compounds

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Organometallic compounds are utilized for a wide range of applications:

• Manufacturing of pharmaceutical drugs like cisplatin.
• Use for agricultural applications like manufacturing of ferrocene which is utilized for soil remediation, crop protection, and herbicides and fungicides.
• Used as stoichiometric reagents in research-oriented and industrial chemical reactions
• Employed as catalysts to increase reaction rates (e.g., in the use of homogeneous catalysis).
• Used in the bulk hydrogenation processes like the production of margarine.
• Organometallic Compounds aid in the oxygen transfer in the bloodstream and the conversion of carbon dioxide to oxygen and organic matter.
• Used in the production of Light Emitting Diodes (LEDs).
• Complexes formed from organometallic compounds are used to form various other organic compounds.
• Manufacturing of semiconductors that require the use of compounds such as trimethylaluminum, trimethylgallium, trimethyl antimony, etc.

Read more: Inner Transition Metals

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Things to Remember

• Organometallic compounds are organic molecules having carbon-metal linkages which are covalent in nature.
• The 18-electron rule governs the formation and structure of organometallic compounds. 
• Organometallic compounds act as reducing agents, facilitate spontaneous combustion, and are known for their toxic nature.
• The thermal stability and reactivity of organometallic compounds are dependent on the carbon-metal bond strength.
• The broad classification of organometallic compounds is the main group, transition metal, lanthanide, and actinides. 
• Based on the nature of the bond, organometallic compounds are classified into sigma bonded, pi bonded and a combination of sigma and pi bonded compounds.
• Organometallic compounds are widely used as stoichiometric catalysts, for the production of semiconductors, and LEDs.
• Many organometallic compounds exhibit reactivities due to bond polarity, making them useful in chemical synthesis

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