What Are Organometallic Compounds? Core Concepts & Practical Examples
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.
FAQs on Organometallic Compounds Explained: Structure, Types & Uses
1. What are organometallic compounds, and what is their defining characteristic?
Organometallic compounds are chemical compounds that contain at least one direct metal-to-carbon bond. This bond can be with an organic group or molecule. The nature of this M-C bond, which can range from covalent to ionic, is the defining characteristic that dictates the compound's properties and reactivity. Common examples include Grignard reagents (R-MgX) and Ferrocene [Fe(C₅H₅)₂].
2. How are organometallic compounds classified based on the nature of their bonding?
Organometallic compounds are primarily classified into three types based on their bonding nature:
- Ionic Bonded: These are formed by highly electropositive metals like those from Group 1 (alkali) and Group 2 (alkaline earth). They are salt-like solids and are highly reactive. Example: Sodium acetylide (NaC₂H).
- σ–bonded (Sigma-bonded): These compounds feature a standard two-electron covalent bond between a metal atom and a carbon atom of an organic group. Examples include Tetraethyllead, Pb(C₂H₅)₄, and Dimethylzinc, Zn(CH₃)₂.
- π–bonded (Pi-bonded): In these compounds, a metal atom is bonded to the π-electrons of an unsaturated organic molecule such as an alkene, alkyne, or benzene. Examples include Zeise's salt, K[PtCl₃(C₂H₄)], and Ferrocene.
3. What is the importance of organometallic compounds in industrial and synthetic chemistry?
Organometallic compounds are extremely important due to their unique reactivity. Their main applications include:
- Catalysis: They are used as catalysts in major industrial processes. For example, Ziegler-Natta catalysts (an organoaluminium compound with a titanium salt) are used for polymerising ethylene and propylene to produce plastics like polyethylene and polypropylene.
- Organic Synthesis: Reagents like Grignard reagents (RMgX) and organolithium compounds (RLi) are invaluable for forming new carbon-carbon bonds, which is a fundamental step in building complex organic molecules.
- Commercial Products: Organotin compounds are used as PVC stabilisers and antifouling agents, while some organosilicon compounds (silicones) are used in lubricants, sealants, and medical implants.
4. How is a Grignard reagent, a common organometallic compound, prepared?
A Grignard reagent is synthesised by reacting an alkyl or aryl halide (like bromoethane) with magnesium metal. The reaction must be carried out in a solvent of dry (anhydrous) ether, such as diethyl ether or tetrahydrofuran (THF). It is crucial to exclude all water and air, as Grignard reagents are highly reactive and will be destroyed by protonolysis (reaction with water) or oxidation.
5. Why is a compound like sodium ethoxide (NaOCH₂CH₃) not considered an organometallic compound?
Sodium ethoxide is not an organometallic compound because it lacks a direct metal-to-carbon bond. In sodium ethoxide, the sodium (Na) atom is bonded to an oxygen (O) atom, which is in turn bonded to the ethyl group (-CH₂CH₃). The defining feature of an organometallic compound is the M-C bond. Since the bond here is M-O-C, it is classified as a metal alkoxide, not an organometallic compound.
6. What does the term hapticity mean in the context of organometallic compounds?
Hapticity (symbol: η, pronounced 'eta') describes how a group of continuous atoms in a ligand are coordinated to a central metal atom. It is denoted by ηⁿ, where 'n' is the number of atoms in the ligand directly bonded to the metal. For example, in ferrocene, [Fe(C₅H₅)₂], each cyclopentadienyl ring is bonded through all five of its carbon atoms, so its hapticity is 5, denoted as (η⁵-C₅H₅).
7. How does the stability of organometallic compounds typically vary for main group elements versus transition metals?
For main group elements (s-block and p-block), the thermal stability of organometallic compounds often decreases down a group due to weaker metal-carbon bonds. For instance, tetramethylsilicon, Si(CH₃)₄, is much more stable than tetraethyllead, Pb(C₂H₅)₄. Conversely, for d-block (transition) metals, the strength of the M-C bond, and thus the stability of the compound, often increases as one descends a group.
8. Explain the structure and bonding in Zeise's salt, a classic example of a π-bonded compound.
Zeise's salt, K[PtCl₃(C₂H₄)], is a key example of a π-bonded organometallic compound. Its structure consists of a central platinum (Pt) atom bonded to three chloride (Cl⁻) ligands and one ethylene (C₂H₄) molecule. The bonding involves a donation of electron density from the π-orbital of the ethylene molecule to an empty orbital on the platinum atom. Simultaneously, there is a back-donation of electrons from a filled d-orbital of platinum into the empty π* (antibonding) orbital of the ethylene. This two-way interaction, known as synergic bonding, strengthens the overall metal-ligand bond.
