Organometallic compounds form one very important and interesting group of chemical substances standing at the edge between organic and inorganic chemistry. This family consists of compounds containing at least one bond between a carbon atom and a metal atom. They can range from alkali metals, like lithium and sodium, to transition metals like palladium and platinum. Each of these gives a different character to the compound it forms.
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Organometallics are a class of chemical species defined by the presence of at least one bond between a carbon atom and a metal atom. Species of this kind are of interest and show properties that clearly distinguish them from organic and inorganic pure compounds. They can be as simple as alkali and alkaline earth metals or connected with different transition metals and metalloids, such as silicon and tin. The very metal-carbon bond is different in nature; it changes from covalent to ionic and even multicenter, which alters the reactivity and stability of the compounds.
The role of organometallic compounds in chemistry cannot be overestimated. They entered this field most importantly in the area of catalysis, where chemical reactions are enabled to proceed at extremely high turnover long before any mediators are consumed. Another striking example in this respect is Grignard reagents and metal carbonyls that have been used in organic synthesis with wider applications. Thus, the platform is provided by the chemist or scientist for understanding the properties and behavior of such compounds to further develop processes and optimize them.
Compounds that contain at least one carbon-metal bond are called organometallic compounds. Zeise, in 1830, prepared the first organometallic compound by the action of ethylene on a solution of potassium chloroplatinate(II). In the last four decades, enormous work has been done in this field and many fascinating compounds have been synthesized and investigated. Grignard reagent, RMgX is a familiar example of organometallic compounds where R is an alkyl group. Diethyl zinc [Zn(C2H5)2], lead tetraethyl [Pb(C2H5)4], ferrocene [Fe(C5H5)2], dibenzene chromium[Cr(C6H6)2], metal carbonyls are other examples of organometallic compounds. The compounds of metalloids such as germanium and antimony and non-metallic elements such as boron and silicon are also included under this classification.
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Organometallic compounds may be classified into three classes:
These complexes contain a metal and carbon atom attached with a sigma bond e.g. Tetramethyl Tin, Trimethyl aluminum etc.
Bonding in Trimethyl aluminum is shown below
These complexes contain a metal and carbon atom attached with a Pi bond. e.g. Ferrocene, Dibenzene Chromium, etc. Bonding in Ferrocene and Dibenzene Chromium is shown below:
These complexes contain both $\sigma$ as well as $\pi$ bonding characteristics. e.g. Metal Carbonyls. The $M-C \sigma$ bond is formed by the donation of the lone pair of electrons of the carbonyl group into the vacant d orbital of metal while the $M-C \pi$ bond is formed by the back donation of the lone pair of electrons from the metal into vacant antibonding $\pi^*$ molecular orbital of CO. This synergic bonding leads to the formation of stronger bonds and stable metal carbonyl complexes. The bonding in metal carbonyls is shown below:
Organometallic compounds can be classified depending on the particular metal or the type of carbon-metal bond.
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The importance of organometallic compounds does not merely lie within the laboratories; they are also relevant in industries and real-life applications.
Example 1
Question: $Mn_2(CO)_{10}$ is an organometallic compound due to the presence of:
1) Mn-C bond
2) Mn-Mn bond
3) Mn-O bond
4) C-O bond
Solution: $Mn_2(CO)_{10}$ is classified as an organometallic compound because it contains a Mn-C bond. Therefore, the correct answer is option (1).
Example 2
Question: The number of bridging CO ligands in $[Mn_2(CO)_{10}]$ is __________.
1) 0
2)1
3)3
4)2
Solution: There are zero bridging CO ligands present in $[Mn_2(CO)_{10}]$. Thus, the answer is 0.
Example 3
Question: In the cobalt-carbonyl complex $[Co_2(CO)_8]$, the number of Co-Co bonds is "X" and terminal CO ligands is "Y". What is X + Y = __________?
1) 7
2)1
3)4
4) 6
Solution: In $[Co_2(CO)_8]$, there is 1 Co-Co bond (X = 1) and 6 terminal CO ligands (Y = 6). Therefore, X + Y = 7. The correct answer is 7.
Example 4
Question: The number of complexes that will exhibit synergic bonding among $[Cr(CO)_6]$, $[Mn(CO)_5]$, and $[Mn_2(CO)_{10}]$ is ________.
1) 3
2) 4
3) 8
4) 5
Solution: All three complexes are metal carbonyl complexes and exhibit synergic bonding. Hence, the answer is 3.
Example 5
Question: The oxidation states of iron atoms in compounds (A), (B), and (C), respectively, are x, y, and z. What is the sum of x, y, and z?
1) 6
2) 9
3) 4
4) 5
Solution: The sum of the oxidation states of iron atoms in compounds (A), (B), and (C) is 6. Therefore, the correct answer is 6.
In summary, organometallic compounds relate to a specific and very exciting area of chemistry, one that is built around the special bond between carbon and metal atoms. Many special properties come under their roof—property factors due to this special bonding, which render them irreplaceable in a lot of applications. We are going to learn the main definitions, and features of organometallic compounds, together with their classification and main-group and transition metal compounds.
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NCERT Chemistry Notes:
Tetramethyl aluminium
Tetraethyl lead
Ziese’s salt
Cis-Platin
Answer: d) Cis-platin
The chemical formula of cis-platin is [PtCl2(NH3)2][PtCl2(NH3)2]. Even though this compound has a metal it lacks a carbon atom. Hence it is not an organometallic compound.
Sodium ethoxide
Ethyl magnesium bromide
Ferrocene
Grignard reagent
Answer: d) sodium ethoxide
Organometallic compounds are those compounds in which carbon atoms and metal are directly bonded. But in sodium ethoxide oxygen is attached to sodium metal so it is not an organometallic compound.
Organometallic compounds can be defined as those chemical species in which the central atoms are bonded directly to one or more carbon atoms of organic functional groups.
a) Cis-Platin
b) Ferrocene
c) Sodium ethoxide
Answer: b) Ferrocene
Hapticity is the number of carbon atoms through which a ligand is attached. It is represented by the symbol ‘ηn’ where n is the number of carbon atoms of the ligand attached with the metal.
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