Definition bonding structure preparation reactions and 18 electron rule in metal carbonyls
What are Metal Carbonyls?
Metal Carbonyls can be defined as compounds that are volatile and have low melting points. They are made from the compound of Mx(Co)y that decomposes into carbon monoxide and metal on heating. They can be toxic when in contact with skin. They can also be toxic if inhaled or ingested due to their property of carbonylate haemoglobin which converts it into carboxyhemoglobin that further prevents the binding of oxygen in the blood cells. Furthermore, in a metal carbonyl, both characters, σ, and π, are possessed by the metal-carbon bond. This bond is further strengthened by the synergic effect produced by the metal-ligand bond.
Metal Carbonyls and Their Structure
When learning about metal carbonyls, it is important to study their structure as well. One of the unique properties of metal carbonyls is that they exist in two types of bonding. The first kind exists when there is a donation of electrons by the carbonyl molecules to the vacant orbitals of the metal, thereby forming a metal-carbon σ bond.
The second type is when there is a donation of a pair of electrons from a filled d orbital metal into the vacant anti bonding π* orbital of a carbonyl ligand. This way a metal-carbon π bond is formed. The 18-electron rule is generally used in predicting the stability of metal carbonyls. According to this rule, the electrons are gained from the ligands by the metal atom to reach the nearest noble gas configuration.
What Are Metal Carbonyls Organometallics?
Metal Carbonyl Organometallics can be defined as compounds that consist of at least one metal-to-carbon bond. Moreover, the carbon in the metal-to-carbon bond is part of an organic group. The organometallic compounds play a major role in the development of the science of chemistry.
An example of an organometallic compound is ferrocene in which an iron atom is in between two hydrocarbon rings. There are great variations amongst the physical and chemical properties of organometallic compounds. Most of them are solid, specifically those with ring-shaped hydrocarbon groups.
Some organometallic compounds are present both in liquid and gaseous states. They can also be flammable, particularly the compounds of electropositive elements such as lithium, aluminium, and sodium. Major organometallic compounds are highly toxic and volatile. Some common examples of organometallics are Grignard Reagent - RMgX, Gilman Reagent - R2CuLi, Dimethylmagnesium - Me2Mg, Triethylborane - Et3B, Ferrocene, Cobaltocene.
Properties of Metal Carbonyls Organometallics
The 18-electron rule followed by metal carbonyl is surprisingly not followed by metal carbonyls organometallics. Some other basic properties of metal carbonyl organometallics are specified below:
Organometallics are not soluble in water
Instead, they are soluble in ether
Metal Carbonyls Organometallics has a relatively low melting point
Another interesting property of organometallics is their electronegativity. While metals have a lower electronegativity of 20, the organometallic carbon compound has an electronegativity of 2.5.
Organometallic compounds are also highly reactive. That is the reason why they are generally kept in organic solvents.
Uses of Metal Carbonyl Organometallic
The discovery of organometallic compounds has led to their application in various fields. Some of the popular uses of organometallics are specified below:
The most common use of organometallic compounds is as a reagent.
Organoarsenic compounds are also used in the treatment of a common sexually transmitted disease called syphilis.
Grignard reagent, which is a popular organometallic compound is used for various purposes such as in the synthesis of a secondary alcohol, aldehydes, etc.
Another use of organometallic is as an additive
They are also useful for various industrial purposes.
Cis-plastin, an organometallic compound, is used as an anticancer drug
For hydrogenation alkenes, Wilkison’s catalyst is used
FAQs on Metal Carbonyls in Organometallic Chemistry
1. What are metal carbonyls in organometallic chemistry?
Metal carbonyls are organometallic compounds in which carbon monoxide (CO) is directly bonded to a transition metal through a metal–carbon bond.
- They contain at least one M–CO bond, where M is usually a transition metal.
- CO acts as a neutral ligand and donates electron density via the carbon atom.
- Examples include Ni(CO)4, Fe(CO)5, and Cr(CO)6.
- They are important in organometallic chemistry, catalysis, and metal–ligand bonding studies.
2. How does carbon monoxide bond to a metal in metal carbonyls?
Carbon monoxide bonds to a metal through synergic bonding involving σ-donation from CO to the metal and π-backbonding from the metal to CO.
- σ-Donation: The lone pair on carbon donates electron density to an empty metal orbital.
- π-Backbonding: Filled metal d-orbitals donate electron density back into the π* antibonding orbital of CO.
- This synergic effect strengthens the M–C bond and weakens the C≡O bond.
- It explains the stability and unique bonding in metal carbonyl complexes.
3. What is the 18-electron rule in metal carbonyls?
The 18-electron rule states that many stable metal carbonyl complexes have a total of 18 valence electrons around the metal center.
- Transition metals can accommodate 18 electrons (s, p, and d orbitals).
- Each CO ligand donates 2 electrons as a neutral ligand.
- Example: In Fe(CO)5, Fe (8 valence electrons) + 5 CO (10 electrons) = 18 electrons.
- Complexes obeying this rule are often particularly stable.
4. What are the different types of metal carbonyls?
Metal carbonyls are classified as mononuclear, polynuclear, and bridged carbonyl complexes based on their structure.
- Mononuclear: Contain one metal atom, e.g., Ni(CO)4.
- Polynuclear: Contain two or more metal atoms, e.g., Fe2(CO)9.
- Bridging carbonyls: CO bridges between two metals (μ-CO).
- Classification depends on the number of metal centers and bonding mode of CO.
5. Why are metal carbonyls considered organometallic compounds?
Metal carbonyls are considered organometallic compounds because they contain a direct bond between a metal and the carbon atom of carbon monoxide.
- The defining feature of organometallic chemistry is a metal–carbon bond.
- In metal carbonyls, CO is bonded through its carbon atom to the metal.
- This M–C bond distinguishes them from simple coordination compounds without metal–carbon bonding.
6. What is the structure and hybridization of Ni(CO)4?
Ni(CO)4 has a tetrahedral structure with sp3 hybridization at the nickel center.
- Nickel is in the zero oxidation state.
- It forms four σ-bonds with four CO ligands.
- The geometry is tetrahedral with bond angles of approximately 109.5°.
- It obeys the 18-electron rule (10 from Ni + 8 from four CO).
7. How are metal carbonyls prepared?
Metal carbonyls are commonly prepared by the direct reaction of a finely divided metal with carbon monoxide under controlled temperature and pressure.
- Example: Ni(s) + 4CO(g) → Ni(CO)4(g) (at moderate temperature and pressure).
- Example: Fe(s) + 5CO(g) → Fe(CO)5(l) (at high pressure and temperature).
- Some are formed by reduction of metal salts in the presence of CO.
- Conditions vary depending on the metal and its reactivity.
8. What is the oxidation state of the metal in metal carbonyls?
In most simple metal carbonyls, the metal is in the zero oxidation state because CO is a neutral ligand.
- Carbon monoxide donates electrons but carries no charge.
- In Fe(CO)5, Fe has oxidation state 0.
- In Cr(CO)6, Cr also has oxidation state 0.
- However, substituted or ionic carbonyl complexes may have different oxidation states.
9. What is the difference between terminal and bridging carbonyl ligands?
A terminal carbonyl ligand is bonded to only one metal atom, while a bridging carbonyl ligand connects two or more metal atoms.
- Terminal (CO): Found in mononuclear complexes like Cr(CO)6.
- Bridging (μ-CO): Present in polynuclear complexes like Fe2(CO)9.
- Bridging carbonyls typically show lower C–O stretching frequencies in IR spectroscopy.
10. What are the uses of metal carbonyls in chemistry and industry?
Metal carbonyls are widely used as catalysts, metal refining agents, and precursors in organometallic synthesis.
- Ni(CO)4 is used in the Mond process for nickel purification.
- Carbonyl complexes serve as catalysts in hydroformylation and other industrial reactions.
- They act as starting materials for synthesizing other organometallic compounds.
- They are important in research on metal–ligand bonding and homogeneous catalysis.
