Metal Carbonyls – Organometallics

Carbonyls are coordination complexes that are formed by transition metals and the carbonyl ligand i.e. carbon-monoxide ligand. These are also termed homoleptic carbonyls. These complexes have a simple and well-defined structure which is generally tetrahedral and octahedral in nature. Nomenclature of Coordination Compounds is done using specific rules.

Structure of Metal Carbonyls

Bonding in Metal Carbonyl is achieved by the formation of both σ and π bonds. Thus the formation of metal carbonyl consists of two steps:

• Formation of bond: This bond is formed due to the donation of electrons by the carbonyl molecules in the vacant orbitals of the metal. This results in the overlap of the non-bonding orbitals of the Carbon atom with the vacant orbitals of the metal atom.

Formation of σ bond

• Formation of bond due to back bonding: This bond is formed due to the donation of pair of electrons from the filled d-orbitals of the metal atom to the vacant π^∗ (π antibonding) orbitals of the Carbonyl ligand or molecule. As electrons from the metal fill the π-antibonding orbital of CO, they then weaken the carbon-oxygen bond compared with the free carbon monoxide, while the metal-carbon bond is strengthened. This bond is formed due to back bonding as the metal atom donates a pair of electrons from its filled d-orbitals to the empty π– antibonding orbitals of the CO ligand. This type of bonding is also known as synergic bonding.

Formation of π bond

Bonding in Metal Carbonyls

The carbon monoxide ligand is a unique ligand which is different from other ligands. The bond between transition metals and metal carbonyls is achieved by “synergistic π star back-bonding.” The bonding is partially similar to the triple bond. The carbonyl (CO) ligand is unsaturated, i.e. it not only donates electrons to form σ– bond but also accepts electrons in its π^∗ orbital from d_π metal orbitals. Thus the CO ligand behaves as an acidic ligand.

Properties of Metal Carbligandonyls

The important properties of metal carbonyls are discussed below in the article:

• Metal carbonyls exist as solids at standard temperature and pressure i.e. at room temperature and standard atmospheric pressure (STP).
• They are toxic in nature and can be fatal if inhaled as the carbonyl ion in the metal carbonyl binds with the haemoglobin in the blood to form carboxyhaemoglobin which hinders the oxygen binding capacity of haemoglobin in the blood cells.
• They are volatile in nature as they have low melting points.
• They are soluble in both polar and non-polar solvents.
• On heating, they decompose to the respective metal and carbon monoxide.
• Both metal and Carbonyl ions are responsible for the reactivity of the metal carbonyls.
• They are carcinogenic in nature.

Types of Metal Carbonyls

Metal carbonyls can be classified into various types:

Based on Types of Ligands

• Homoleptic Metal Carbonyls: In homoleptic carbonyls, only one type of ligands i.e. CO ligands are present. They are represented using the general formula M_x​(CO)_y​
• Heteroleptic Metal Carbonyls: Heteroleptic carbonyls contain CO ligands along with other types of ligands such as -Cl, -Br, -I, etc. They are represented using the M_x​L_m​(CO)_y​ general formula.

Based on The Number of Metal Centres Involved in Forming The Complex

• Mononuclear metal carbonyl: These metal carbonyls have only one metal centre. The general formula of mononuclear metal carbonyls is M(CO)_x
• Polynuclear metal carbonyl: These metal carbonyls have two or more metal centres. The general formula of mononuclear metal carbonyls is M_x​N_y​(CO)_z​

Uses of Metal Carbonyls

• Nickel carbonyl is used to produce pure Nickel using the Mond process.
• Iron carbonyls are used to prepare dietary supplements.
• They are used in organic synthesis and as catalysts.
• They are being used as drugs to release CO.
• They are also used to create aldehydes.

Metal Carbonyls Organometallics

These compounds have at least one metal-carbon bond and the carbon atom is a part of an organic group. Some common examples of organometallics are Grignard Reagent – RMgX, Gilman Reagent – R₂CuLi, Dimethylmagnesium – Me₂Mg, Triethylborane – Et₃B, Ferrocene, Cobaltocene. These compounds are very important in the field of chemistry.

Properties of Organometallics

The properties of Organometallic compounds are:

• Most of them exist in a solid state.
• Some of them may be present in both liquid and gaseous states.
• Organometallics formed by electropositive elements such as lithium, aluminium can be flammable in nature.
• They are very toxic and volatile in nature.
• They are not soluble in water but in ethers.
• They are very reactive in nature.
• They have high electronegativity.

Uses of Organometallics

The uses of Organometallic compounds are:

• They are used as reagents in organic reactions.
• They are used in the treatment of syphilis.
• Grignard reagent which is an organometallic is widely used in organic chemistry.
• Cis-Plastin is used as an anti-cancer drug.
• They are used as additives.

FAQs on Metal Carbonyls

Question 1: Define metal carbonyls with examples.

Answer:

Metal carbonyls are coordination complexes that are formed by transition metals and the carbonyl ligand i.e. carbon-monoxide ligand. Ni(CO)₄ is an example of metal carbonyl.

Question 2: Give any 2 properties of metal carbonyls.

Answer:

• They are toxic in nature and can be fatal if inhaled as the carbonyl ion in the metal carbonyl binds with the haemoglobin in the blood to form carboxyhaemoglobin which hinders the oxygen binding capacity of haemoglobin in the blood cells.
• They are volatile in nature as they have low melting points.

Question 3: List any 3 uses of metal carbonyls R₂CuLi

Answer:

Uses of metal carbonyls are:

• Nickel carbonyl is used to produce pure Nickel using the Mond process.
• They are also used to create aldehydes.
• Iron carbonyls are used to prepare dietary supplements.

Question 4: What are the stages in the formation of metal carbonyls?

Answer:

The stages in the formation of metal carbonyls are:

• Formation of a sigma bond
• Formation of pi bond through back bonding

Question 5: What are organometallics?

Answer:

Those compounds that have at least one metal-carbon bond and the carbon atom is a part of an organic group is termed organometallics. Some common examples of organometallics are Grignard Reagent – RMgX, Gilman Reagent -R₂CuLi

Related Resources

• Molecular Orbital Theory
• Valance Bond Theory
• Bohr’s Atomic Model