Bonding in Metal Carbonyls: Definition, Examples and Diagram

The complex compounds that contain only carbonyl ligands (Carbon Monoxide) are termed as homoleptic carbonyls or Metal Carbonyls. Out of all the transition metals, most of them do form homoleptic carbonyls or metal carbonyls. These metal carbonyls have simple and well-defined shapes and structures.The metal carbon bond in a metal carbonyl that possess both sigma and pi character and the strength of the bond between carbonyl molecule and the metal is due to the synergic effect.

Keyterms: Complex Compounds, Carbonyl, Carbon Monoxide, Metal carbon Bond, Electron, Sigma Bond, pi Bond, Synergic Effect, Complex Nomenclature

Bonding in Metal Carbonyls

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In Metal Carbonyls, the metal and carbon bond (M-C) possess both s and p character. To form the M-C bond, Carbon Monoxide (CO) as a ligand binds itself to the metal atoms with atom atom. CO is a weak donor.

The M-C sigma bond is formed by donating the lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal and the M-C pi bond is formed by donating the lone pair of electrons from a filled d orbital of metal to the vacant antibonding pi* orbital of the carbon monoxide. This back bonding property which stablises the metal ligand interaction is termed as the Synergic Bonding. 

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Structure of Metal Carbonyls

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The metal carbonyls have a very simple and well-defined shapes and structures. For example:- tetracarbonylnickel(0) is Tetrahedral, pentacarbonyliron(0) is trigonal bipyramidal and hexacarbonyl chromium (0) is octahedral.

• A metal-carbon sigma bond is formed when the carbonyl molecule donate electrons to the vacant orbitals of the metal.
• A metal-carbon pi bond is formed when a pair of electrons are donated from a filled d orbital of a metal into the vacant anti bonding π* orbital (this type of bonding is also known as back bonding of the carbonyl group) of carbonyl ligand.

• Here, the sigma bond strengthens the pi bond and pi bond strengthens the sigma bond (vice-versa).

Stability of Carbonyl Compounds

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• In a solution, the stability of coordination compounds mainly depend on the degree of association between the species involved in the state of equilibrium. 
• The magnitude of the equilibrium constant for the formation of the compound says about the stability of that complex. 

           For example, 

                    P + 4Q → PQ₄

• Here, the amount of PQ₄ present in the solution depends upon the value of equilibrium constant,k. This k is also well known as stability constant and the dissociation constant of the complexes which is also known as instability constant of the complexes is the reciprocal of the equilibrium constant (stability constant) of the reaction.
• So, the larger the value of stability constant, the higher is the proportion of PQ4 that is in the solution. 
• Free metal ions are rare in the solution so that P is surrounded by solvent molecules which will compete with the ligand molecules Q.

Physical Characteristics of Carbonyl Compounds

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• Most of the carbonyl compounds are colourless or pale yellow in colour. Di and polymetallic Carbonyls possess deeper colour when compared to others. Triiron dodecacarbonyl forms deep green crystals
• They are generally volatile in nature.
• They can be found as liquids and solids which are flammable and toxic. Example:- Vanadium Hexacarbonyl. 
• Crystalline metal carbonyl compounds usually undergo sublimation in the vacuum. 
• Metal Carbonyls are soluble in non-polar as well as polar organic solvents. example:- benzene, acetone, glacial acetic acid, diethyl ether and carbon tetrachloride. Few salts of cationic and anionic metal Carbonyls are soluble even in water or lower alcohols.

Explanation:- 

• Let's take Carbon Monoxide bonding (with transition metal) to understand the bonding in Metal Carbonyls 
• Using the synergistic pi* back-bonding, carbon monoxide bonds with transition metals.

• A partial triple bond is formed due to three components.
• Here, a sigma bond arises due to the overlap of the non-bonding (weakly anti-bonding) sp hybridised electron pair on carbon with a mix of s-,p-,d- orbitals on the metal.
• A pair of pi bonds are formed due to the overlapping of filled d-orbitals on the metal with a pair of π* anti-bonding orbitals coming from the carbon atom of carbon monoxide. This kind of bonding requires the metal to have d-electrons and metal is in relatively low oxidation state which makes the back-donation of electron density favorable. 
• Electrons from the metal fill the π-antibonding orbital of Carbon Monoxide and this weakens the carbon-oxygen bond compared with free carbon monoxide and at the same time metal-carbon bond strength is increased.

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