Bonding in Metal Carbonyls

Homoleptic carbonyls, also known as metal carbonyls, are complex compounds that contain only carbonyl ligands (carbon monoxide). Most of the transition metals generate homoleptic carbonyls or metal carbonyls. The geometries and structures of these metal carbonyls are characterised. The synergic effect is responsible for forming the metal-carbon bond in a metal carbonyl that has both sigma and pi character and for strengthening the link between the carbonyl molecule and the metal.

Bonding in Metal Carbonyls 

Metal carbonyls are coordination complexes formed by transition metals and carbon mono-oxide ligands. Ludwig Mond created the first metal carbonyl compound, Ni(CO)4, in 1884. Carbon monoxide is a unique ligand. It absorbs electrons from dπ metal orbitals in its π∗ orbital. Thus, the CO ligand behaves as an acidic ligand. Additionally, CO is a weak ligand compared to other ligands such as H2O or alkoxides (RO–). Let us examine the bonding of the CO ligand with carbonyl compounds.

A lone pair of electrons is present on the carbon and oxygen atoms in the carbon monoxide ligand. Carbonyls are created when carbon atoms donate electrons to the transition metals. 

Metal carbonyls are compounds where the metal and carbon bond (M-C) has both s and p character. Carbon monoxide (CO), acting as a ligand, attaches itself to the metal atoms, forming the M-C bond. CO is a poor donor.

A lone pair of electrons on the carbonyl carbon must be donated into a vacant orbital of metal to form the M-C sigma bond. In contrast, to form the M-C π bond, a lone pair of electrons must be donated from a filled d orbital of metal to the vacant antibonding π∗  orbital of carbon monoxide. Synergic bonding describes this back bonding ability that helps to stabilise the metal-ligand contact.

Structure of Metal Carbonyls 

Tetracarbonylnickel (0), pentacarbonyl iron (0) and hexacarbonyl chromium(0) have the shapes of an octahedron.

• Sigma bonds between metals and carbon are created when the carbonyl molecule donates electrons to empty orbitals in the metal’s atoms.
• In the case of carbonyl ligands, a metal-carbon pi bond is produced when two electrons are donated from a metal’s filled d orbital into the vacant anti-bonding π* orbital (this sort of bonding is also referred to as back bonding of the carbonyl group).
• In this case, the sigma bond strengthens the pi bond and increases the sigma bond (vice-versa).

Stability of Metal Carbonyls

• The stability of coordination compounds in a solution is primarily determined by the degree of association between the species involved in the state of equilibrium.
• The value of the equilibrium constant for creating the compound describes the stability of the complex in question.

For example,

P + 4Q → PQ4

Where- 

P – is central metal atom or ion

Q – is monodentate ligand 

• The amount of PQ4 present in the solution is dependent on the value of the equilibrium constant, k, in this situation. The dissociation constant of complexes, known as the instability constant, is the reciprocal of the equilibrium constant (and thus, the stability constant of the reaction) in a given reaction.
• Consequently, the greater the stability constant value, the higher the amount of PQ4 present in the solution.
• Because free metal ions are scarce in solution, P is surrounded by solvent molecules, which will compete with the ligand molecules Q..

Physical Characteristics of Metal Carbonyls

• Most of the carbonyl compounds are either colourless or pale yellow in appearance. Compared to other carbonyls, di and polymetallic carbonyls have a rich colouration. Triiron dodecacarbonyl [ Fe3(CO)12 ] crystallises to form dark green crystals.
• Metal carbonyls are typically volatile.
• They can be found as liquids and solids and are combustible and hazardous in their natural state, for example, vanadium hexacarbonyl.
• Crystalline metal carbonyl compounds are most frequently sublimed without oxygen.
• Metal carbonyls are soluble in both nonpolar and polar organic solvents. They are particularly soluble in water. For instance, benzene, acetone, glacial acetic acid, diethyl ether, and carbon tetrachloride are all organic compounds. Few salts of cationic and anionic metal carbonyls are soluble in water or lower alcohol concentrations.

Explanation:

• Let us consider carbon monoxide bonding (with a transition metal) to explain the bonding in metal carbonyls.
• Carbon monoxide binds with transition metals via the synergistic π* back-bonding.
• Three components combine to produce a partial triple bond.
• A sigma bond is formed when a non-bonding (weakly anti-bonding) sp hybridised electron pair on carbon overlaps with a mixture of s-, p-, d- orbitals on the metal.
• The overlapping of full d-orbitals on the metal with a pair of π∗  anti-bonding orbitals from the carbon atom of carbon monoxide results in a pair of π bonds. This type of bonding needs the presence of d-electrons in the metal and the metal being in a relatively low oxidation state, favouring the reverse-donation of electron density.
• Metal electrons fill the antibonding orbital of carbon monoxide, weaken the carbon-oxygen bond relative to free carbon monoxide and increase the strength of the metal-carbon bond.

Conclusion

The CO ligand is a specific ligand of organometallic chemistry. The CO ligands attach securely to the metal centre via a synergistic mechanism involving the donation of the CO ligand lone pair to the metal’s vacant orbitals. It is subsequently followed by a π back donation from a filled metal d orbital to a vacant CO ligand σ∗ orbital.