What do you know about carbonyl ligands? Or is it the first time you are actually coming across this term? Well, do not worry! We will help you in that. In this chapter, we will look at the concept of bonding in various metal carbonyls. However, before we proceed, let us first look at what carbonyls are?
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Brief about Metal Carbonyls
We are aware of many complex compounds having carbonyl ligands, We call them homoleptic carbonyls. We can, otherwise, call them metal carbonyls. So, which elements form these carbonyls? Maximum of the transition metals specialise in forming homoleptic carbonyls or metal carbonyls. These have simple and very well-defined structures. For example, their structures are usually tetrahedral, octahedral etc.
In a metal carbonyl, the metal-carbon bond has the characteristics of both σ and π bonds. The bond between the carbonyl molecule and the metal becomes stronger by the synergic effect that the metal-ligand bond produces. We will explain the two types of bonding in metal carbonyls in the section below.
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Structure of Metal Carbonyls
The formation of a metal-carbon σ bond takes place because of the donation of electrons by the carbonyl molecules. To what do they donate these electrons? They donate it to the vacant orbitals of the metal. This is one way of formation of the metal carbonyls.
The other form is the creation of a metal-carbon π bond because of the donation of a pair of electrons from a filled d orbital metal into the vacant antibonding π* orbital of carbonyl ligand.
Stability of Carbonyl Compounds
How stable are these coordination compounds? Well, we have practical evidence that suggests these compounds to break down or dissociate in various solutions. The stability of a coordination compound in a solution mainly depends on upon the degree of association between the two species involved in the state of equilibrium.
Quantitatively speaking, the stability of any complex is the magnitude of the equilibrium constant for the formation of the compound. A general example is given below:
A  +  4B  →  AB4
In the above example, the amount of AB4 molecule in the solution depends on upon the value of the equilibrium constant, k. This k is also what we know as the stability constant. On the other hand, the instability constant or the dissociation constant of complexes is nothing but the reciprocal of the equilibrium constant of the formation reaction.
Solved Example for You
Q: Write a note on the physical characteristics of carbonyl compounds.
Ans:
- Most of these carbonyl compounds are colourless or pale yellow in colour and are generally volatile in nature.
- We can find them as liquids or solids that are flammable and toxic. For example, Vanadium hexacarbonyl, which is a stable 17-electron metal carbonyl, is a blue-black solid. It is a unique compound. Di- and polymetallic carbonyls have deeper colours as compared to the others. The crystalline metal carbonyls usually undergo sublimation in the vacuum.
- Metal carbonyls are soluble in both non-polar and polar organic solvents. These include benzene, diethyl ether, and carbon tetrachloride. Some salts of metal carbonyls are soluble in water or lower alcohols.
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