Which is not true about metal carbonyls?

Here, p­π − p­π back bonding takes place

The answer is option 4. pπ − pπ back bonding takes place.

Metal carbonyl complexes are coordination compounds where carbon monoxide \((CO)\) acts as a ligand, binding to a central transition metal atom. These complexes are notable for their unique bonding interactions, which involve both the metal center and the \(CO\) ligands. Let's delve into the details:

Bonding in Metal Carbonyls

CO as a Ligand:

Carbon monoxide, despite being a neutral molecule, acts as a ligand in metal carbonyl complexes by donating a pair of electrons from its lone pair on carbon to form a coordinate bond with the metal center. CO is a strong field ligand, meaning it induces significant splitting of the metal d-orbitals in the complex.

Metal as a Lewis Acid and Lewis Base:

Lewis Acid: The metal center in metal carbonyls acts as a Lewis acid because it accepts electron density from the lone pair on the CO ligand.

Lewis Base: Conversely, the metal center can act as a Lewis base when it donates electron density back to the CO ligands through dπ ­− pπ back bonding.

Back Bonding (dπ ­− pπ Back Bonding):

One of the distinctive features of metal carbonyls is \(dπ ­− pπ\) back bonding. This occurs when electrons from filled d-orbitals on the metal (dπ electrons) interact with the \(π\)-antibonding orbitals of the CO ligands (\(pπ^*\) orbitals).

Mechanism: The metal donates electron density from its d-orbitals into the \(π^*\) antibonding orbitals of the CO ligands.

Purpose: This interaction stabilizes the complex by strengthening the metal-ligand bond through partial electron sharing, thereby reducing the overall bond order of the CO ligands.

Effect: It also affects the electronic structure and properties of the metal center, influencing its reactivity and spectroscopic properties.

Absence of \(pπ ­− pπ\) Back Bonding:

In contrast to \(dπ ­− pπ\) back bonding, \(pπ ­− pπ\) back bonding between CO ligands themselves is not typically observed in metal carbonyl complexes. CO ligands have filled π orbitals that can interact with empty \(π^*\) antibonding orbitals of adjacent CO ligands, but this interaction does not typically contribute significantly to the bonding in metal carbonyls. Therefore, the statement suggesting pπ ­− pπ back bonding takes place in metal carbonyls is not true.

Summary:

Metal carbonyl complexes are characterized by their complex bonding interactions involving CO ligands acting as both Lewis bases and Lewis acids, and metal centers engaging in \(dπ ­− pπ\) back bonding with CO ligands. This back bonding is crucial for understanding the stability and reactivity of metal carbonyls in various chemical processes and catalysis.

In conclusion, the correct option that is not true about metal carbonyls is: Here, \(pπ ­− pπ\) back bonding takes place. This option is incorrect because \(pπ ­− pπ\) back bonding between CO ligands themselves is not a typical feature of metal carbonyl complexes.