Animated Solution for Chemistry - Coordination Compounds: The theory that can completely/properly explain the nature of bonding in $[Ni(CO{)}_4]$ is :

Imagine you are looking at a molecule of Nickel tetracarbonyl, mathematically written as $[Ni(CO{)}_4]$. It is a fascinating coordination compound that exhibits extraordinary stability. For decades, chemists tried to explain the nature of the bond between the central metal atom and the carbon monoxide ligands using classical theories.

Werner's Theory, Valence Bond Theory (VBT), and Crystal Field Theory (CFT) are fantastic tools. They perfectly describe how a ligand donates a lone pair of electrons to a central metal atom to form a simple coordinate $\sigma$-bond. However, when applied to metal carbonyls, these theories fall short. They simply cannot account for the immense strength and unique properties of the metal-carbon bond in these complexes.

Synergic Bonding

To truly understand what is happening, we have to look deeper into a phenomenon called Synergic Bonding. This is a beautiful, two-way dance of electrons between the metal and the ligand.

First, the carbon atom of the $CO$ ligand donates its lone pair of electrons into an empty orbital of the Nickel atom. This forms a standard coordinate $\sigma$-bond. If the story ended here, the bond would be relatively weak.

But here is the catch: Nickel is rich in electron density, possessing filled $d$-orbitals. At the same time, the carbon monoxide molecule has empty ${\pi }^{\ast }$ (pi-star) antibonding orbitals. In a brilliant display of chemical reciprocity, the Nickel atom back-donates its electron density from its filled $d$-orbitals into the empty ${\pi }^{\ast }$ antibonding orbitals of the $CO$ ligand. This forms a $\pi$-bond, commonly referred to as $\pi$-backbonding.

This mutual push-and-pull of electrons strengthens the bond between the metal and the carbon atom, creating a synergic effect. But which theory actually allows us to talk about these mysterious ${\pi }^{\ast }$ antibonding orbitals?

Neither VBT nor CFT incorporates the concept of antibonding orbitals. The existence of bonding and antibonding molecular orbitals is the crowning achievement of Molecular Orbital Theory (MOT). Because the synergic bonding in metal carbonyls fundamentally relies on the involvement of these ${\pi }^{\ast }$ antibonding orbitals, only Molecular Orbital Theory can completely and properly explain the nature of bonding in $[Ni(CO{)}_4]$.

The Way Forward: Always remember that as the extent of $\pi$-backbonding increases, electron density is pumped into the antibonding orbital of the $CO$ ligand. This weakens the $C-O$ bond, decreasing its bond order and increasing its bond length. This is a high-yield concept for competitive exams!