Metal-to-Ligand Charge Transfer (MLCT) & Synergic Bonding

The Structural Loop:

$$\begin{aligned} \text{Increased Back-Bonding (MLCT)} &\implies \text{Strengthens M-C bond} \implies \text{M-C bond order increases, bond length shortens} \\ &\implies \text{Weakens C-O bond} \implies \text{C-O bond order decreases, bond length stretches} \end{aligned}$$

Concept: Describes a self-strengthening covalent system where a neutral $$\sigma$$ -donor ligand ($$\text{CO}$$) shares its electron pair with the central metal, which simultaneously pumps electron density backward into the empty antibonding molecular orbital ($$\pi^*$$) of the ligand using its filled $$d$$-orbitals.

Notation:

• $$\pi^* \text{ orbital}$$ = Target destination of back-donated electron density;
• $$\text{Synergy}$$ = Two-way cooperative mechanism where $$\sigma$$ -donation enhances $$\pi$$ -back donation and vice versa.