Synergic Bonding
Bonding in Metal Carbonyls | Coordination Chemistry
1. What is Synergic Bonding?
Definition: In metal carbonyls, the metal-carbon bond possesses both sigma ($\sigma$) and pi ($\pi$) character. This self-strengthening interaction, where the ligand donates electrons to the metal and the metal simultaneously donates electrons back to the ligand, is called the Synergic Effect.
Ligand involved: Carbon Monoxide ($CO$). It is a $\pi$-acid or $\pi$-acceptor ligand.
2. Mechanism of Bonding
The bonding involves two simultaneous components:
A. Formation of $\sigma$-bond
- Donation of a lone pair of electrons from the Carbon atom of the $CO$ molecule into a vacant d-orbital of the metal.
- Direction: Ligand ($L$) $\rightarrow$ Metal ($M$).
- Orbital: HOMO of CO ($\sigma_{2s}$) $\rightarrow$ Empty d-orbital of M.
B. Formation of $\pi$-bond (Back Bonding)
- Donation of a lone pair of electrons from a filled d-orbital of the metal into the empty antibonding $\pi^*$ orbital of Carbon.
- Direction: Metal ($M$) $\rightarrow$ Ligand ($L$).
- Orbital: Filled $d\pi$ of M $\rightarrow$ LUMO of CO ($\pi^*$).
Synergy: The $\sigma$-donation tends to make the metal negative. The $\pi$-back donation removes this negative charge. They strengthen each other.
3. Consequences of Synergic Bonding
The donation of electrons into the antibonding ($\pi^*$) orbital of $CO$ weakens the $C-O$ bond order.
| Parameter | Effect of Synergic Bonding | Reason |
|---|---|---|
| Metal-Carbon ($M-C$) Bond | Becomes Stronger & Shorter | Due to partial double bond character. |
| Carbon-Oxygen ($C-O$) Bond | Becomes Weaker & Longer | Electrons enter Antibonding $\pi^*$ of CO, reducing Bond Order. |
| C-O Stretching Frequency ($\nu_{CO}$) | Decreases | Bond becomes weaker (shifts from triple towards double bond). |
4. Factors Affecting Back Bonding
The extent of back bonding depends on the electron density on the central metal atom.
$$ \text{Negative Charge on Metal} \propto \text{Back Bonding} \propto \frac{1}{\nu_{CO}} $$
Trends in Isoelectronic Series:
Consider $[V(CO)_6]^-$, $Cr(CO)_6$, and $[Mn(CO)_6]^+$.
- $[V(CO)_6]^-$: Metal has -ve charge $\rightarrow$ Max Back Bonding $\rightarrow$ Weakest C-O Bond $\rightarrow$ Lowest $\nu_{CO}$.
- $[Mn(CO)_6]^+$: Metal has +ve charge $\rightarrow$ Min Back Bonding $\rightarrow$ Strongest C-O Bond $\rightarrow$ Highest $\nu_{CO}$.
Order of $\nu_{CO}$: $[Mn(CO)_6]^+ > Cr(CO)_6 > [V(CO)_6]^-$
Practice Quiz
Test your knowledge on Metal Carbonyls.
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