Question

Metal-ligand $\pi$-bond formation in Mn$_2$(CO)$_{10}$ and [MnO$_4$]$^-$ requires electron-pair donation between metal and ligand orbitals. Which one of the following represents the direction of electron-pair donation?

• A. Mn$_2$(CO)$_{10}$: metal orbital $\rightarrow$ ligand orbital; [MnO$_4$]$^-$: ligand orbital $\rightarrow$ metal orbital
• B. Mn$_2$(CO)$_{10}$: ligand orbital $\rightarrow$ metal orbital; [MnO$_4$]$^-$: ligand orbital $\rightarrow$ metal orbital
• C. Mn$_2$(CO)$_{10}$: metal orbital $\rightarrow$ ligand orbital; [MnO$_4$]$^-$: metal orbital $\rightarrow$ ligand orbital
• D. Mn$_2$(CO)$_{10}$: ligand orbital $\rightarrow$ metal orbital; [MnO$_4$]$^-$: metal orbital $\rightarrow$ ligand orbital

Hint:

Remember:
- Metal Carbonyls ($\text{M-CO}$) $\rightarrow$ Back-bonding ($\text{M} \rightarrow \text{CO}$) is the source of the $\pi$-bond!
- Metal Oxo complexes with metals in high oxidation states (like $\text{CrO}_4^{2-}$, $\text{MnO}_4^-$) $\rightarrow$ Oxygen acts as a strong $\pi$-donor, donating electron density from ligand to metal ($\text{L} \rightarrow \text{M}$).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
We need to distinguish between $\pi$-back-bonding (metal to ligand) and $\pi$-donation from ligands (ligand to metal).

Step 2: Detailed Explanation:
$\bullet$ $Mn_{2(CO)_{10}$ (Metal Carbonyl):} Carbon monoxide is a $\pi$-acid ligand. It accepts electron density from the metal's filled d-orbitals into its empty $\pi^{*}$ orbitals. This is called back-donation. Direction: Metal $\rightarrow$ Ligand.
$\bullet$ $[MnO_{4}]^{-$ (Permanganate):} In this oxo-complex, Manganese is in its highest oxidation state ($+7$, $d^{0}$). It has no electrons to give. Instead, the oxo ligands ($O^{2-}$) act as $\pi$-donors, donating electron density from their filled p-orbitals into the empty d-orbitals of the metal. Direction: Ligand $\rightarrow$ Metal.

Step 3: Final Answer:
Direction for Carbonyl is M$\rightarrow$L; direction for Permanganate is L$\rightarrow$M.
This matches option (A).