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 Question:
This question asks for the direction of electron-pair transfer during the formation of $\pi$-bonds in two manganese complexes: the neutral organometallic dimer $\text{Mn}_2(\text{CO})_{10}$ and the permanganate anion $[\text{MnO}_4]^-$.

Step 2: Detailed Explanation:
Let's analyze the bonding mechanism in each coordination species:

• 1. Mn$_2$(CO)$_{10$ (Metal Carbonyl Complex):}
- Bonding in metal carbonyls involves a synergistic interaction.
- First, a $\sigma$-bond is formed by the donation of a lone pair from the carbonyl carbon to a vacant $d$-orbital on the metal (ligand $\rightarrow$ metal).
- Second, a $\pi$-bond is formed via back-donation (or back-bonding).
- In this step, filled $d$-orbitals of the low-valent Manganese metal donate electron density into the vacant, low-lying antibonding $\pi^*$ orbitals of the carbon monoxide ($\text{CO}$) ligand.
- Thus, the formation of the $\pi$-bond occurs via the direction: metal orbital $\rightarrow$ ligand orbital.

• 2. [MnO$_4$]$^-$ (Permanganate Ion):
- In $[\text{MnO}_4]^-$, Manganese is in its highest oxidation state of $+7$ ($\text{Mn}^{7+}$).
- The electronic configuration of $\text{Mn}^{7+}$ is $d^0$, meaning it has completely empty $d$-orbitals and a very high positive charge density.
- The oxo ligands ($\text{O}^{2-}$) act as strong $\pi$-donors.
- The $\pi$-bonds are formed by the overlap of filled $2p$ orbitals of the oxygen ligands with the vacant $3d$ orbitals of the metal center. This is also responsible for ligand-to-metal charge transfer (LMCT) transitions.
- Thus, the direction of electron-pair donation for $\pi$-bonding is: ligand orbital $\rightarrow$ metal orbital.

Step 3: Final Answer:
For $\text{Mn}_2(\text{CO})_{10}$, the $\pi$-bond is metal $\rightarrow$ ligand. For $[\text{MnO}_4]^-$, the $\pi$-bond is ligand $\rightarrow$ metal. This corresponds to option (A).