Which is not true about metal carbonyls?

Options

A. Here CO acts as a Lewis base as well Lewis acid
B. here metal acts as lewis base as well as lewis acid
C. here `d pi- p pi` back bonding takes place
D. Here `p pi-p pi` ack bonding takes place

Explanation

To determine which statement is not true about metal carbonyls, we need to analyze the properties and characteristics of these compounds. Here’s a step-by-step solution:

### Step 1: Understand Metal Carbonyls
Metal carbonyls are coordination compounds formed when transition metals bond with carbon monoxide (CO) ligands. The CO molecule has a carbon atom that is bonded to an oxygen atom via a double bond (C=O), and it can act as a ligand.

**Hint:** Recall that ligands donate electron pairs to the metal center in coordination compounds.

### Step 2: Identify the Nature of CO as a Ligand
CO is known to act as both a Lewis base and a Lewis acid. As a Lewis base, it donates a pair of electrons to the metal, while as a Lewis acid, it can accept electrons back from the metal through back bonding.

**Hint:** Consider the dual role of CO in bonding and how it interacts with the metal.

### Step 3: Examine Back Bonding
In metal carbonyls, back bonding occurs where the metal donates electron density to the π* (antibonding) orbitals of the CO ligand. This dπ-pπ back bonding is a significant feature of metal carbonyls and stabilizes the metal-ligand bond.

**Hint:** Focus on the types of bonding interactions that occur between the metal and the CO ligand.

### Step 4: Analyze the Statements
1. CO acts as a Lewis base and Lewis acid - True.
2. The metal acts as a Lewis base and Lewis acid - True.
3. dπ-pπ back bonding occurs - True.
4. Pπ-Pπ back bonding occurs - Not true.

**Hint:** Look for inconsistencies in the bonding types mentioned in the statements.

### Conclusion
The statement that is not true about metal carbonyls is that Pπ-Pπ back bonding occurs. This is incorrect because the bonding in metal carbonyls primarily involves dπ-pπ back bonding between the metal and the CO ligand.

### Final Answer
The statement that is not true about metal carbonyls is: "Pπ-Pπ back bonding occurs."