Question

The Vaska’s complex trans-IrCl(CO)(PPh₃)₂ shows a band at 1967 cm⁻¹ for the \( \nu_{\text{CO}} \) stretching vibration in its infrared spectrum. The complex(es) that will show an increase in the \( \nu_{\text{CO}} \) stretching vibration from 1967 cm⁻¹ is/are:

• A.
• B.
• C.
• D.

Hint:

To predict shifts in CO stretching frequency, check the ligands: electron-donating ligands enhance back-donation (lower \( \nu_{{CO}} \)), while electron-withdrawing ligands reduce back-donation (higher \( \nu_{{CO}} \)).

Answer 1

The Correct Option is C, D

The carbonyl stretching frequency \( \nu_{\text{CO}} \) in metal carbonyl complexes is influenced by back-donation from the metal \( d \)-orbitals to the antibonding \( \pi^* \)-orbitals of CO.

• Increased back-donation \( \Rightarrow \) weaker C–O bond \( \Rightarrow \) lower \( \nu_{\text{CO}} \)
• Decreased back-donation \( \Rightarrow \) stronger C–O bond \( \Rightarrow \) higher \( \nu_{\text{CO}} \)

Let's evaluate each complex:

• (A) has strong donor phosphines, but overall electron density is similar to the original. Not a major increase expected.
• (B) contains an electron-donating –OMe group, increasing electron density at Ir and enhancing back-donation to CO \( \Rightarrow \) decrease in \( \nu_{\text{CO}} \).
• (C) has a strongly electron-withdrawing ligand (carbonyl itself bound as an L-type ligand), reducing back-donation \( \Rightarrow \) increase in \( \nu_{\text{CO}} \).
• (D) has an N-bound isocyanide \( \text{C} \equiv \text{N} \), a poor \( \pi \)-acceptor and weak back-donation environment, thus reduces metal-to-CO back-donation \( \Rightarrow \) increase in \( \nu_{\text{CO}} \).

\[ \boxed{\text{Correct options: (C), (D)}} \]