Question

Which one of the following species responds to an external magnetic field ?

• A. $[Fe(H_2O)_6]^{3+}$
• B. $[Ni(CN)_4]^{2-}$
• C. $[Co(CN)_6]^{3-}$
• D. $[Ni(CO)_4]$

Hint:

Weak field ligands (halides, water, etc.) often result in high-spin paramagnetic complexes, especially for ions like \(Fe^{3+}\).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A species responds to an external magnetic field if it is paramagnetic, which requires the presence of one or more unpaired electrons.
Step 2: Key Formula or Approach:
Apply Crystal Field Theory (CFT) to determine the electronic configuration of the central metal ion.
Step 3: Detailed Explanation:
1. \([Fe(H_2O)_6]^{3+\):} \(Fe^{3+}\) is \(d^5\). \(H_2O\) is a weak field ligand (WFL). No pairing occurs. Configuration: \(t_{2g}^3 e_g^2\). 5 unpaired electrons. Paramagnetic.
2. \([Ni(CN)_4]^{2-\):} \(Ni^{2+}\) is \(d^8\). \(CN^-\) is a strong field ligand (SFL). The complex is square planar. All electrons are paired. Diamagnetic.
3. \([Co(CN)_6]^{3-\):} \(Co^{3+}\) is \(d^6\). \(CN^-\) is an SFL. Pairing occurs in \(t_{2g}\). Configuration: \(t_{2g}^6 e_g^0\). Diamagnetic.
4. \([Ni(CO)_4]\): \(Ni\) is \(d^{10}\) in this complex. All orbitals are filled. Diamagnetic.
Only \([Fe(H_2O)_6]^{3+}\) is paramagnetic.
Step 4: Final Answer:
The correct species is (A).