Question

Match the complexes with their magnetic nature:

• A. A–I, B–II
• B. A–II, B–I
• C. A–II, B–II
• D. A–I, B–I

Hint:

Strong-field ligands (such as \(\text{CN}^-\), \(\text{CO}\)) typically force pairing, resulting in low-spin diamagnetic complexes.
Weak-field ligands (such as \(\text{F}^-\), \(\text{Cl}^-\)) do not force pairing, which often leads to high-spin paramagnetic complexes.
Knowing the spectrochemical series helps you quickly determine the magnetic nature of complexes.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:

This matching question requires us to determine whether the given coordination complexes are paramagnetic (containing unpaired electrons) or diamagnetic (containing only paired electrons).

Step 2: Key Formula or Approach:

We use Crystal Field Theory (CFT) to determine:

• The oxidation state of the metal ion
• The \(d\)-electron configuration
• The pairing behavior in the presence of strong-field or weak-field ligands

\[ \text{Paramagnetic: Unpaired electrons present } (n > 0) \] \[ \text{Diamagnetic: All electrons paired } (n = 0) \]
Step 3: Detailed Explanation:

• Analyzing Complex (A): \([\text{Fe}(\text{CN})_6]^{4-}\)

Let the oxidation state of iron be \(x\).

• \[ x + 6(-1) = -4 \] \[ x = +2 \]

Therefore, the metal ion is \(\text{Fe}^{2+}\).

Electronic configuration of \(\text{Fe}^{2+}\):

• \[ [Ar]\,3d^6 \]

The cyanide ion \((\text{CN}^-)\) is a strong-field ligand. It causes pairing of electrons in the \(t_{2g}\) orbitals.

Electron arrangement:

• \[ t_{2g}^{6} e_g^{0} \]

All electrons are paired, so the complex is diamagnetic.

Hence, (A) matches with (II).

• Analyzing Complex (B): \([\text{CoF}_6]^{3-}\)

Let the oxidation state of cobalt be \(y\).

• \[ y + 6(-1) = -3 \] \[ y = +3 \]

Therefore, the metal ion is \(\text{Co}^{3+}\).

Electronic configuration of \(\text{Co}^{3+}\):

• \[ [Ar]\,3d^6 \]

The fluoride ion \((\text{F}^-)\) is a weak-field ligand. It does not cause electron pairing.

Electron arrangement:

• \[ t_{2g}^{4} e_g^{2} \]

This configuration contains unpaired electrons, so the complex is paramagnetic.

Hence, (B) matches with (I).

Step 4: Final Answer:

The correct matching is:

\(A \rightarrow II\) 
\(B \rightarrow I\)

Therefore, the correct answer is Option (B).