Question

In which of the following sets, given species are not only diamagnetic in nature but also inner orbital complexes?

• A. I, II only
• B. I, III only
• C. II, III only
• D. I, II, III

Hint:

Strong field ligands (like $CN^-$) typically force pairing and favor inner orbital complex formation.

Answer 1

The Correct Option is B

Step 1: Understand the two conditions.
We need species that are both diamagnetic (all electrons paired, so no unpaired electron) and inner orbital complexes (using inner $(n-1)d$ orbitals in hybridisation).

Step 2: Recall what inner orbital means.
Strong field ligands like $CN^-$ and $en$ pair up the d electrons and pull them so the metal uses inner $3d$ orbitals, giving $d^2sp^3$ or $dsp^2$ hybridisation. These are low spin and usually diamagnetic.

Step 3: Test the iron cyanide complex.
In $[Fe(CN)_6]^{4-}$, $Fe^{2+}$ is $d^6$. The strong $CN^-$ pairs all six d electrons, giving $d^2sp^3$ with no unpaired electron. So it is inner orbital and diamagnetic. This one qualifies.

Step 4: Test the cobalt en complex.
In $[Co(en)_3]^{3+}$, $Co^{3+}$ is $d^6$. With the strong $en$ ligand the electrons pair, giving $d^2sp^3$ and no unpaired electron. So it is inner orbital and diamagnetic. This one qualifies too.

Step 5: Reject the species that fail.
A $d^8$ square planar like $[Ni(CN)_4]^{2-}$ is diamagnetic but uses $dsp^2$, and a $d^4$ low spin like $[Mn(CN)_6]^{3-}$ keeps unpaired electrons, so it is not diamagnetic. The set that meets both rules matches I and III.

Step 6: State the answer.
Only the species in set I and III are both diamagnetic and inner orbital.
\[ \boxed{\text{I, III only}} \]