JEE Chemistry 2023 Question with Solution

Given: The complexes are $[CoF_6]^{3-}$, $[CoCl_6]^{3-}$, $[Co(NH_3)_6]^{3+}$ and $[NiCl_4]^{2-}$.

Find: The octahedral diamagnetic low-spin complex.

From the solution, the ligand field strength determines whether the complex is high spin or low spin.

For $[Co(NH_3)_6]^{3+}$, the coordination number is $6$, so it is octahedral. Ammonia, $NH_3$, is a strong field ligand.

Cobalt has electronic configuration

Therefore, $Co^{3+}$ has configuration

In the presence of strong field ligand $NH_3$, electrons pair up to give

All electrons are paired, so the complex is diamagnetic and it is a low-spin octahedral complex.

However, the solution states "The Correct Option is A" while its own working and final answer identify $[Co(NH_3)_6]^{3+}$, which corresponds to option C in the given options. Using the solution working as the authority, the correct option is C.

Given: the solution contains an internal mismatch.

Find: Which option label matches the compound concluded by the working.

The step-by-step explanation explicitly concludes that $[Co(NH_3)_6]^{3+}$ is diamagnetic, low-spin and octahedral.

Mapping the listed options:

1. $[CoF_6]^{3-}$ $\rightarrow$ A
2. $[CoCl_6]^{3-}$ $\rightarrow$ B
3. $[Co(NH_3)_6]^{3+}$ $\rightarrow$ C
4. $[NiCl_4]^{2-}$ $\rightarrow$ D

So although the solution says "The Correct Option is A", the worked solution clearly supports option C.

Therefore, the defensible answer from the solution content is C.