JEE Chemistry 2025 Question with Solution

Given: We must identify the homoleptic complexes in which the central metal has an odd number of $d$-electrons.

Find: Which listed species satisfy both conditions, and then choose the matching option.

Check each complex using oxidation state and $d$-electron count:

• (A) $[FeO_4]^{2-}$: The oxidation state of iron is $+6$, so iron has $d^0$ configuration. This is an even number of $d$-electrons, so it does not qualify.
• (B) $[Fe(CN)_6]^{3-}$: The oxidation state of iron is $+3$, so iron has $d^5$ configuration. Since $5$ is odd, this complex qualifies.
• (C) $[Fe(CN)_6]^{2-}$: The oxidation state of iron is $+2$, so iron has $d^6$ configuration. Since $6$ is even, this does not qualify.
• (D) $[CoCl_4]^{2-}$: The oxidation state of cobalt is $+2$, so cobalt has $d^7$ configuration. Since $7$ is odd, this complex qualifies.
• (E) $[Co(H_2O)_6]^{3+}$: The oxidation state of cobalt is $+3$, so cobalt has $d^6$ configuration. Since $6$ is even, this does not qualify.

Thus, the homoleptic complexes with odd number of $d$-electrons are (B) and (D) only.

Therefore, the correct option is A.

There is a discrepancy in the provided the solution: one section states "The Correct Option is A" while another writes the final answer as (1) (B) and (D) only. These are consistent because option A corresponds to choice (1).

Given: Determine the oxidation state of the metal in each complex and then count the remaining $d$-electrons.

Find: Which complexes are homoleptic and contain an odd number of $d$-electrons.

For (A) $[FeO_4]^{2-}$:

Iron is $Fe^{6+}$, so it has $d^0$ configuration. Not odd.

For (B) $[Fe(CN)_6]^{3-}$:

Iron is $Fe^{3+}$, so it has $d^5$ configuration. Odd.

For (C) $[Fe(CN)_6]^{2-}$:

the solution states this species has even $d$-electron count and excludes it from the answer. Hence it is not selected.

For (D) $[CoCl_4]^{2-}$:

Cobalt is $Co^{2+}$, so it has $d^7$ configuration. Odd.

For (E) $[Co(H_2O)_6]^{3+}$:

Cobalt is $Co^{3+}$, so it has $d^6$ configuration. Even.

Therefore, only (B) and (D) satisfy the condition, so the correct option is A.