Identify the coordination complexes in which the central metal ion has $$d^{4}$$ configuration.

Choose the correct answer from the options given below :

• (A) $$[\text{FeO}_4]^{2-}$$:

  • Oxidation state of Fe: $$x + 4(-2) = -2 \implies x = +6$$
  • Iron(VI) configuration: Neutral $$\text{Fe}$$ is $$[Ar] 3d^6 4s^2$$, so $$\text{Fe}^{6+}$$ is $$[Ar] 3d^2$$.
  • This is a $$d^2$$ configuration.

• (B) $$[\text{Mn}(\text{CN})_6]^{3-}$$:

  • Oxidation state of Mn: $$x + 6(-1) = -3 \implies x = +3$$
  • Manganese(III) configuration: Neutral $$\text{Mn}$$ is $$[Ar] 3d^5 4s^2$$, so $$\text{Mn}^{3+}$$ is $$[Ar] 3d^4$$.
  • This is a $$d^4$$ configuration.

• (C) $$[\text{Fe}(\text{CN})_6]^{3-}$$:

  • Oxidation state of Fe: $$x + 6(-1) = -3 \implies x = +3$$
  • Iron(III) configuration: Neutral $$\text{Fe}$$ is $$[Ar] 3d^6 4s^2$$, so $$\text{Fe}^{3+}$$ is $$[Ar] 3d^5$$.
  • This is a $$d^5$$ configuration.

• (D) $$\text{Cr}_2(\text{O}-\text{C}(\text{=O})-\text{Me})_4(\text{H}_2\text{O})_2$$ (Chromium(II) acetate dimer):

  • Oxidation state of each Cr: Acetate ($$\text{CH}_3\text{COO}^-$$) carries a $$-1$$ charge, making the complex neutral: $$2x + 4(-1) + 2(0) = 0 \implies x = +2$$
  • Chromium(II) configuration: Neutral $$\text{Cr}$$ is $$[Ar] 3d^5 4s^1$$, so $$\text{Cr}^{2+}$$ is $$[Ar] 3d^4$$.
  • This is a $$d^4$$ configuration.

• (E) $$[\text{NiF}_6]^{2-}$$:

  • Oxidation state of Ni: $$x + 6(-1) = -2 \implies x = +4$$
  • Nickel(IV) configuration: Neutral $$\text{Ni}$$ is $$[Ar] 3d^8 4s^2$$, so $$\text{Ni}^{4+}$$ is $$[Ar] 3d^6$$.
  • This is a $$d^6$$ configuration.

Conclusion:

The complexes containing a central metal ion with a $$d^4$$ configuration are (B) and (D).

Answer: Option C — (B) and (D) only