Identify the coordination complexes in which the central metal ion has d⁴ configuration.
(A) [FeO₄]²⁻
(B) [Mn(CN)₆]³⁻
(C) [Fe(CN)₆]³⁻
(D) Cr₂(O-C(∥(O))-Me)₄(H₂O)₂ (E) [NiF₆]²⁻
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Find each metal's oxidation state first, then take electrons from 4s before 3d to count the d electrons.

1. Oxidation state. Ligand charges plus the metal charge must add up to the charge on the complex. O²⁻, CN⁻, F⁻ and acetate (CH₃COO⁻) each carry their own negative charge. Water is neutral.
2. Count d electrons. When a transition metal forms a cation, the 4s electrons leave first. So the d count is the group number minus the oxidation state. For example, Mn is in group 7, so Mn³⁺ is 7 - 3 = 4, that is 3d⁴.

   Complex	Metal oxidation state	Configuration	d count
   (A) [FeO₄]²⁻	x - 8 = -2, so Fe(+6)	[Ar] 3d²	d²
   (B) [Mn(CN)₆]³⁻	x - 6 = -3, so Mn(+3)	[Ar] 3d⁴	d⁴
   (C) [Fe(CN)₆]³⁻	x - 6 = -3, so Fe(+3)	[Ar] 3d⁵	d⁵
   (D) Cr₂(CH₃COO)₄(H₂O)₂	2x - 4 = 0, so Cr(+2)	[Ar] 3d⁴	d⁴
   (E) [NiF₆]²⁻	x - 6 = -2, so Ni(+4)	[Ar] 3d⁶	d⁶

3. So. Only Mn³⁺ in (B) and Cr²⁺ in (D) are d⁴, which is option (d).

The idea to keep. For a 3d metal ion, the d count is the group number minus the oxidation state. Get the oxidation state from the ligand charges, and remember that the 4s electrons are always removed before the 3d ones.

Answer from the NTA final answer key. Solution by PrepWiser.