The metal complex that is diamagnetic is (Atomic number: Fe, 26 ; Cu, 29)

Solution

Chemistry ma’am · 3 steps

Find each metal's oxidation state, then count d electrons: only Cu⁺ has a full d¹⁰ set.

1. What diamagnetic needs. A diamagnetic complex has no unpaired electrons. So find the metal's oxidation state, write its d-electron count, and check whether all of them can pair up.
2. Oxidation states. Each K⁺ is +1, and CN⁻ and Cl⁻ are -1 each. For example, in K₃[Cu(CN)₄] the complex ion is -3, so x - 4 = -3 and Cu is +1.

   Complex	Metal ion	d electrons	Unpaired electrons
   K₃[Cu(CN)₄]	Cu⁺	3d¹⁰, all paired	0, diamagnetic
   K₂[Cu(CN)₄]	Cu²⁺	3d⁹, odd	1
   K₃[Fe(CN)₄]	Fe⁺	3d⁷, odd	at least 1
   K₄[FeCl₆]	Fe²⁺	3d⁶, weak-field Cl⁻, high spin	4

3. So. Only K₃[Cu(CN)₄] has every electron paired, because Cu⁺ is d¹⁰. The answer is (a).

The idea to keep. Diamagnetic means every electron is paired. An odd d count can never pair up fully, whatever the ligand, and a d¹⁰ ion is always diamagnetic.

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