In which of the following pairs do the two complexes have DIFFERENT spin-only magnetic moments?

Step-by-step solution

The spin-only magnetic moment depends on the number of unpaired electrons in the central metal ion. For d^1, d^2, d^3 and d^8, d^9, d¹⁰ octahedral complexes, the number of unpaired electrons remains the same regardless of the ligand field strength (strong or weak). [Cr(H₂O)₆]³⁺ and [Cr(CN)₆]³⁻ : Cr³⁺ is a d^3 ion. Both complexes have 3 unpaired electrons ( t_ 2g ^3 e_g^0 ). [Ni(H₂O)₆]²⁺ and [Ni(en)₃]²⁺ : Ni²⁺ is a d^8 ion. Both complexes have 2 unpaired electrons ( t_ 2g ^6 e_g^2 ). [Cu(H₂O)₆]²⁺ and [Cu(en)₃]²⁺ : C