The correct match of spin-only magnetic moment for the complexes cis-$[\mathrm{Fe(phen)_2(NCS\text{-}N)_2}]$ (A) and $[\mathrm{Fe(phen)_3}]\mathrm{Cl_2}$ (B) at 300 K is (phen=1,10-phenantholine)
(a)4.89 BM for both A and B
(b)0 BM for both A and B
(c)4.89 BM for A and 0 BM for B
(d)0 BM for A and 4:59 BM for B
Answer
Answer: C ✓ checked by 4AB · confidence high
Answer: 4.89 BM for A and 0 BM for B.
Explanation
Answer: 4.89 BM for A and 0 BM for B — cis-[Fe(phen)₂(NCS)₂] is a spin-crossover complex (T½ ≈ 176 K), so at 300 K it is high-spin d⁶ with 4 unpaired electrons (4.89 BM). [Fe(phen)₃]²⁺ is low-spin d⁶ (t₂g⁶), so 0 BM.