Which one of the following complexes will exhibit the least paramagnetic behaviour ? [Atomic number, Cr = 24, Mn = 25, Fe = 26, Co = 27]
- [Fe(H2O)6]2+
- [Mn(H2O)6]2+
- [Co(H2O)6]2+
- [Cr(H2O)6]2+
Answer
(3) [Co(H2O)6]2+
Solution
H2O is a weak-field ligand ⇒ all are high spin; = n(n + 2) BM (n = unpaired electrons)
(A): Fe2+, 3d6 ⇒ n = 4, = 4.90 BM
(B): Mn2+, 3d5 ⇒ n = 5, = 5.92 BM
(C): Co2+, 3d7 ⇒ n = 3, = 3.87 BM
(D): Cr2+, 3d4 ⇒ n = 4, = 4.90 BM
∴ fewest unpaired electrons ⇒ [Co(H2O)6]2+ is least paramagnetic
Answer: (3)