Which one of the following complexes will have o = 0 and = 5.96 B.M.?
- [FeF6]4-
- [Mn(SCN)6]4-
- [Co(NH3)6]3+
- [Fe(CN)6]4-
Answer
(2) [Mn(SCN)6]4-
Solution
The condition " o = 0" here means zero crystal field stabilisation energy, and 5.9 B.M. means 5 unpaired electrons ( 5 7 = 5.92 B.M.).
(A) Fe2+, 3d6, F- weak field: t2g4 eg2, 4 unpaired, CFSE = -0.4 o
(B) Mn2+, 3d5, SCN- weak field: t2g3 eg2, 5 unpaired
CFSE = 3(-0.4 o) + 2(+0.6 o) = 0
(C) Co3+, 3d6, NH3: t2g6 eg0, 0 unpaired, CFSE = -2.4 o
(D) Fe2+, 3d6, CN- strong field: t2g6 eg0, 0 unpaired, CFSE = -2.4 o
[Mn(SCN)6]4- has zero CFSE and = 5.92 5.96 B.M.
Answer: (2)