Question

The octahedral complex ions $[FeC{l}_6{]}^{3-}$ and $\left[Fe\right(CN{)}_6{]}^{3-}$ are both paramagnetic but the former is high spin and the latter is low spin. In the low spin complex, $\left[Fe\right(CN{)}_6{]}^{3-}$ has the d-configuration as :

• A. $t_{2g}^3{e}_g^2$
• B. $t_{2g}^5$
• C. $t_{2g}^2$
• D. $e_g^5$

Answer 1

Answer: (B)

$t_{2g}^5$

Low spin complex $[FeC{N}_6{]}^{3-}$ has the d-configuration as $t_{2g}^5$

$F{e}^{3+}$ configuration is $\left[Ar\right]3d^5$ hence according to the CFSE for octahedral geometry(low spin) all the five electrons will go to $t_{2g}$ level.