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.
Thus, CFSE for the complexes, $[FeC{l}_6{]}^{3-}$ and $\left[Fe\right(CN{)}_6{]}^{3-}$ are ___________ .

• A. $0,0$
• B. $-2{\Delta }_o,0$
• C. $0,-2{\Delta }_o$
• D. $-2{\Delta }_o,-2{\Delta }_o$

Answer 1

The correct option is C $0,-2{\Delta }_o$

High spin complex $[FeC{l}_6{]}^{3-}$ has the d-configuration as $t_{2g}^3{e}_g^2$.

$CFSE=3\times (-0.4)+2\times (+0.6)=0$

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

$CFSE=5\times (-0.4)=-2{\Delta }_o$

Therefore, from the above calculation, C is the correct option.