Question

Among the following complexes, the one which shows zero crystal field stabilisation energy (CFSE) is

• A. $[Mn(H_2 O)_6]^{3+}$
• B. $[Fe(H_2O)_6]^{3+}$
• C. $[Co (H_2O)_6]^{2+}$
• D. $Co (H_2O)_6]^{3+}$

Answer 1

The Correct Option is B

The CFSE for octahedral complex is given by
CFSE = $[-0.4t_{2g}e^- + 0.6e_ge^-]$
For $Mn^{3+},[3d^4] \rightarrow t^3_{2g}e^1_g$
$\therefore\, \, \, \, \, \, CFSE = [(-0.4 \times 3) + (0.6 \times 1)] = - 0.6$
$For Fe^{3+},[3d^5] \rightarrow t^3_{2g} e^2_g$
$\, \, \, \, \, \, $CFSE =$ [-(0.4 \times 3) + (0.6 \times 2)] = 0$
For $Co^{2+}, [3d^7]\rightarrow t_{2g}^5 e^2_g$
$\, \, \, \, \, \, $CFSE =$ [(-0.4 \times 5) + (2 \times 0.6)] = 0$
= -0.8
For $Co^{3+},[3d^6]\rightarrow t^4_{2g}e^2_g$
CFSE = $[(-0.4 \times 4) + (2 \times 0.6)]$
= -0.4