Question

Arrange the following cobalt complexes in the order of increasing crystal field stabilisation energy (CFSE) value.

Complexes :
A . ${\left[Co{F}_6\right]}^{3-}$
B . ${\left[Co{\left(H_{2}O\right)}_6\right]}^{2+}$
C . ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$
D . ${\left[Co{\left(en\right)}_3\right]}^{3+}$

Choose the correct option :

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• A. C < D < B < A
• B. A < B < C < D
• C. B < C < D < A
• D. B < A < C < D

Answer 1

The correct option is D B < A < C < D

CFSE value increases as the strength of the ligand increases also with increase in positive charge of central atom. According to spectrochemical series, the order of ligand strength is $en>N{H}_3>H_{2}O>F^{-}$ . So , the CFSE value of the given complexes should be $A<B<C<D$. But as complex $A$ contain cobalt in +3 oxidation state, $A>B$. Final order is $B<A<C<D$.
Hence, option (d) is correct.