Coordination Compounds 2 Question 20

####22. The complex that has highest crystal field splitting energy $(\Delta)$, is

(a) $\left[Co\left(NH _3\right) _5 Cl\right] Cl _2$

(b) $\left[Co\left(NH _3\right) _5\left(H _2 O\right)\right] Cl _3$

(c) $K _3\left[Co(CN) _6\right]$

(d) $K _2\left[CoCl _4\right]$

(2019 Main, 9 Jan II)

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Answer:

Correct Answer: 22. (c)

Solution:

  1. All of the complex given are the octahedral complexes of Co (III) except $K _2\left[CoCl _4\right]$, which is a tetrahedral complex of $Co$ (II) ( $s p^{3}$-hybridised).

We know,

$$ \Delta _t<\Delta _o $$

$$ \left[\because \Delta _t=\frac{4}{9} \Delta _o\right] $$

So, the octahedral complexes $(a, b, c)$ have higher $\Delta _o$ values than that of tetrahedral, $K _2\left[CoCl _4\right]$.

Now, for the complexes, $a, b$ and $c$,

the magnitude of $\Delta _o \propto$ ligand strength, which is based on their positions in the spectrochemical series.

$Cl^{-}<H _2 O<NH _3<CN^{-}$

Hence, $K _3\left[Co(CN) _6\right]$ will have the highest $\Delta$ value.



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