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:
- 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.
