Question

In which of the following coordination entities the magnitude of ${\Delta }_o$ (CFSE in octahedral feild ) will be maximum? (Atomic number of $Co=27$)

• A. $\left[Co\right(H_{2}O{)}_6{]}^{3+}$
• B. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$
• C. $\left[Co\right(CN{)}_6{]}^{3+}$
• D. $\left[Co\right(C_2{O}_4{)}_3{]}^{3+}$

Answer 1

The correct option is C $\left[Co\right(CN{)}_6{]}^{3+}$

In all the given complexes the oxidation state of cobalt is +3.
Thus, the configuration of $C{o}^{3+}$ is $d^6$
The magnitude of ${\Delta }_o$ (the orbital splitting energy) is decided by the nature of ligand. In strong field ligand, the splitting will be more.
has higest ${\Delta }_o.$ The increasing field strength of ligand is as
$I^{-}<B{r}^{-}<C{l}^{-}<F^{-}<O{H}^{-}<C_2{O}_4^{2-}<H_{2}O<N{H}_3<en<N{O}_2^{-}<C{N}^{-}$

This series is termed as spectrochemical series.

The $C{N}^{-}$ is the strongest ligand among these, hence the magnitude od ${\Delta }_o$ will be maximum in $\left[Co\right(CN{)}_6{]}^{3-}$.