Question

The absorption maxima of some complexions are as follows:

$[CrC{l}_6{]}^{3-}-760nm$

$\left[Cr\right(N{H}_3{)}_6{]}^{3+}-470nm$

$\left[Cr\right(H_{2}O{)}_6{]}^{3+}-695nm$

The order of ${\Delta }_o$ for these ions is

• A. $\left[Cr\right(N{H}_3{)}_6{]}^{3+}>\left[Cr\right(H_{2}O{)}_6{]}^{3+}>[CrC{l}_6{]}^{3-}$
• B. $[CrC{l}_6{]}^{3-}>[Cr(H_{2}O{)}_6{]}^{3+}>[Cr(N{H}_3{)}_6{]}^{3+}$
• C. $\left[Cr\right(H_{2}O{)}_6{]}^{3+}>\left[Cr\right(N{H}_3{)}_6{]}^{3+}>[CrC{l}_6{]}^{3-}$
• D. $[CrC{l}_6{]}^{3-}>[Cr(N{H}_3{)}_6{]}^{3+}>[Cr(H_{2}O{)}_6{]}^{3+}$

Answer 1

Correct option: (A)

The ${\Delta }_o$ represents the crystal field splitting energy.

The value of ${\Delta }_o$ is inversely related to the wavelength of maximum absorption, i.e.,

$E=hv=\frac{hc}{\lambda }$.

So, smaller the wavelength of $\lambda ,$ greater will be the ${\Delta }_o$ value.

Hence, $\left[Cr\right(N{H}_3{)}_6{]}^{3+}({\lambda }_{max}=470nm)$ has the highest ${\Delta }_o$ value.

$\left[Cr\right(H_{2}O{)}_6{]}^{3+}({\lambda }_{max}=695nm)$ has the higher ${\Delta }_o$ than $\left[Cr\right(Cl{)}_6{]}^{3-}$ and lower than $\left[Cr\right(N{H}_3{)}_6{]}^{3+}$.

$\left[CrC{l}_6{]}^{3-}\right({\lambda }_{max}=760nm)$ has the lowest ${\Delta }_o$ value.

Hence, the decreasing order of crystal field splitting energy is:

$\left[Cr\right(N{H}_3{)}_6{]}^{3+}>\left[Cr\right(H_{2}O{)}_6{]}^{3+}>\left[Cr\right(Cl{)}_6{]}^{3-}$

Hence, the correct option is (A).