Question

Among the following statements $\mathrm{A}-\mathrm{D}$, the incorrect ones are:

A. Octahedral $\mathrm{Co}\left(\mathrm{III}\right)$ complexes with strong field ligands have high magnetic moments.

B. When ${\mathrm{\Delta }}_{\mathrm{o}}<\mathrm{P}$, the d- electron configuration of $\mathrm{Co}\left(\mathrm{III}\right)$ in an octahedral complex is ${\mathrm{t}}^{4}2\mathrm{g},{\mathrm{e}}^2\mathrm{g}$.

C. Wavelength of light absorbed by $[\mathrm{Co}{\left(\mathrm{en}\right)}_3{]}^{3+}$is lower than that of ${\left[{\mathrm{CoF}}_6\right]}^{3-}$.

D. If the ${\mathrm{\Delta }}_0$ for an octahedral complex of $\mathrm{Co}\left(\mathrm{III}\right)$ is $18000{\mathrm{cm}}^{-1}$, the ${\mathrm{\Delta }}_{\mathrm{t}}$ for its tetrahedral complex with the same ligand will be $16000{\mathrm{cm}}^{-1}$ .

• A. $\mathrm{B}$ and $\mathrm{C}$ only
• B. $\mathrm{A}$ and $\mathrm{B}$ only
• C. $\mathrm{A}$ and $\mathrm{D}$ only
• D. $\mathrm{C}$ and $\mathrm{D}$ only

Answer 1

The correct option is C

$\mathrm{A}$ and $\mathrm{D}$ only

The explanation for the correct option:

(C) $\mathbf{A}$ and $\mathbf{D}$ only

Statement $\mathbf{A}$:

1. Octahedral $\mathrm{Co}\left(\mathrm{III}\right)$ complexes with strong field ligands have high magnetic moments.
2. Crystal field splitting of octahedral $\mathrm{Co}\left(\mathrm{III}\right)$ complexes with strong field ligands can be shown as follows:
3. From the splitting it can be observed that Octahedral $\mathrm{Co}\left(\mathrm{III}\right)$ complexes does not have any unpaired electrons hence, the magnetic moment for these complexes is $0BM$.
4. Hence, statement $\mathrm{A}$ is incorrect.

Statement $\mathbf{D}$:

1. If the ${\mathrm{\Delta }}_0$ for an octahedral complex of $\mathrm{Co}\left(\mathrm{III}\right)$ is $18000{\mathrm{cm}}^{-1}$, the ${\mathrm{\Delta }}_{\mathrm{t}}$ for its tetrahedral complex with the same ligand will be $16000{\mathrm{cm}}^{-1}$ .
2. The formula for calculating ${\mathrm{\Delta }}_{\mathrm{t}}$ using the value of ${\mathrm{\Delta }}_0$ is as follows: ${∆}_t=\frac{4}{9}{∆}_0$
3. Putting the value of ${\mathrm{\Delta }}_0$ to calculate ${\mathrm{\Delta }}_{\mathrm{t}}$: $$\begin{gathered} {∆}_t=\frac{4}{9}\times 18000 \\ {∆}_t=8000{\mathrm{cm}}^{-1} \end{gathered}$$
4. So the ${\mathrm{\Delta }}_{\mathrm{t}}$ for tetrahedral complex is $8000{\mathrm{cm}}^{-1}$.
5. Hence, statement $\mathrm{D}$ is incorrect.

Explanation for incorrect options:

(A) $\mathbf{B}$ and $\mathbf{C}$ only

Statement $\mathbf{B}$:

1. When ${\mathrm{\Delta }}_{\mathrm{o}}<\mathrm{P}$, the d- electron configuration of $\mathrm{Co}\left(\mathrm{III}\right)$ in an octahedral complex is ${{\mathrm{t}}^4}_{2\mathrm{g}},{{\mathrm{e}}^2}_{\mathrm{g}}$.
2. The crystal field splitting of $\mathrm{Co}\left(\mathrm{III}\right)$ in an octahedral complex when ${\mathrm{\Delta }}_{\mathrm{o}}<\mathrm{P}$ can be shown as follows:
3. From the splitting it is clear that the number of electrons in ${\mathrm{t}}_{2\mathrm{g}}\mathrm{and}{\mathrm{e}}_{\mathrm{g}}$ orbitals are four and two respectively.
4. Hence, statement $\mathrm{B}$ is correct.

Statement $\mathbf{C}$:

1. Wavelength of light absorbed by $[\mathrm{Co}{\left(\mathrm{en}\right)}_3{]}^{3+}$is lower than that of ${\left[{\mathrm{CoF}}_6\right]}^{3-}$.
2. Ethylenediamine is a strong ligand but fluorine is a weak ligand so, more energy is absorbed by $[\mathrm{Co}{\left(\mathrm{en}\right)}_3{]}^{3+}$ complex than ${\left[{\mathrm{CoF}}_6\right]}^{3-}$ complex.
3. Thus, the wavelength of light absorbed is more for $[\mathrm{Co}{\left(\mathrm{en}\right)}_3{]}^{3+}$than${\left[{\mathrm{CoF}}_6\right]}^{3-}$.
4. Hence, statement $\mathrm{C}$ is correct.

(B) $\mathbf{A}$ and $\mathbf{B}$ only

It can be observed from the above discussion that statement $\mathrm{A}$ is incorrect and statement $\mathrm{B}$ is correct.

(D) $\mathbf{C}$ and $\mathbf{D}$ only

It can be observed from the above discussion that statement $\mathrm{D}$ is incorrect and statement $\mathrm{C}$ is correct.

Hence, option $\mathbf{C}$ is correct, statement $\mathrm{A}$ and $\mathrm{D}$ are incorrect.