Question

Which of the following complex ions is expected to absorb visible light?

(Atomic number $Zn=30$, $Sc=21$, $Ti=22$, $Cr=24$)

• A. $\left[Zn\right(N{H}_3{)}_6{]}^{2+}$
• B. $\left[Sc\right(H_{2}O{)}_3(N{H}_3{)}_3{]}^{3+}$
• C. $\left[Ti\right(en{)}_2(N{H}_3{)}_2{]}^{4+}$
• D. $\left[Cr\right(N{H}_3{)}_6{]}^{3+}$

Answer 1

Answer: (D)

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

Transition metal complexes absorb visible light only when they have an incomplete d nutshell.

Also, when an electron can go from lower energy orbital to higher energy orbital, then they can absorb visible light.

In $\left[Cr\right(N{H}_3{)}_6{]}^{3+}$, $Cr$ is present as $C{r}^{+3}$

Electronic configuration of $C{r}^{{}_{+}3}=\left[Ar\right]3d^{3}4s^0$

Now, since the complex has $3$ unpaired electrons, excitation of electrons is possible & therefore, it will absorb visible light.