Question

Which of the following complex ion is expected to absorb light in the visible region?

• A. $\left[Ti\right(en{)}_3{]}^{4+}$
• B. $\left[Sc\right(N{H}_3{)}_4(H_{2}O{)}_2{]}^{3+}$
• C. $[Cr{O}_4{]}^{2-}$
• D. $\left[Zn\right(CN{)}_2{]}^{2-}$

Answer 1

Answer: (C)

$[Cr{O}_4{]}^{2-}$

Only those transition metal complexes are expected to absorb visible light, in which d- subshell is incomplete and excitation of an electron from a lower energy orbital to higher energy orbital is possible.

(a) In ${\left[Sc\right(H_{2}O\left)\right({NH}_3{)}_3]}^{3+}$ Sc is present in ${Sc}^{3+}$=$\left[Ar\right]3d^0,4s^0$

Since in this complex excitation of the electron is not possible, it will not absorb visible light.

(b) ${\left[Ti\right(en{)}_2\left(N{H}_3{)}_2\right]}^{4+},$ Ti is present as ${Ti}^{4+}$

${Ti}^{4+}=\left[Ar\right]3d^0,4s^0$

Hence it will not absorb visible light

(c) In ${\left[Cr\right({NH}_3{)}_6]}^{3+}$ Cr is present as ${Cr}^{3+}$

${Cr}^{3+}$=$\left[Ar\right]3d^3,4s^0$

Since this complex has three unpaired electrons excitation of electrons is possible and thus, it is expected that this complex will absorb visible light.

(d) In ${\left[Zn\right(N{H}_3{)}_6]}^{2+}$ Zn is present as ${Zn}^{2+}$

${Zn}^{2+}$=$\left[Ar\right]3d^{10},4s^0$

Hence , this complex will not absorb visible light.

The correct option is C.