NEET UG 2009 — Chemistry Inorganic Chemistry

Key Idea Only those transition metal complexes are expected to absorb visible light, in which d-subshell is incomplete (ie, has unpaired electron) and excitation of electron from a lower energy arbital to higher energy orbital is possible. (a) In $\left[\mathrm{Sc}\left(\mathrm{H}_2 \mathrm{O}\right)_3\left(\mathrm{NH}_3\right)_3\right]^{3+}, \mathrm{Sc}$[Sc(H2​O)3​(NH3​)3​]3+,Sc is present as $\mathrm{Sc}^{3+}$Sc3+. $$\mathrm{Sc}^{3+}=[\mathrm{Ar}] 3 \mathrm{~d}^0, 4 \mathrm{~s}^0$$Sc3+=[Ar]3 d0,4 s0 Since, in this complex excitation of electron is not possible, it will not absorb visible light. (b) In $\left[\mathrm{Ti}(\mathrm{en})_2\left(\mathrm{NH}_3\right)_2\right]^{4+}, \mathrm{Ti}$[Ti(en)2​(NH3​)2​]4+,Ti is present as $\mathrm{Ti}^{4+}$Ti4+. $$\mathrm{Ti}^{4+}=[\mathrm{Ar}] 3 \mathrm{~d}^0, 4 \mathrm{~s}^0$$Ti4+=[Ar]3 d0,4 s0 Hence, it will not absorb visible light. (c) In $\left[\mathrm{Cr}\left(\mathrm{NH}_3\right)_6\right]^{3+}, \mathrm{Cr}$[Cr(NH3​)6​]3+,Cr is present as $\mathrm{Cr}^{3+}$Cr3+. $$\mathrm{Cr}^{3+}=[\mathrm{Ar}] 3 \mathrm{~d}^3, 4 \mathrm{~s}^0$$Cr3+=[Ar]3 d3,4 s0

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 $\left[\mathrm{Zn}\left(\mathrm{NH}_3\right)_6\right]^{2+}, \mathrm{Zn}$[Zn(NH3​)6​]2+,Zn is present as $\mathrm{Zn}^{2+}$Zn2+. $$\begin{gathered} \mathrm{Zn}^{2+}=[\mathrm{Ar}] 3 \mathrm{~d}^{10}, 4 \mathrm{~s}^0 \\ {\left[\mathrm{Zn}\left(\mathrm{NH}_3\right)_6\right]^{2+}=[\mathrm{Ar}] 3 d^{10}} \end{gathered}$$Zn2+=[Ar]3 d10,4 s0[Zn(NH3​)6​]2+=[Ar]3d10​

Hence, this complex will not absorb visible light.