Out of $\mathrm{TiF}_6^{2-}, \mathrm{CoF}_6^{3-}, \mathrm{Cu}_2 \mathrm{Cl}_2$ and $\mathrm{NiCl}_4^{2-}$ (…

Out of $\mathrm{TiF}_6^{2-}, \mathrm{CoF}_6^{3-}, \mathrm{Cu}_2 \mathrm{Cl}_2$ and $\mathrm{NiCl}_4^{2-}$ ( $\mathrm{Z}$ of $\mathrm{Ti}=22, \mathrm{Co}=27, \mathrm{Cu}=29, \mathrm{Ni}=28$ ) the colourless species are
  1. $\mathrm{TiF}_6^{2-}$ and $\mathrm{CoF}_6^{3-}$
  2. $\mathrm{Cu}_2 \mathrm{Cl}_2$ and $\mathrm{NiCl}_4^{2-}$
  3. $\mathrm{TiF}_6^{2-}$ and $\mathrm{Cu}_2 \mathrm{Cl}_2$
  4. $\mathrm{CoF}_6^{3-}$ and $\mathrm{NiCl}_4^{2-}$

Solution

Key Idea Species without unpaired electrons are colourless because transition of electron is not passible. (a) In $\mathrm{TiF}_6^{2-}, \mathrm{Ti}$ is present as $\mathrm{Ti}^{4+}$. $\mathrm{Ti}^{4+}=[\mathrm{Ar}] 3 \mathrm{~d}^0 4 \mathrm{~s}^0$ Hence, $\mathrm{TiF}_6^{2-}$ is colourless. (b) In $\mathrm{CoF}_6^{3-}$, Co is present as $\mathrm{Co}^{3+}$ Due to presence of unpaired electrons, $\mathrm{CoF}_6^{3-}$ is coloured. (c) In $\mathrm{Cu}_2 \mathrm{Cl}_2, \mathrm{Cu}$ is present as $\mathrm{Cu}^{+}$. Due to absence of unpaired electron, $\mathrm{Cu}_2 \mathrm{Cl}_2$ is colourless. (d) In $\mathrm{NiCl}_4^{2-}, \mathrm{Ni}$ is present as $\mathrm{Ni}^{2+}$. Since, unpaired electrons are present, $\mathrm{NiCl}_4^{2-}$ is coloured. Hence, $\mathrm{TiF}_6^{2-}$ and $\mathrm{Cu}_2 \mathrm{Cl}_2$ are colourless species.

Asked in: NEET 2009 (Screening)

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