Out of TiF_6^ 2- , CoF_6^ 3- , Cu_2 Cl_2 and NiCl_4^ 2- ( Z of...

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 $\mathrm{Co}^{3+}<img src="/api/proxy-image?url=https$ \\ 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 $\mathrm{Cu}^{+}$.$<img src="/api/proxy-image?url=https$ \\ 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 $\mathrm{Ni}^{2+}$.$<img src="/api/proxy-image?url=https$ \\ 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.