Amongst `TiF_6^(2-), CoF_6^(3-), Cu_2Cl_2" and " NiCl_4^(2-)`, which are the colourless species ? (atomic number of Ti = 22 , Co = 27, Cu = 29, Ni = 28)

Explanation

(A). In `[TiF_(6)]^(2-)` the titanium is in +4 oxidation state having electronic configuration `[Ar]^(18)3d^(0)4s^(0)` similarly in `Cu_(2)Cl_(2)` The copper is in +1 oxidation state having the electronic configuraiton `[Ar]^(18)d^(10)4s^(0)`
As they do not have any unpaired electrons for d-d transiton, they are thereofre colourless.
In `[NiCl_(4)]^(2-)` the nickel is in +2 oxidation state and electroniconfiguration is `[Ar]^(18)3d^(8)4s^(0)`. As it has two unpaired electrons, so the complex is coloured.
In `[CoF_(6)]^(3-)`, the cobalt is in +3 oxidation state having electron configuration `[Ar]3d^(6)4s^(0)`. As it has four unpaired electrons, so the complex is coloured.