Out of TiF ₆²⁻, CoF ₆³⁻, Cu ₂ Cl ₂ and NiCl ₄²⁻ ( Z of Ti =22, Co =27, Cu =29, Ni =28 ) the colourless species are

Step-by-step solution

Key Idea Species without unpaired electrons are colourless because transition of electron is not passible. (a) In TiF ₆²⁻, Ti is present as Ti ⁴⁺ . Ti ⁴⁺=[ Ar ] 3 ~d ^0 4 ~s ^0 Hence, TiF ₆²⁻ is colourless. (b) In CoF ₆³⁻ , Co is present as Co ³⁺ Due to presence of unpaired electrons, CoF ₆³⁻ is coloured. (c) In Cu ₂ Cl ₂, Cu is present as Cu ⁺ . Due to absence of unpaired electron, Cu ₂ Cl ₂ is colourless. (d) In NiCl ₄²⁻, Ni is present as Ni ²⁺ . Since, unpaired electrons are present, NiCl ₄²⁻ is coloured. Hence,