Among TiF6 2-, CoF6 3-, Cu2Cl2 and NiCl4 2- (atomic numbers of Ti = 22, Co = 27, Cu = 29, Ni = 28), the colourless species are

Colour is shown by those transition element complexes which have unpaired d–electrons and are paramagnetic in nature. The electronic transitions of these unpaired electrons impart colour to the solution. 2 0 6 10 2 2 TiF due d configuration diamagnetic&colorless Cu Cl due to d configuration