In [Ni(H2O)6]2+ is a weak field ligand. Therefore there are unpaired electrons in Ni2+. In this complex the cl electrons from the lower energy level can be excited to the higher energy level is the possiblity of d- d transition is present. Hence [Ni(H2O)6]2+ is coloured.
In [Ni(CN)4]2- CN is a strong field ligand. Therefore, d- d transition is not possible due to absence of unpaired electron (CN initiates pairing of d elcctrons ). Hence it is colourless.
