The colour of the transition metal ions is mainly attributed to d-d transition. d-d transition takes place because of the presence of unpaired electrons. The metal ions having same number of unpaired electrons may give same colour in their aqueous solution. In VOCl 2 , V IV ion having 3d 1 configuration has one unpaired electron. Similarly in CuCl 2 , the Cu 2+ ion having 3d 9 configuration also has one unpaired electron. So VOCl 2 and CuCl 2 are expected to exhibit same colour i.e. blue colour Mn 2+ has five unpaired and Fe 2+ has four unpaired electrons