Electronic configuratrions of metal ions (i) Cu+=[Ar]3d104s0 (ii) Zn2+=[Ar]3d104s0 (iii) Sc3+=[Ar]3d04s0 (iv) V4+=[Ar]3d14s0 As it is clear from the electronic configuration of metal ions that V4+ has one unpaired electron in its d-subshell. The d-subshells are non-degenerated in presence of ligands. On exposure to visible light, the excitation of unpaired d-electrons takes place from lower to higher energy d-sublevels. The d-d excitation during complex formation permits the absorbance of required wavelength and rest light is transmitted out. Thus, transition metals or metal ions having unpaired d-electrons appears coloured.