Co-ordination compounds absorb a particular wavelength following certain rules.
Wavelength of light absorbed ∝Oxidation state of metal ion1
∝Strength of ligand1
Ligand field strength - CN−>NH3>H2O>Cl−
| List I Coordination entity | | List II Wavelength of light absorbed in nm |
| A | [CoCl(NH3)5]2+ | I. | 535 |
| B | [Co(NH3)6]3+ | II. | 475 |
| C | [Co(CN)6]3− | III. | 310 |
| D | [Cu(H2O)4]2+ | IV. | 600 |