The absorption spectra of transition and inner transition elements in the UV-Visible region arise mainly from electronic transitions between different energy levels of the partially filled d- and f-orbitals. UV-Visible spectroscopy is therefore particularly useful for studying the electronic structure, oxidation states, coordination compounds and magnetic properties of these elements. The nature of the spectra of transition elements differs considerably from that of inner transition elements because d–d transitions and f–f transitions are governed by different electronic environments and selection rules. Transition elements are characterized by the presence of partially filled d-orbitals in their atoms or in at least one of their common oxidation states. When a transition-metal ion is placed in a ligand environment, the five d-orbitals no longer remain degenerate. In a free ion, all five d-orbitals have approximately the same energy, but interaction with surrounding ligands produces crystal-field or ligand-field splitting. For example, in an octahedral complex, the five d-orbitals split into two groups, namely the lower-energy t₂g set and the higher-energy eᵍ set. In a tetrahedral complex, the pattern is reversed. The energy difference between these sets often corresponds to radiation in the visible region. When a transition-metal ion absorbs visible or ultraviolet radiation, an electron may be promoted from a lower-energy d-level to a higher-energy d-level. Such transitions are called d–d transitions. The wavelength of maximum absorption depends upon the magnitude of crystal-field splitting, usually represented by Δ. Since Δ depends on the nature of the metal ion, its oxidation state, the geometry of the complex and the nature of the ligands, different transition-metal complexes absorb at different wavelengths. The absorption spectra of transition-metal ions are generally broad rather than sharp. This is because the electronic energy levels are affected by vibrational and rotational motions of the molecule or complex and by interactions with the surrounding solvent or ligands. Consequently, several closely spaced vibrational levels may be associated with an electronic transition, producing a broad absorption band. The exact position and intensity of the band can also vary with temperature, solvent, ligand field and molecular geometry. The colour of many transition-metal compounds is directly related to their absorption