When light passes through a solution containing transition metal complexes, we see those wavelengths of light that are transmitted. The solutions of most octahedral Cu (II) complexes are blue. The visible spectrum for an aqueous solution of Cu (II), [Cu(H2O6]2+, shows that the absorption band spans the red-orange-yellow portion of the spectrum and green, blue and violet are transmitted.
The absorption band corresponds to the energy required to excite an electron from the t2g level to the eg level.
Recall, the energy possessed by a light wave is inversely proportional to its wavelength. The Cu(II) solution transmits relatively high energy waves and absorbs the low energy wavelengths. This indicates that the band gap between the two levels is relatively small for this ion in aqueous solution.
d-Orbital Splitting
The magnitude of the splitting of the d-orbitals in a transition metal complex depends on three things:
• the geometry of the complex
• the oxidation state of the metal
• the nature of the ligands
