Consider a d6 octahedral complex, such as IrBr63−), whose t2g levels are filled. As a consequence, an intense absorption is observed around 250 nm corresponding to a transition from ligand σ MO to the empty eg MO. However, in IrBr62− that is a d5 complex two absorptions, one near 600 nm and another near 270 nm, are observed. This is because two transitions are possible, one to t2g (that can now accommodate one more electron) and another to eg. The 600 nm band corresponds to transition to the t2g MO and the 270 nm band to the eg MO.
Charge transfer bands may also arise from transfer of electrons from nonbonding orbitals of the ligand to the eg MO.