LMCT complexes arise from transfer of electrons from MO with ligand-like character to those with metal-like character. This type of transfer is predominant if complexes have ligands with relatively high-energy lone pairs (example S or Se) or if the metal has low-lying empty orbitals. Many such complexes have metals in high oxidation states (even d0). These conditions imply that the acceptor level is available and low in energy.
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.