To have an A- and B-term in the MCD spectrum, a molecule must contain degenerate excited states (A-term) and excited states close enough in energy to allow mixing (B-term). One case exemplifying these conditions is a square planar, d complex such as [(n-CH)N]Pt(CN). In addition to containing A- and B-terms, this example demonstrates the effects of spin-orbit coupling in metal to ligand charge transfer (MLCT) transitions. As shown in figure 1, the molecular orbital diagram of [(n-CH)N]Pt(CN) reveals MLCT into the antibonding π* orbitals of cyanide. The ground state is diamagnetic (thereby eliminating any C-terms) and the LUMO is the a. The dipole-allowed MLCT transitions are a-a and e-a. Another transition, b-a, is a weak (orbitally forbidden singlet) but can still be observed in MCD.
Because A- and B-terms arise from the properties of states, all singlet and triplet excited states are given in figure 2.
Mixing of all these singlet and triplet states will occur and is attributed to the spin orbit coupling of platinum 5d orbitals (ζ ~ 3500 cm), as shown in figure 3. The black lines on the figure indicate the mixing of A with E to give two A states. The red lines show the E, E, A, and B states mixing to give four E states. The blue lines indicate remnant orbitals after spin-orbit coupling that are not a result of mixing.