It is well known that there is no direct coupling between the quintet and singlet electronic states, whose spin momentum differ by ∆S = 2. However the weaker second order spinorbit coupling can still be effective to make the transition possible, with the involvement of a triplet state, namely, the 3 T1g (in octahedral symmetry). Buhks et al. suggested that the electronic matrix element could be approximated on the basis of this as