The inversion of the lowest excited singlet and triplet states involves breaking the Hund's rule and deserves some explanation. Typically, triplets have lower energy than corresponding singlets because of the exchange energy. Accordingly, the first requirement for ST inversion is a small exchange energy between HOMO and LUMO. To this end, a possible strategy consists in having non-overlapping HOMO and LUMO, so that the two orbitals have sizable weight on two different subsets of atoms. Systems with disconnected HOMO and LUMO belong to the large and interesting family of MR-CT dyes. However, our results show quite explicitly that the disjoint nature of the HOMO and LUMO is not a strict prerequisite for ST inversion. A more reliable indication of the propensity to the breaking of the Hund's rule is recognized in the HOMO-LUMO exchange energy itself.