What is the difference between the manganese porphyrin and corrole analogues of cytochrome P450's compound I?

Density functional calculations on oxo-manganese complexes of corrole (1) and porphyrin (2 and 3) show a fundamental difference. The ground state of I is the singlet manganese(V) state, 1A(MnV), in which corrole is a closed shell. In contrast, 2 and 3 have high-spin manganese(IV) states, 1A1u and 3A2u respectively. This difference and the state ordering for each system are rationalized based on the competition between the intrinsic tendency of manganese to prefer high-spin electronic configurations, vis-à-vis the general tendency to prefer double occupancy in the low-lying orbitals. The outcome of this competition is determined primarily by the identity of the macrocycle, corrole versus porphyrin. Corrole with a small cavity holds the MnO moiety with a high off-plane displacement, and thereby prefers the low-spin state. On the other hand, porphyrin with the wider cavity holds the MnO moiety closer to the plane, and thereby prefers high-spin states.