Complex 9 was further characterized by an X-BAND EPR measurement carried out on its freshly prepared DCM solution frozen to 5 K. Similarly to several other S = 3/2 Mn(IV) complexes, the recorded EPR spectrum of 9 featured a narrow peak in the low field region (1400 Oe) and a very broad one in the high field region (4200 Oe) (Figure , black trace). Simulation of the experimental data for an S = 3/2 system with the spin Hamiltonian parameters given in Table (entry 1) resulted in a reasonable agreement with the recorded spectrum (Figure , red trace). In particular, the simulated axial ZFS value (D = -0.83 cm -1 ) occurs within the range typical for pseudo-octahedral N,O-coordinated Mn(IV) species (D=|0.67| -|2.5| cm -1 ). Although due to the large broadening of the EPR signal well-resolved hyperfine interaction with the 55 Mn nucleus could not be observed (a well-known phenomenon for Mn(IV) species), to simulate the experimental spectrum, we considered nearly axial hyperfine coupling constants (Table , entry 1, last column). The corresponding Aiso = 233 MHz value is in good agreement with the literature. To verify that the recorded EPR spectrum is consistent with the structure of 9, the simulated spin Hamiltonian parameters were corroborated by ab-initio/CASSCF/NEVPT2 calculations on the model complex 9*. Since spin Hamiltonian parameters are well-known to be highly sensitive to the size of the active space and the number of electronic states used in such calculations, several different active space/electronic state combinations were employed (see Table for detailed results). The minimal CAS (3,5) active space, considering only the Mn 3d orbitals (Fig. ), yielded a close axial ZFS value (D), but failed to correctly predict the observed rhombicity value (E/D, Table , entry 2). A better agreement with both axial and rhombic experimental ZFS parameters (Table , entry 3) was obtained with a larger CAS (11,9) active space which takes into account all metal-and ligand-based orbitals contributing to the metal's first coordination sphere (Fig. ). Table . Simulated and calculated (CASSCF/NEVPT2) spin Hamiltonian parameters for complexes 9 and 9*, respectively.