The resulting lowest energy levels up to 1400 cm -1 are shown in Fig. together with the corresponding magnetic moment expectation values ⟨µ z ⟩ along the main magnetic axis. The latter coincides with the long molecular axis in both cases, which is given by the intersection of the ligand planes. Origin of Strong Zero-Field Splitting. It is well-established that 1 is a strongly axial system with an unusually strong ZFS. The energy difference between the lowest two KDs is 253 cm -1 in our calculations, which is very similar to the experimental excitation energy of ca. 235 cm -1 for peak 1A (Table ). Projecting these states onto a pseudo-spin 3/2 system, according to the spin Hamiltonian ('model m1')