with E as the second rank transverse ZFS parameter, and the other parameters as introduced for eq. 1, we obtain a value of g z = 3.3 for the main magnetic axis, while the two other components are close to 2.0. The ZFS tensor is aligned with the main magnetic axes and we compute D = -127 cm -1 and E = 0.5 cm -1 (Table ). Hence, E is very small, and in the following we neglect the E term. The origin of the strong ZFS has already been discussed in Ref. 24 in terms of a perturbative expression stemming from ligand field theory. In this context, the very small splitting to the first excited quartet state was noted, as well as that this fact makes quantitative use of the perturbation theory expression questionable. The small energetic difference to the next-higher KDs is also clear from Fig. ), where further KDs are calculated at 847 and 1160 cm -1 . Analyzing these states at 847 cm -1 and 1160 cm -1 in the pseudo-spin 3/2 formalism results in parameter values of D = +156 cm -1 , E = 0.5 cm -1 and a very small g z tensor component of g z = -0.8 (g x and g y values are again close to 2.0), which reflects the very small ⟨µ z ⟩ expectation values of these states, as indicated in Fig. ).