*Dalton Trans.*, **47**, 16429-16444 (2018). (DOI:10.1039/C8DT02988A)

The fictitious spin-1/2 (effective spin-1/2) Hamiltonian approach has been the putative method to analyze the conventional fine-structure/hyperfine ESR spectra of high spin metallocomplexes with sizable zero-field splitting (ZFS) tensors since the early 1950s, and the approach gives salient principal *g*^{eff}-values far from *g* = 2 without explicitly affording their ZFS values in most cases. The experimental *g*^{eff}-values thus determined, however, never agree with those (*g*^{true}-values) of the true principal **g**-tensors, which are obtainable from reliable quantum chemical calculations. We have recently derived exact or extremely accurate analytical expressions for the *g*^{eff}/*g*^{true} relationships for the spin quantum number *S*‘s up to *S* = 7/2 (T. Yamane *et al.*, *Phys. Chem. Chem. Phys.*, 2017, **19**, 24769–24791). In this work, we have removed the limitation of the collinearity between **g**– and ZFS tensors and derived the generalized *g*^{eff}/*g*^{true} relationships. To illustrate the usefulness of the present approach, we have revisited important typical high spin systems with large ZFS values such as picket fence metalloporphyrins with Mn^{II} (*S* = 5/2) (Q. Yu *et al.*, *Dalton Trans.*, 2015, **44**, 9382–9390), a 6th ligand coordinated porphyrin with Fe^{III} (*S* = 5/2) (Y. Ide *et al.*, *Dalton Trans.*, 2017, **46**, 242–249) and a pseudo-octahedral Co^{II} (*S* = 3/2)(hfac)_{2} complex (D. V. Korchagin *et al.*, *Dalton Trans.*, 2017, **46**, 7540–7548), completing the ESR spectral and magnetic susceptibility analyses and gaining significant physical insights into their electronic structures. The off-principal axis extra peaks overlooked in the documented spectra of the picket fence Mn^{II} porphyrins have fully been assigned, affording their accurate true **g**-, hyperfine and ZFS tensors, for the first time. For the Co^{II} complex, the occurrence of the non-collinearity between the **g**– and ZFS tensors has been discussed by using the generalized *g*^{eff}/*g*^{true} relationships. We have attempted to carry out reliable DFT-based and *ab initio* quantum chemical calculations of their magnetic tensors, in which spin–orbit couplings are incorporated, reproducing the experimental true tensors. We emphasize that the incorporation of multi-reference nature in the electron configuration is important to interpret the magnetic tensors for the Co^{II} complex.