A remarkably large magnetic anisotropy energy of 305 K is computed by quantum chemistry methods for divalent Fe²⁺ d⁶ substitutes at Li-ion sites with D_6h point-group symmetry within the solid-state matrix of Li₃N. This is similar to values calculated by the same approach and confirmed experimentally for linearly coordinated monovalent Fe¹⁺ d⁷ species, among the largest so far in the research area of single-molecule magnets. Our ab initio results therefore mark a new exciting exploration path in the search for superior single-molecule magnets, rooted in the configuration of d⁶ transition-metal ions with linear or quasilinear nearest-neighbor coordination. This d⁶ axial anisotropy may be kept robust even for symmetries lower than D_6h, provided the ligand and farther-neighbor environment is engineered such that the splitting remains large enough.