The dependence of the optical and spin-Hamiltonian parameters of NiF₆^5- and NiF₄^3- D_4h units, with a b_1g* ( approximately x²-y²) unpaired electron, upon the equatorial (R_eq) and axial (R_ax) Ni⁺-F^- distances has been studied through multiple-scattering X alpha and self-consistent charge extended Huckel methods. Both methods lead to the following main conclusions: (i) A charge-transfer transition like e_u( pi + sigma ,eq) to b_1g*( approximately x²-y²) (termed E_u) is more sensitive than a crystal-field one like b_2g* ( approximately xy) to b_1g*( Delta ₁) or e_g* to b_1g*( Delta ₂) to variations in R_eq. The change experienced by E_u mainly reflects that of V_el(M)-V_el(L), where V_el(M) and V_el(L) are the electrostatic potentials experienced by an electron placed on metal and ligands, respectively, (ii) As regards the unpaired spin densities onto n_Lp and n_Ls ligand valence orbitals (termed f_sigma and f_s respectively), it is found that f_sigma>>f_s but f_s is much more sensitive than f_sigma to changes in R_eq. The microscopic origin of this relevant fact is explained in detail. (iii) The removal of axial ligands, keeping R_eq constant, induces a decrease of E_u as a result of the diminution of the electrostatic repulsion, V_el (M), and produces a slight increase of Delta ₁ and a more important one of Delta ₂. (iv) The dependence of _g//-g₀ and g_{perpendicular to}-g₀ on R_eq essentially reflects that of Delta ₁^-1 and Delta ₂^-1, respectively, because of the low covalency (f_sigma approximately=2%).