The dipole moment-covalency-polarisability correlation ligand-field model is proposed, after a number of experimental spectra data have been analysed. In this model, once the mu - kappa - alpha correlations are established, there is no need to introduce the fitting parameters in order to investigate the optical and magnetic properties for a set of Ni²⁺-6X^- clusters. Using this model and a parametrisation d-orbit theory, the theoretical absorption spectra of Ni²⁺-6X^- clusters, such as RbNiF₃, KNiF₃, NiF₂, BaNiF₄, RbNiCl₃, CsNiCl₃, NiCl₂, (CH₃)₄NNiCl₃, CsNiBr₃, NiBr₂, (CH₃)₄NNiBr₃, RbNiBr₃, CsNiI₃ and NiI₂ are calculated without any fitting parameters. The calculation results agree well with the experimental findings. The calculated absorption spectra, EPR parameters and magnetic susceptibility are in good agreement with experimental results, in particular for CsNiCl₃.