The nickel(II) complex of the encapsulating ligand 1-methyl-8-amino-3,13-dithia-6,10,16,19-tetraazabicyclol[6.6.6]-icosane, [Ni(AMN(4)S(2)sar)](2+), is reported. Crystals of the complex are monoclinic, space group P2(1)/c, with Z = 4, a = 9.167(6) Angstrom, b = 19.885(4) Angstrom, c = 13.068(8) Angstrom, beta = 95.37(3)degrees, and R = 0.055. The low-temperature (similar to 10 K) Single-crystal absorption spectrum and ligand field analysis of the complex are reported. The spin-forbidden (3)A(2g) --> (1)E(g),(1)A(1g) transitions were observed at low temperature, and from the spectroscopic analysis, a significant differential nephelauxetic effect is shown to occur resulting in much lower values for the Racah B and C parameters for these spin-forbidden transitions in relation to the spin-allowed transitions.