The copper(II) complex of the N4S2 encapsulating ligand 1-methyl-8-ammonio-3,13-dithia-6,10,16,19-tetraazabicyclo-[6.6.6]icosane, [Cu(AMN(4)S(2)sarH)](3+), is reported. Crystals of the complex are monoclinic, of space group P2(1)/a (Z = 4, a = 16.463(3) Angstrom, b = 11.073(3) Angstrom, c = 17.138(3) Angstrom, beta = 106.20(1)degrees, R = 0.053 (3449 F)). The coordination about the copper(II) atom is a distorted version of a 4 + 2 elongated octahedron with the pronounced tetragonal distortion arising from long Cu-N and Cu-S axial bonds of 2.447(5) and 2.600(2) Angstrom, respectively. The presence of longer Cu-S bonds (2.436(2), 2.600(2) Angstrom) cis to shorter Cu-N bonds (2.012(5), 2.029(5) Angstrom) results in a tilting of the capping groups in relation to the pseudo-3-fold axis of the metal complex, the two caps being eclipsed with respect to one another. The randomly orientated frozen solution EPR spectrum and low-temperature (similar to 10 K) absorption spectrum are indicative of an orthorhombically distorted complex with a pronounced tetragonal elongation, consistent with the crystal structure determination and room-temperature magnetic moment of 1.75 mu(B). Computer simulation of the randomly orientated frozen solution X-band EPR spectrum yielded g(z) = 2.189, g(x) = 2.048, g(y) = 2.074, and A(z) = 160.5 x 10(-4) cm(-1). Ligand nitrogen hyperfine coupling was not resolved on the M(I) = -1/2 parallel resonance at S-band microwave frequencies, presumably a consequence of the low symmetry around the Cu(II) ion. From the analysis of the absorption spectrum, angular-overlap-model ligand-field parameters of e(sigma)(N) = 6000, e(sigma)(S) = 6100 cm(-1) (for the shortest Cu-N and Cu-S bonds), zeta = 600 cm(-1) and k = 0.69 were obtained, resulting in calculated g values in very good agreement with those found from EPR measurements.