The synthesis of the N5S ligand 5-methyl-5-(4-amino-2-thiabutyl)-3,7-diazanonane-1,9-diamine, and its complex with cobalt(III), is reported. Reaction of this complex with nitromethane and formaldehyde results in the nitrocapped encapsulated complex [(1-methyl-8-nitro-3-thia-6,10,13,16,19)-pentaazabicyclo[6.6.6]icosane)cobalt(III)] perchlorate, [Co(NON(3)Ssar)](ClO4)(3) The NH3+-capped complex [Co(AMN(5)SsarH)]Br-4.2H(2)O has been characterized by a single-crystal X-ray study : crystals are monoclinic, space group P2(1)/a, with Z = 4, a = 10.735(3) Angstrom, b = 18.699(7) Angstrom, c = 12.692(3) Angstrom, beta = 91.25(2)degrees, and R = 0.031 (1480 F). The low-temperature (11 K) absorption spectrum of [Co(AMN(5)SsarH)](4+) has been measured in Nafion film, and from the observed positions of both spin-allowed T-1(1g), T-1(2g) and spin-forbidden T-3(1g), T-3(2g) states; octahedral ligand-field parameters of B = 565 cm(-1), C = 3300 cm(-1), and 10Dq = 22900 cm(-1) were determined. Comparison with other sar type N-6, N4S2, N3S3, and S-6 encapsulated complexes indicates that B is reduced by approximately 30 cm(-1) on replacement of a nitrogen with a thioether donor.