The reactions of cobalt(II) complexes of tetraazamacrocyclic tropocoronand (TC) ligands with nitric oxide (NO) were investigated. When [Co(TC-5,5)] was allowed to react with NO(g), the {CoNO}(8) mononitrosyl {Co(NO)(TC-5,5)} was isolated and structurally characterized. In contrast, a {Co(NO)(2)}(10) species formed when [Co(TC-6,6)] was exposed to NO(g), and the nitrito [Co(NO2)(TC-6,6)] complex was structurally and spectroscopically characterized from the reaction mixture. The {Co(NO)(2)}(10) species was assigned as the bis(cobalt dinitrosyl) complex [Co-2(NO)(4)(TC-6,6)] by spectroscopic comparison with independently synthesized and characterized material. These results provide the first evidence for the influence of tropocoronand ring size on the nitric oxide reactivity of the cobalt(II) complexes.