The size-dependent reactivity of cobalt tropocoronands [TC-n,n](2-) is manifest in the NO chemistry of the cobalt(III) nitrite complexes [Co(eta(2)-NO2)(TC-n,n)] (n = 4-6), the synthesis and characterization of which are reported for the first time. Complete conversion of [Co(eta(2)-NO2)(TC-4,4)] to the cobalt mononitrosyl [Co(NO)(TC-4,4)] occurs upon exposure to NO(g). In contrast, addition of NO(g) to [Co(eta(2)-NO2)(TC-5,5)] generates both cobalt mono- and dinitrosyl adducts, and addition of nitric oxide to [Co(eta(2)-NO2)(TC-6,6)] converts this complex to the dicobalt tetranitrosyl species [Co-2(NO)(4)(TC-6,6)]. In the latter complex, two tetrahedral cobalt dinitrosyl units are bound to the aminotroponeiminate poles of the [TC-6,6](2-) ligand. These results significantly broaden the chemistry of cobalt tropocoronands with nitric oxide and the nitrite anion.