A number of new [Co(cyclen)(NH3)X](3+/2+) complexes have been prepared (X = OH2, Cl, N-3, OH), and their stereochemistries, interconversions, and substitution reactions have been explored. All have the syn,anti configuration about the two meridional amine centers of the cis-coordinated cyclen ligand (syn(NH3),anti(X)and syn(X),anti(NH3)-isomers for X = OH2, Cl, N-3, OH; designated as l-X and 2-X respectively) but a syn,syn intermediate is proposed for a number of the reactions. OH--catalyzed hydrolysis (base hydrolysis) dominates the substitution chemistry in aqueous solution (k(OH)/M-1 s(-1) values reported) and results from deprotonation at one of the relatively very acidic meridional NH centers (k(H)/M-1 s(-1) values reported). A syn orientation of the NH proton in the reactant is considered important in these reactions, with the resulting lone pair assisting in the displacement of X, and with this proton being transferred to X = OH to facilitate an unusually fast and spontaneous solvent replacement reaction (k(ex) = 8 +/- 2 s(-1), I = 1.0 M (NaClO4), 25 degrees C). Anation by N-3(-) has been studied, and this proceeds largely via 2-OH for both isomers. A crystal structure of syn(NH3),anti(N-3)-[Co(cyclen)(NH3)-N-3]Cl-0.5(ClO4)(1).(5). H2O is reported; monoclinic, P2(1)/n (No. 14), a = 9.008(6) Angstrom, b = 28.690(15) Angstrom, c = 14.528(7) Angstrom, a = 90 degrees, beta = 104.12(7)degrees, gamma = 90 degrees, Z = 8, R = 0.0695.