Substitution and exchange reactions of cis- and trans-L-1(H2O)RhH2+ (L-1 = 1,4,8,11-tetraazacyclotetradecane = [14]aneN(4)) were studied in aqueous solutions by UV-vis and H-1 NMR spectroscopies. At pH 1 and 25 degreesC, the substitution of SCN- for the coordinated molecule of water is rapid and thermodynamically favorable. Spectrophotometric determinations yielded the equilibrium constants K = 1.49 x 10(3) M-1 (cis) and 1.44 x 10(3) (trans). H-1 NMR studies in D2O revealed a rapid dynamic process, interpreted as the exchange between coordinated water and X- (X = Cl, Br, or l). On the other hand, no line broadening was observed for the strongly bound ligands CN- and SCN-. The complex trans-L-1(D2O)RhH2+ undergoes a base-catalyzed H/D exchange of the hydride in D2O with a rate constant of (1.45 +/- 0.02) x 10(3) M-1 s(-1). The exchange in the cis isomer is very slow under similar conditions. The complex cis-[(LClRhH)-Cl-1](ClO4) crystallizes in the centrosymmetric P (1) over bar space group, unit cell dimensions a = 8.9805(11) Angstrom, b = 9.1598(11) Angstrom, c = 10.4081(13) Angstrom, alpha = 81.091(2)degrees, beta = 81.978(2)degrees, gamma = 88.850(2)degrees. The rhodium atom resides in a slightly distorted octahedral environment consisting of the four N atoms of the cyclam, a wstereochemically active hydrogen, and a chlorine atom.