The novel organometallic aqua complex [(CO)(3)Mn(H2O)(3)](+) (1(+)) was obtained through hydrolysis of the analogous acetone complex. IR [nu(CO) = 2051, 1944 cm(-1)] and O-17 NMR spectroscopy revealed the presence of a fac tricarbonyl unit. Potentiometric titrations established that the trimer [(CO)(3)Mn-3(OH)(4)](-) was the principal condensation product in the pH range >6 prior to slow formation of the tetramer [{(CO)(3)Mn}(OH)](4). Water exchange in 1(+), determined by NMR line broadening as k(ex) = 19 +/- 4 s(-1) at 298 K, is four orders faster than with the analogous Re complex. The activation volume DeltaV(double dagger) = -4.5 +/- 0.4 cm(3) mol(-1) is indicative of an associatively activated (I-a) process.