The mixed valence dodecanuclear complex [Mn12O12(O(2)CEt)(16)(H2O)(3)] may be reduced by one electron to give (PPh(4))[Mn12O12(O(2)CEt)(16)(H2P)(4)] Reduction of the complex occurs at one Mn-III ion to give an unusual trapped-valence (MnMn7Mn4IV)-Mn-II-Mn-III complex. This formulation is supported by X-ray crystallography at both low temperature and room temperature. DC (2.0-4.0 K, 20-50 kG fields) and AC (2.0-30 K, 1.0 and 0.0050G fields, 25-1000 Hz frequencies) magnetic susceptibility measurements indicate ground states of S = 9 and S = 19/2 for the neutral and reduced species, respectively. (PPh(4))[Mn12O12(O(2)CEt)(16)(H2O)(4)] exhibits an out-of-phase component of the AC magnetic susceptibility (chi(M) ") which has a frequency dependent maximum. This is a very unusual property for a discrete molecular species to display. The neutral analog shows two peaks in the chi(M) " vs. T plot. DC measurements also show the presence of prominent hysteresis loops, and field-cooled and zero-field-cooled magnetization data are not superimposed at low temperatures. The unusual magnetic relaxation properties persist even in polystyrene-doped samples of the complexes, indicating that this behavior arises from intramolecular rather than intermolecular properties.