The mixed valence dodecanuclear complex [Mn12O12(O2CEt)16(H2O)3may be reduced by one electron to give (PPh4)[Mn12O12(O2CEt)16(H2O)4. Reduction of the complex occurs at one MnIIIion to give an unusual trapped-valence MnIIMnIII7MnIV4complex. This formulation is supported by X-ray crystallography at both low temperature and room temperature. DC (2.0–4.0K, 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. (PPh4)[Mn12O122(O2CEt)16(H2O)4exhibits an out-of-phase component of the AC magnetic susceptibility (XM) 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 theXMvs.Tplot. 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.