The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn-4(hmp)(6)Br-2-(H2O)(2)]Br-2. 4H(2)O (2) and [Mn-4(6-me-hmp)(6)Cl-4]. 4H(2)O (3), where hmp(-) is the anion of 2-hydroxymethylpyridine and 6-me-hmp(-) is the anion of 6-methyl-2-hydroxymethylpyridine. Complex 2 . 4H(2)O crystallizes in the space group P2(1)/c, with cell dimensions at -160 degreesC of a = 10.907(0) Angstrom, b = 15.788(0) Angstrom, c = 13.941(0) Angstrom, = 101.21(0)degrees, and Z = 2. The cation lies on an inversion center and consists of a planar Mn-4 rhombus that is mixed-valence, (Mn2Mn2II)-Mn-III. Them hmp(-) ligands function as bidentate ligands and as the only bridging ligands in 2 . 4H(2)O. Complex 3 . 4H(2)O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 degreesC of a = 17.0852(4) Angstrom, b = 20.8781(5) Angstrom, c = 14.835(3) Angstrom, beta = 90.5485(8)degrees, and Z = 4. This neutral complex also has a mixed-valence (Mn2Mn2II)-Mn-III composition and is best described as having four manganese ions arranged in a bent chain. An mu (2)-oxygen atom of the 6-me-hmp(-) anion bridges between the manganese ions; the Cl- ligands: are terminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex 2 . 4H(2)O has a S = 9 ground state whereas complex 3 . 4H(2)O has S = 0 ground state. Fine structure patterns are seen in the HFEPR spectra, and in the case of 2 . 4H(2)O it was possible to simulate the fine structure assuming S = 9 with the parameters g = 1.999, axial zero-field splitting of D/k(B) = -0.498 K, quartic longitudinal zero-field splitting of B-4 degrees/k(B) = 1.72 x 10(-5) K, and rhombic zero-field splitting of E/k(B) = 0.124 K. Complex 2 . 4H(2)O exhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complex functions as a SMM. The AC susceptibility data for complex 2 . 4H(2)O were measured in the 0.05-4.0 K range and when fit to the Arrhenius law, gave an activation energy of DeltaE = 15.8 K for the reversal of magnetization. This AE value is to be compared to the potential-energy barrier height of U/k(B) = \D (S) over cap (2)(Z)\ = 40.3 K calculated for 2 . 4H(2)O.