The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn-4(O2CMe)(2)(pdmH)(6)](ClO4)(2) (1). Complex 1 . 2MeCN . Et2O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) Angstrom, b = 15.347(4) Angstrom, c = 9.660(3) Angstrom, alpha = 104.58(1)degrees, beta = 93.42(1)degrees, gamma = 106.06(1)degrees and Z = 1. The cation lies on an inversion center and consists of a planar Mn-4 rhombus that is mixed-valent, (Mn2Mn2II)-Mn-III The pdmH(-) ligands (pdmH(2) is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH-ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1 . 2.5H(2)O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1 2.5B20 and indicate that the desolvated complex has a S = 8 ground state, whereas the hydrated 1 . 2.5H(2)O has a S = 9 ground state. Ferromagnetic interactions between Mn-III-Mn-II and Mn-III-Mn-III pairs result in parallel spin alignments of the S = (5)/(2) Mn-II and S = 2 Mn-III ions. High-frequency EPR spectra were run for complex 1 . 2.5H(2)O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99, D = -0.451 K, and B(4)degrees =2.94 x 10(-5) K for the S = 9 ground state of 1 . 2.5H(2)O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals (chi''(M)) were seen for complexes 1 and 1 . 2.5H(2)O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in chi''(M) VS temperature plots was seen for each frequency; the temperature of the chi''(M) peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S = 8 complex with D = -0.35 K. The 1 . 2.5H(2)O complex with an S = 9 ground state has its chi"(M) peaks at higher temperatures.