Several single-molecule magnets with the composition [Mn12O12(O2CR)(16)(H2O)(x)] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals. one in the 4-7 K region and the other in the 2-3 K region. New Mn-12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn-12 complex with a given R substituent exist where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)(16)(H2O)(3)]. 4H(2)O (space group P (1) over bar) (complex 2a) has been reported previously. The more prevalent needle-form of ]Mn12O12(O2CEt)(16)(H2O)(3)] (complex. 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degreesC has a = 16.462(7) Angstrom, b = 22.401(9) Angstrom, c = 20.766(9) Angstrom, beta = 103.85(2)degrees, and Z = 4. The arrangements of H2O and carboxylate ligands on the Mn-12 molecule are different in the two crystal forms. The complex [Mn12O12-(O2CC6H4-p-Cl)(16)(H2O)(4)]. 8CH(2)Cl(2) (5) crystallizes in the monoclinic space group C2/c, which at -172 degreesC has a = 29.697(9) Angstrom, b = 17.708(4) Angstrom, c = 30.204(8) Angstrom, beta = 102.12(2)degrees, and Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) Angstrom, b = 18.2288(2) Angstrom, c 26.5882(4) Angstrom, beta 125.8359(2)degrees, and Z = 4. [Mn12O12(O2CC6H4-P-Me)(16)(H2O)(4)].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) Angstrom, b = 32.2371(4) Angstrom, c = 29.8738(6) Angstrom, beta = 99.2650(10)degrees, and Z = 8. The Mn-12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H2O ligands. Complex 6 exhibits an out-of-phase ac peak (chi (M)") in the 2-3 K region, whereas the hydrate complex 7 has a chi (M)" signal in the 4-7 K region. In addition, however, in complex 6, one Mn-III ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 6 compared with complex 7. Thus: complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi (M)" signal in the 2-3 K region, whereas complex 7 has its. chi (M)" peak in the 4-7 K region, i.e., the rate of tunneling of magnetization is greater in complex 6 than complex 7. Detailed H-1 NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p methyl-benzoate Mn-12 complexes and confirm the structural integrity of the (Mn12O12) complexes upon dissolution. In solution there is rapid ligand exchange and no evidence for the different isomeric forms of Mn-12 complexes seen in the solid state.