The two new Mn-III dinudear compounds [{Mn-(H2O)(phen)}(2)(mu-4-CH3C6H4COO)(2)(mu-O)](ClO4)(2)center dot 3CH(3)CN center dot H2O (1.3CH(3)CN center dot H2O) and [{Mn(H2O)(phen)}(mu-O)](mu-2-BrC6H4COO)(2){Mn(NO3)(phen)}]NO3 (2) have been synthesized. Their structural data reveal significant differences in the shape of the coordination octahedron around the Mn-III ions in both compounds. The different distortions from ideal geometry incite a very different magnetic behavior, affecting both the zero-field splitting parameters of the Mn-III ions (D-Mn and E-Mn) and the magnetic interaction between them. Compound 1, with elongation in the monodentate ligand direction, shows antiferromagnetic coupling (ground state S = 0) and local D-Mn < 0, while compound 2, with compression in the oxo bridge direction, displays a ferromagnetic interaction (ground state S = 4) and local D-Mn > 0. Theoretical CASSCF and DFT calculations corroborate the different magnetic anisotropy and exchange coupling found in both compounds. Moreover, with the help of theoretical calculations, some interesting magneto-structural correlations have been found between the degree of distortion of the coordination octahedra and the magnetic coupling; it becomes more antiferromagnetic when the elongation parameter, Delta, in absolute value is increased.