Orbital overlap and spin polarization effects in Mo and W [M(2)X(9)](3)(-) halide and in [M(2)X'(3)X' '(6)](3)(-) mixed-halide systems have been investigated by means of density-functional calculations performed on the S = 0, S = 3, and reference states of these species. For the regular [M(2)X(9)](3)(-) systems, a strong linear correlation between the two factors has been obtained, and decreasing trends in both the overlap energy and the spin polarization energy upon descending the halide group have been observed. These trends can be related to the changes in the size and covalency of the ligands and in the nature of the metal-bridge interaction. For the mixed-ligand [M(2)X'(3)X' '(6)](3)(-) systems, important deviations (from the behavior of the regular systems), which are apparently the result of particular structural and energetic characteristics, have been observed.