Syntheses and characterizations reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr-) ligand, where Ar is XC(6)H(4)(-), with X as p-OMe (1), H (2), m-OMe (3), p-Cl (4), m-Cl (5), m-CF3 (6), p-COMe (7), p-CF3 (8), or Ar is 3,4-Cl2C6H3- (9) or 3,5-Cl2C6H3- (10). The (quasi)reversible oxidation potentials measured for the Mo-2(5+)/Mo-2(4+) couple were found to correlate with the Hammett constant (sigma(X)) of the aryl substituents according to the following equation : Delta E(1/2) = E(1/2)(X) - E(1/2)(H) = 87(8 sigma(X)) mV. Molecular structure determinations of compounds 1,2,5, and 10 revealed an invariant core geometry around the Mo-2 center, with statistically identical Mo-Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Angstrom, respectively. Magnetic anisotropies for compounds 1-10 estimated on the basis of H-1 NMR data were similar and unrelated to sigma(X). Similarity in UV-vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and H-1 NMR spectra, was interpreted as the existence of a constant upper valence structure across the series. Results of Fenske-Hall calculations performed for several model compounds paralleled the experimental observations.