A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo-2(ArNCHNAr)(3)](2)(mu-O2CC6H4CO2) (Ar = p-XC6H4, with X = OCH3 (1), CH3 (2), F (3), Cl (4), OCF3 (5), and CF3 (6)), has been synthesized and studied in terms of substituent effects on electron delocalization between the two dimetal sites. X-ray structural analyses show that these complexes share the same molecular scaffold with the para-substituents (X) being about 8 angstrom away from the Mo-2 center. It is found that the remote substituents have the capability to tune the electronic properties of the complexes. For the series 1 to 6, the metal-metal bond distances (d(Mo-Mo)) decrease slightly and continuously; the potential separations (Delta E-1/2) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (lambda(max)) increase in order. More interestingly, the two types of methine protons, H-parallel to on the horizontal and H-perpendicular to on the vertical ligands with respect to the plane defined by the Mo-Mo bond vectors and bridging ligand, display separate resonant signals delta(parallel to) and delta(perpendicular to) in the NMR spectra. The displacements of the chemical shifts, Delta delta(parallel to-perpendicular to) = delta(parallel to) - delta(perpendicular to), are getting smaller as the substituents vary from electron-donating to -withdrawing. These results show that the peripheral groups on the [Mo-2] units function to fine-tune the metal-metal interactions crossing the bridging ligand. The experimental parameters, Delta E-1/2, lambda(max), and Delta delta(parallel to-perpendicular to), which are linearly related with the Hammett constants (sigma(x)) of the X groups, can be used to probe the charge density on the two [Mo-2] units and the electronic delocalization between them.