The reactions between the tetra-n-butylammonium salt of [{Mo6I8}I-6](2-) and silver carboxylates RCOOAg (R = CH3 (1), C(CH3)(3) (2), alpha-C4H3O (3), C6H5 (4), alpha-C10H7 (5), or C2F5 (6)) in CH2Cl2 afforded new carboxylate complexes [{Mo6I8}(RCOO)(6)](2-). The complexes were characterized by X-ray single-crystal diffraction and elemental analysis, cyclic/differential pulse voltammetry, and IR, NMR, and UV-visible spectroscopies. The emission properties of the complexes 1-6, and those of the earlier reported complexes with R = CF3 (7) and n-C3F7 (8), were studied both in acetonitrile solution and in the solid state. In deaerated CH3CN at 298 K, all of the complexes 1-8 exhibit intense and long-lived emission with the quantum yield and lifetime being 0.48-0.73 and 283-359 mu s, respectively. The oxidation (E-ox)/reduction (E-red) potentials of the complexes correlate linearly with the pK(a) value of the terminal carboxylate ligands L = RCOO (pK(a)(L)). Reflecting the pK(a)(L) dependences of E-ox/E-red, the emission energy (nu(em)) of the complexes was also shown to correlate with pK(a)(L). The present study successfully demonstrates synthetic tuning of the redox, spectroscopic, and photophysical characteristics of a {Mo6I8}(4+)-based cluster complex with pK(a)(L).