Reaction of (Na2MoO4)-O-VI center dot 2H(2)O with (NH4)(2)SO3 in the mixed-solvent system H2O/CH3CN (pH = 5) resulted in the formation of the tetranuclear cluster (NH4)(4)[(Mo4SO16)-S-VI]center dot H2O (1), while the same reaction in acidic aqueous solution (pH = 5) yielded (NH4)(4)[(Mo5S2O21)-S-VI]center dot 3H(2)O (2). Compound {(H(2)bipy)(2)[(Mo5S2O21)-S-VI]center dot H2O}(x) (3) was obtained from the reaction of aqueous acidic solution of (Na2MoO4)-O-VI center dot 2H(2)O with (NH4)(2)SO3 (pH = 2.5) and 4,4'-bipyridine (4,4'-bipy). The mixed metal/sulfite species (NH4)(7)[Co-III((Mo2O4)-O-V)(NH3)(SO3)(6)]center dot 4H(2)O (4) was synthesized by reacting (Na2MoO4)-O-VI center dot 2H(2)O with CoCl2 center dot 6H(2)O and (NH4)(2)SO3 with precise control of pH (5.3) through a redox reaction. The X-ray crystal structures of compounds 1, 2, and 4 were determined. The structure of compound 1 consists of a ring of four alternately face- and edge-sharing (MoO6)-O-VI octahedra capped by the trigonal pyramidal sulfite anion, while at the base of the Mo-4 ring is an oxo group which is asymmetrically shared by all four molybdenum atoms. Compound 3 is based on the Strandberg-type heteropolyion [(Mo5S2O21)-S-VI](4-), and these coordinatively saturated clusters are joined by diprotonated 4,4'-H(2)bipy(2+) through strong hydrogen bonds. Compound 3 crystallizes in the chiral space group C2. The structure of compound 4 consists of a novel trinuclear [(CoMo2SO32-)-Mo-III-S-V] cluster. The chiral compound 3 exhibits nonlinear optical (NLO) and photoluminescence properties. The assignment of the sulfite bands in the IR spectrum of 4 has been carried out by density functional calculations. The cobalt in 4 is a d(6) octahedral low-spin metal atom as it was evidenced by magnetic susceptibility measurements, cw EPR, BVS, and DFT calculations. The IR and solid-state UV-vis spectra as well as the thermogravimetric analyses of compounds 1-4 are also reported.