In our pursuit of mixed chalcogen-bridged cluster complexes, solids of the compositions Mo3SSe6Br4 and W3SSe6Br4 were prepared using high-temperature synthesis from the elements. Treatment of Mo3SSe6Br4 with Bu4NBr in a vibration mill yielded (Bu4N)(3)([Mo-3(mu(3)-S)(mu(2)-Se-2)(3)Br-6]Br} (I). Its all-selenide analogue (Bu4N)(3){[Mo-3(mu(3)-Se)(mu(2)-Se-2)(3)Br--(6)]Br} (II) was prepared from Mo3Se7Br4 in a similar way. Both compounds were characterized by IR, Raman, and Se-77 NMR spectroscopy. The structure of 11 was determined by X-ray single-crystal analysis. Compound I is isostructural with II and contains the new Mo-3(mu(3)-S)Se-6(4+) cluster core. By treatment of a 4 M Hpts solution of I with PPh3 followed by cation-exchange chromatography, the new mixed chalcogenido-molybdenum aqua ion, [Mo-3(mu S--(3))(mu(2)-Se)(3)(H2O)(9)](4+), was isolated and characterized using UV-vis spectroscopy and, after derivatization into [Mo-3(mu S--(3))(mu(2)-Se)(3)(acac)(3)(py(3))](+), electrospray ionization mass spectrometry. From HCl solutions of the aqua ion, a supramolecular adduct with cucurbit[6]uril (CB[6]), {[Mo-3(mu(3)-S)(mu(2)-Se)(3)(H2O)(6)Cl-3](2)CB[6]}Cl-2 center dot 11H(2)O (III), was isolated and its structure determined using X-ray crystallography. W3SSe6Br4 upon reaction with H3PO2 gave a mixture of all of the [W3SxSe4-x(H2O)(9)](4+) species. After repeated chromatography, crystals of {[W-3(mu(3)-S)(mu(2)-Se)(3)(H2O)(7)Cl--(2)](2)CB[6]}Cl-4 center dot 12H(2)O (IV) were crystallized from the fraction rich in [W-3(mu(3)-S)Se-3(H2O)(9)](4+) and structurally characterized.