In our pursuit of mixed chalcogen-bridged cluster complexes, solids of the compositions Mo(3)SSe(6)Br(4) and W(3)SSe(6)Br(4) were prepared using high-temperature synthesis from the elements. Treatment of Mo(3)SSe(6)Br(4) with Bu(4)NBr in a vibration mill yielded (Bu(4)N)(3){[Mo(3)(mu(3)-S)(mu(2)-Se(2))(3)Br(6)]Br} (I). Its all-selenide analogue (Bu(4)N)(3){[Mo(3)(mu(3)-Se)(mu(2)-Se(2))(3)Br(6)]Br} (II) was prepared from Mo(3)Se(7)Br(4) in a similar way. Both compounds were characterized by IR, Raman, and (77)Se NMR spectroscopy. The structure of II 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 PPh(3) followed by cation-exchange chromatography, the new mixed chalcogenido-molybdenum aqua ion, [Mo(3)(mu(3)-S)(mu(2)-Se)(3)(H(2)O)(9)](4+), was isolated and characterized using UV-vis spectroscopy and, after derivatization into [Mo(3)(mu(3)-S)(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)(H(2)O)(6)Cl(3)](2)CB[6]}Cl(2) x 11 H(2)O (III), was isolated and its structure determined using X-ray crystallography. W(3)SSe(6)Br(4) upon reaction with H(3)PO(2) gave a mixture of all of the [W(3)S(x)Se(4-x)(H(2)O)(9)](4+) species. After repeated chromatography, crystals of {[W(3)(mu(3)-S)(mu(2)-Se)(3)(H(2)O)(7)Cl(2)](2)CB[6]}Cl(4) x 12 H(2)O (IV) were crystallized from the fraction rich in [W(3)(mu(3)-S)Se(3)(H(2)O)(9)](4+) and structurally characterized.