The purple corner-shared double cube [Mo6HgS8(H2O)(18)](8+) derivative of green [Mo3S4(H2O)(9)](4+), obtained under air-free conditions by the reaction with Hg-0 (metal), is also formed with Hg-2(t). The Hg-2(t) reaction is accounted for by the disproportionation Hg-2(t) reversible arrow Hg-0 + Hg-11, which is a source of Hg-0. X-ray crystallographic information on the blue partially Cl- substituted cucurbituril supramolecular assemblies [Mo(6)HgQ(8)Cl(4)(H2O)(14)](C36H36N24O12)Cl-4. 14H(2)O (1) and of the Se analogue [Mo6HgSe8Cl4 (H2O)(14)](C36H36N24O12)Cl-4. 14H(2)O (2) have been determined. The product [W6HgSe8Cl4(H2O)(14)](C36H36N24O12)Cl-4. 14H(2)O (3) has also been obtained, but there is no evidence for [W6HgS8(H2O)(18)](8+) and related forms. The formation of [Mo6HgS8(H2O)(18)](8+) by the reaction of [MO3S4 (H2O)(9)](4+) with HgO under anaerobic conditions maximizes after similar to 40 h in 2.0 M HCl, but requires longer reaction time (similar to 120 h) in 2.0 M Hpts (p-toluenesulfonic acid) and in 2 M HClO4 (similar to6 days). In 2.0 M HCl there is little absorbance increase until [Mo3S4(H2O)(9)](4+) exceeds 1.2 x 10(-3) M, which is explained by a dependence of the formation K (265 M-1) on [Mo3S4(H2O)(9)(4+)](2). Furthermore, on dilution of column-purified [Mo6HgS8(H2O)(18)](8+), Beer's law is not obeyed and equilibria involving 2[Mo3S4(H2O)(9)](4+) are apparent. The kinetics of formation of [Mo6HgS8(H2O)(18)](8+) is first-order in [Mo3S4(H2O)(9)](4+), consistent with rate-determining formation of the single cube [Mo3HgS4(H2O)(x)](4+). The oxidations of [Mo6HgS8(H2O)(18)](8+) with [Fe(H2O)(6)](3+) and [Co(dipic)(2)](-) are complicated by the release of [Hg(H2O)(6)](2+), which also functions as an oxidant. Similar results are obtained for [Mo6HgSe8(H2O)(18)](8+) and the less extensively studied [W6HgSe8(H2O)(18)](8+).