= a transition metal and E = S, Se, Te. The most important members are the sulfides. They are always dark diamagnetic solids, insoluble in all solvents, and exhibit semiconducting properties. Some are superconductors. In terms of their electronic structures, these compounds are usually viewed as derivatives of M4+, where M4+ = Ti4+ (d0 configuration), V4+ (d1 configuration), Mo4+ (d2 configuration). Titanium disulfide was investigated in prototype cathodes for secondary batteries, exploiting its ability to reversibly undergo intercalation by lithium. Molybdenum disulfide is the subject of thousand articles and the main ore of molybdenum, termed molybdenite. It is used as a solid lubricant and catalyst for hydrodesulfurization. The corresponding diselenides and even ditellurides are known, e.g., TiSe2, MoSe2, and WSe2. Transition metals Transition metal dichalcogenides typically adopt either cadmium diiodide or molybdenum disulfide structures. In the CdI2 motif, the metals exhibit octahedral structures. In the MoS2 motif, which is not observed for dihalides, the metals exhibit trigonal prismatic structures. The strong bonding between the metal and chalcogenide ligands, contrasts with the weak chalcogenide—chalcogenide bonding between the layers. Owing to these contrasting bond strengths, these materials engage in intercalation by alkali metals. The intercalation process is accompanied by charge transfer, reducing the M(IV) centers to M(III). The attraction between electrons and holes in 2D tungsten diselenide is 100s of times stronger than in a typical 3D semiconductor. Pyrite and related disulfides In contrast to classical metal dichalcogenides, iron pyrite, a common mineral, is usually described as consisting of Fe2+ and the persulfido anion S22−. The sulfur atoms within the disulfido dianion are bound together via a short S-S bond. "Late" transition metal disulfides (Mn, Fe, Co, Ni) almost always adopt the pyrite or the related marcasite motif, in contrast to early metals (V, Ti, Mo, W) which adopt 4+ oxidation state with two chalcogenide dianions. Tri- and tetrachalcogenides Several metals, mainly for the early metals (Ti, V, Cr, Mn groups) also form trichalcogenides. These materials are usually described as M4+(E22−)(E2−) (where E = S, Se, Te). A well known example is niobium triselenide. Amorphous MoS3 is produced by treatment of tetrathiomolybdate with acid: MoS42− + 2 H+ → MoS3 + H2S The mineral patrónite, which has the formula VS4, is an example of a metal tetrachalcogenide. Crystallographic analysis shows that the material can be considered a bis(persulfide), i.e. V4+,(S22−)2. Main group chalcogenides Chalcogen derivatives are known for all of the main