Transition metal chalcogenides occur with many stoichiometries and many structures. Most common and most important technologically, however, are the chalcogenides of simple stoichiometries, such as 1:1 and 1:2. Extreme cases include metal-rich phases (e.g. Ta2S), which exhibit extensive metal-metal bonding, and chalcogenide-rich materials such as Re2S7, which features extensive chalcogen-chalcogen bonding. For the purpose of classifying these materials, the chalcogenide is often viewed as a dianion, i.e.,
S2−,
Se2−,
Te2−, and
Po2−. In fact, transition metal chalcogenides are highly
covalent, not ionic, as indicated by their semiconducting properties. In most of their chalcogenides, transition metals adopt oxidation states of II or greater. Nonetheless, several examples exist where the metallic atoms far outnumber the chalcogens. Such compounds typically have extensive metal-metal bonding.
Monochalcogenides Metal monochalcogenides have the formula ME, where M = a transition metal and E = S, Se, Te. They typically crystallize in one of two motifs, named after the corresponding forms of
zinc sulfide. In the
zinc blende structure, the sulfide atoms pack in a cubic symmetry and the Zn2+ ions occupy half of the tetrahedral holes. The result is a
diamondoid framework. The main alternative structure for the monochalcogenides is the
wurtzite structure wherein the atom connectivities are similar (tetrahedral), but the crystal symmetry is hexagonal. A third motif for metal monochalcogenide is the
nickel arsenide lattice, where the metal and chalcogenide each have octahedral and trigonal prismatic coordination, respectively. This motif is commonly subject to
nonstoichiometry. Important monochalcogenides include some
pigments, notably
cadmium sulfide. Many minerals and ores are monosulfides.
Dichalcogenides Metal dichalcogenides have the formula ME2, where M = 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 thousands of 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 persulfido 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==