9.6: Bonding in TiS₂, MoS₂, and Pyrite Structures

Many layered dichalcogenides, such as TiS₂ and ZrS₂, have the CdI₂ structure. In these compounds, as we have noted above, the metal ions are octahedrally coordinated by S. Interestingly, the structures of MoS₂ and WS₂, while they are also layered, are different. In these cases, the metal is surrounded by a trigonal prism of sulfur atoms. NbS₂, TaS₂, MoSe₂, MoTe₂, and WSe₂ also have the trigonal prismatic molybdenite structure, which is shown below alongside a platy crystal of MoS₂.

The coordination of the metal ions by a trigonal prism of chalcogenide ions is sterically unfavorable relative to octahedral coordination. There are close contacts between the chalcogenide ions, which are eclipsed in the stacking sequence AbA/BaB/AbA/BaB... (where "/" indicates the van der Waals gap between layers). What stabilizes this structure?

The molybdenite structure occurs most commonly in MX₂ compounds with a d¹ or d² electron count. The figure below compares the splitting of d-orbital energies in the octahedral and trigonal prismatic coordination environments:

The trigonal prismatic structure is stabilized in MoS₂ by filling the lowest energy band, the d_z2. The d_z2 orbital which points vertically through the triangular top and bottom faces of the trigonal prism, has the least interaction with the sulfide ligands and therefore the lowest energy. The d_xz and d_yz orbitals, which point at the ligands, have the highest energy. The d_z2 orbital is lower in energy in this structure than the t_2g orbitals are in the octahedral structure of TiS₂.

d-orbital splittings and energy bands in TiS2 and MoS2. MoS2 is a semiconductor with a 1.3 eV gap between its filled and empty bands.

Because it has an unfilled t_2g band, TiS₂ is relatively easy to reduce by intercalation with Li. For this reason, LiTiS₂ was one of the first intercalation compounds studied by Stanley Whittingham, who developed the concept of the non-aqueous lithium ion battery in the early 1970's.^[3] Because it has a filled d_z2 band, MoS₂ is harder to reduce, but it can be intercalated by reaction with the powerful reducing agent n-butyllithium to make Li_xMoS₂ (x < 1). Atoms in the van der Waals planes of these compounds are relatively unreactive, which gives MoS₂ its good oxidative stability and enables its application as a high temperature lubricant. Atoms at the edges of the crystals are however more reactive and in fact are catalytic. High surface area MoS₂, which has a high density of exposed edge planes, is used as a hydrodesulfurization catalyst and is also of increasing interest as an electrocatalyst for the reduction of water to hydrogen.

Layered metal dichalcogenides, including MoS₂, WS₂, and SnS₂, can form closed nanostructures that take the shape of multiwalled onions and multiwalled tubes. These materials were discovered by the group of Reshef Tenne in 1992, shortly after the discovery of carbon nanotubes. Since then nanotubes have been synthesized from many other materials, including vanadium and manganese oxides.

The pyrite (FeS2) crystal structure. The structure is related to NaCl, with Fe^2^+ and S2^2^- ions occupying the cation and anion sites.

Although early (TiS₂) and late (PtS₂) transition metal disulfides have layered structures, a number of MS₂ compounds in the middle of the transition series, such as MnS₂, FeS₂ and RuS₂, have three-dimensionally bonded structures. For example, FeS₂ has the pyrite structure, which is related to the NaCl structure. The reason is that FeS₂ is not Fe⁴⁺(S^2-)₂, but is actually Fe²⁺(S₂^2-), where S₂^2- is the disulfide anion (which contains a single bond like the peroxide anion O₂^2-). S^2- is too strong a reducing agent to exist in the same compound with Fe⁴⁺, which is a strong oxidizing agent. Because FeS₂ is actually Fe²⁺(S₂^2-), it is a 1:1 compound and adopts a 1:1 structure.