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Molybdenum-sulfur compounds

In addition to its affinity for oxygen molybdenum in its higher oxidation states, (III) to (VI), has an appreciable affinity for sulfur, a property which is important in its biochemistry and in a number of applications. The affinity of molybdenum for sulfur is illustrated by its occurrence as the sulfide ore, MoS2, and in molybdoenzymes; its precipitation in qualitative analysis as a sulfide rather than as a hydroxide or hydrated oxide. Molybdenum disulfide is black.

It has a layer structure with Mo atoms in trigonal prismatic coordination sandwiched between sheets of S atoms. It is unreactive towards water and acids and alkalis. However, molybdenum disulfide and other Mo-S compounds oxidise when heated in air to molybdenum trioxide. Because of the unreactivity of molybdenum disulfide Mo-S compounds are made via reaction with molybdates as in the preparation of hydrodesulfurisation catalysts. Molybdenum disulfide is used, per se, as a dry lubricant.

Molybdenum forms complexes with sulfide ([MoS4] 2-) and many organosulfur ligands. These compounds unlike their oxygen counterparts are often brightly coloured, e.g. [MoS4] 2- is deep orange-red. The dithiocarbamates and dithiophosphates are applied as oil soluble lubricant additives.

Octahedral and trigonal prismatic structures

Octahedral and trigonal prismatic structures viewed through triangular faces. A sulfur atom is located at each vertex. A molybdenum atom is located between each pair of triangles. MoO3 and molybdates with linked [MoO6] groups have the octahedral structure; MoS2 is trigonal prismatic.