Molybdenum (Mo)

Molybdenum shows a wide range of oxidation states from \(-2\) to \(+6\), with \(+6\) and \(+4\) being the most common in inorganic chemistry.

• +6: Oxo species such as \(\mathrm{MoO_3}\) and the tetrahedral molybdate anion \(\mathrm{MoO_4^{2-}}\).
• +5/+4: Mixed-valent oxo clusters and oxides; \(\mathrm{MoO_2}\) (Mo(IV)) is a common solid.
• 0 and negative states: Observed in carbonyls and sulfide clusters (e.g., \(\mathrm{Mo(CO)_6}\), \(\mathrm{Mo_2S_{10}^{2-}}\)).

Solid-solution strengthening and carbide formation improve hot hardness, creep resistance, and corrosion resistance. Mo forms stable carbides (e.g., \(\mathrm{Mo_2C}\)) and refines grain boundaries, which helps steels retain strength at elevated temperatures (power plants, aerospace, oil & gas).

The ground-state configuration is \([\mathrm{Kr}]\,4d^5\,5s^1\), which is an example of a half-filled \(d\)-subshell stabilization (an "anomalous" departure from the naive \(4d^4\,5s^2\)). This favors exchange energy and overall lower energy.

All three are Group 6 elements (Cr, Mo, W). Trends down the group include increasing atomic mass, higher melting points, and more stable high-oxidation-state oxo-chemistry:

• Chromium: Strong oxidant in \(\mathrm{Cr_2O_7^{2-}}\), but lower melting point than Mo/W.
• Molybdenum: Balance of high-temperature strength and workable density; versatile oxo- and sulfide chemistry.
• Tungsten: Highest melting point in the group; heavier and denser, often chosen for extreme heat applications.

Key compounds include:

• Molybdenum trioxide, \(\mathrm{MoO_3}\): precursor for many Mo chemicals and catalysts.
• Ammonium heptamolybdate, \(\mathrm{(NH_4)_6Mo_7O_{24}\cdot4H_2O}\): lab source of \(\mathrm{MoO_4^{2-}}\).
• Molybdate ion, \(\mathrm{MoO_4^{2-}}\): tetrahedral, dominant in alkaline/neutral solutions.
• Molybdenum disulfide, \(\mathrm{MoS_2}\): layered solid lubricant and key HDS catalyst support phase.

Hydrodesulfurization (HDS) catalysts: Sulfided Mo (often promoted with Co or Ni) on high-surface-area supports removes sulfur from petroleum fractions.

Solid lubricant: Layered \(\mathrm{MoS_2}\) has weak interlayer forces, enabling low friction in vacuum/high-temperature environments where oils fail.

Yes, as part of the molybdenum cofactor (Moco) bound to the organic ligand molybdopterin. Enzymes such as xanthine oxidase, aldehyde oxidase, and sulfite oxidase use Mo to mediate oxygen-atom transfer reactions.

In biology, the active site often cycles between \(\mathrm{Mo^{VI}}\) and \(\mathrm{Mo^{IV}}\) via oxo/hydroxo ligation, commonly depicted as \(\mathrm{Mo=O}\) cores within the cofactor framework.

In alkaline to neutral solutions, \(\mathrm{MoO_4^{2-}}\) predominates and remains relatively soluble. At lower pH, polymolybdates form via condensation (e.g., \(\mathrm{[Mo_7O_{24}]^{6-}}\)), and under very acidic conditions, molybdic acid species appear. Speciation can be summarized as:

Mo is commonly obtained from the mineral molybdenite (\(\mathrm{MoS_2}\)). Processing steps include flotation concentration, roasting to \(\mathrm{MoO_3}\), and reduction to metal powder:

Mo is used in:

• Steels & superalloys for turbines, boilers, tools.
• Electronics (thin films, contacts) due to thermal stability and conductivity.
• Catalysts for refining (HDS) and specialty chemical transformations.
• Lubricants (\(\mathrm{MoS_2}\)) in extreme environments.

Metallic Mo is of relatively low toxicity, but certain soluble hexavalent compounds (e.g., \(\mathrm{MoO_4^{2-}}\)) can be harmful at elevated exposure. Handle powders to avoid inhalation, follow workplace exposure limits, and manage catalyst/waste streams appropriately. In the environment, molybdate mobility increases under alkaline, oxidizing conditions.