Ammonium tetrathiomolybdate

Ammonium tetrathiomolybdate is the chemical compound with the formula [NH₄]₂[MoS₄]. This bright red ammonium salt is an important reagent in the chemistry of molybdenum and has been used as a building block in bioinorganic chemistry. The thiometallate anion has the distinctive property of undergoing oxidation at the sulfur centers concomitant with reduction of the metal from Mo(VI) to Mo(IV).

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Preparation and structure

The salt contains the tetrahedral [MoS₄]^2- anion. The compound is prepared by treating solutions of molybdate, [MoO₄]^2- with hydrogen sulfide in the presence of ammonia:^[1]

[NH₄]₂[MoO₄] + 4 H₂S → [NH₄]₂[MoS₄] + 4 H₂O

Reactions

The anion is also an excellent ligand. For example, with Ni(II) sources, it forms [Ni(MoS₄)₂]^2-. Much of the chemistry of the thiomolybdate results from studies on salts of quaternised organic cations, such as [NEt₄]₂[MoS₄] and [PPh₄]₂[MoS₄] (Et = C₂H₅, Ph = C₆H₅).^[2] These organic salts are soluble in polar organic solvents such as acetonitrile and dmf.

The thermal decomposition of [NH₄]₂[MoS₄] leads to sulphides of composition MoS_x (2< x< 3), which are of interest as catalysts for hydrodesulfurization.^[3]

Related compounds

Several related thio and seleno anions are known including (A = alkali metal cation, [PPh₄]⁺, [NEt₄]⁺)

• A₃[VS₄]^[4]
• A₃[NbS₄]^[4]
• A₃[TaS₄]^[4]
• A₂[MoSe₄]
• A₂[WS₄]^[5]
• A₂[WSe₄]
• A[ReS₄]^[6]

More complex tetrahedral anions include A₂[MoS_4-xO_x] and A₂[WS_4-xO_x]

References

1. ^ Müller, A.; Diemann, E.; Jostes, R.; Bögge, H. (1981). "Transition Metal Thio Anions: Properties and Significance for Complex Chemistry and Bioinorganic Chemistry". Angewandte Chemie International Edition in English 20: 934. doi:10.1002/anie.198109341.
2. ^ Coucouvanis, D. (1998). "Syntheses, Structures, and Reactions of Binary and Tertiary Thiomolydate Complexes Containing the (O)Mo(S_x) and (S)Mo(S_x) Functional Groups (x = 1, 2, 4)". Advances in Inorganic Chemistry 45: 1-73.
3. ^ Alonso, G.; Berhault, G.; Aguilar, A.; Collins, V.; Ornelas, C.; Fuentes, S.; Chianelli, R. R. (2002). "Characterization and HDS Activity of Mesoporous MoS₂ Catalysts Prepared by in Situ Activation of Tetraalkylammonium Thiomolybdates". J. Catal. 208: 359-369. doi:10.1006/jcat.2002.3553.
4. ^ ^{a b c} Lee, S. C.; Li, J.; Mitchell, J. C.; Holm, R. H., (1992). "Group 5 Tetrathiometalates: Simplified Syntheses and Structures". Inorg. Chem. 31: 4333-4338. doi:10.1021/ic00047a021.
5. ^ Srinivasan, B. R.; Poisot, M.; Näther, C.; Bensch, W. (2004). "Diammonium tetrathiotungstate(VI), [NH₄]₂[WS₄], at 150 K". Acta Crystallographica Section E: Structure Reports Online E60: i136-i138.
6. ^ Goodman, J. T.; Rauchfuss, T. B., (2002). "Tetraethylammonium-tetrathioperrhenate [Et₄N][ReS₄]". Inorganic Syntheses 33: 107-110.