MOLYBDENUM

PHILIP C. H. MITCHELL, UNIVERSITY OF READING, ENGLAND

I
was introduced to molybdenum in the 1950s by R. J. P. Williams, my Oxford D. Phil. supervisor. We knew that the enzymes nitrogenase (which catalyzes the reduction of nitrogen to the ammonium ion), nitrate reductase (nitrate to nitrite), and xanthine oxidase (hydroxylation of xanthine to uric acid) were molybdenum dependent. At the Chester Beatty Cancer Research Institute in London, R. C. Bray was using electron spin resonance (ESR) to investigate the kinetics of xanthine oxidase. ESR showed that molybdenum was coordinated by sulfur.

Subsequently, X-ray crystallographers showed that the oxidase enzymes are built around oxomolybdenum centers ligated with sulfur. The structures are familiar from the model oxomolybdenum sulfur complexes that I and others studied. My group prepared the first Mo-cysteine complex, Na2[MoV2O4{SCH2CH(NH2)COO}2]5H2O. However, the dimeric anion turned out to be a poor model of monomeric molybdenum in enzymes. What could not have been predicted were the unique Mo-Fe-S clusters central to nitrogenase. Researching the Mo-enzyme chemistry greatly extended knowledge of molybdenum coordination chemistry.

My first task as a graduate student was to get a feel for molybdenum chemistry. Back then, the source was Nevil V. Sidgwick's classic "The Chemical Elements and Their Compounds." The analytical chemistry of molybdenum--gravimetrically as lead molybdate, PbMoO4, or the 8-hydroxyquinoline complex, [MoVIO2(C9H6NO)2]; colorimetrically as the purple, diamagnetic thiocyanate, [MoV2O3(NCS)6]2–, or the emerald green toluene-3,4-dithiolate, [Mo(C7H6S2)3]; volumetrically via reduction to Mo(V) or Mo(III)--also provided insights. Molybdenum is extraordinarily versatile: It forms compounds with most inorganic and organic ligands and has oxidation states from (–II) to (VI) and coordination numbers from 4 to 8. Therein lies its challenge and excitement.

MOLYBDENUM AT A GLANCE
Name: From the Greek molybdos, lead. Its primary ore was once confused with a lead compound.
Atomic mass: 95.94.
History: Discovered in 1778 by Swedish chemist Carl Welhelm Scheele.
Occurrence: Found primarily in the ore molybdenite (MoS2).
Appearance: Silvery white, hard metal.
Behavior: Molybdenum compounds have low toxicity.
Uses: Essential to life in trace amounts. Has a role in nitrogen fixation and in some enzymes. The metal is used as an alloy in stainless and other steels.
8136element_molybden1

VIEW FROM THE EDGE Computer-generated picture of MoS2. COURTESY OF PHILIP C.H. MITCHELL

The chemistry of molybdenum in its higher oxidation states (IV to VI) is dominated by oxo-species--molybdates [MoVIO4]2–; poly- and heteropolymolybdates; and, in complexes, MoVIO2, MoVO, MoV2O3, MoV2O4, MoIVO, and MoIVO2 as central cations. The oxide MoO3, the molybdenum blues, the polymolybdates, and the remarkable molybdenum wheels of Achim Muller are built from linked [MoOx] polyhedra. Oxomolybdenum redox chemistry is exploited in selective oxidation catalysis: In the oxidase enzymes and the heterogeneous catalysts bismuth molybdate and iron molybdate, molybdenum shuttles between oxidation states (VI) and (IV) while transferring O or HO to substrate molecules [J. Inorg. Biochem., 28, 107 (1986)].

Molybdenum has an extensive sulfur chemistry. The sulfide MoS2 is the main molybdenum ore. Its layer structure confers lubricating properties like graphite to it. Complexes of dithiocarbamates, R2NCS2, and of dithiophosphates, (RO)2PS2, are used as oil-soluble lubricant additives, decomposing at rubbing surfaces to MoS2. My group developed Mo-S chemistry in contracts with Shell and, later, with Esso, synthesizing many Mo-S compounds and lubricant additives [Wear, 100, 281 (1984)]. The Mo-dithiolate complex was an excellent friction modifier and wear reducer (and the colored oil a beautiful green) but was expensive. The water-soluble Mo-cysteine complex has potential in metalworking applications.

Molybdenum-sulfur chemistry underpins one of the most important industrial catalysts, the MoS2-based hydrodesulfurization catalyst used in removing sulfur compounds from petroleum by reaction with hydrogen and conversion to H2S. I was introduced to these catalysts by E. R. Braithwaite of Climax Molybdenum Co. The synergic relationship with Climax continues now through A. W. Armour (who, while a graduate student with me, achieved the notable feat of determining the X-ray structure of ammonium dimolybdate, (NH4)2Mo2O7, with a crystal taken directly from the Climax manufacturing plant at Rotterdam).

In its lower oxidation states, molybdenum has extensive organometallic chemistry exemplified by the well-known hexacarbonyl [Mo0(CO)6]. A feature of Mo(II) is strong Mo-Mo bonding, as in the acetate, Mo2(CH3CO2)4, and the so-called dichloride, Mo6Cl12.

The fundamental challenge of molybdenum chemistry, and the source of its continuing interest, is the subtle interplay of oxidation state, coordination number, and ligating atom, and their impact on structure and reactivity, as well as the potential for applications of molybdenum compounds.

Currently, I am involved in maintaining the Molybdenum Environmental Database for the International Molybdenum Association (http://www.imoa.info). The database is a primer for molybdenum biochemistry. MoS2, MoO3, and the molybdates have low toxicity. Molybdenum replaces more toxic elements in some applications--chromates in corrosion inhibitors and antimony in polyvinyl chloride smoke suppressants.


Philip C. H. Mitchell is Leverhulme Emeritus Fellow and formerly a reader in chemistry in the School of Chemistry at the University of Reading, England. He is a consultant for Climax Molybdenum Co. and the International Molybdenum Association.




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