IT'S ELEMENTAL!
MOLYBDENUM PHILIP C. H. MITCHELL, UNIVERSITY OF READING, ENGLAND
I
WAS INTRODUCED TO MOLYBDENUM
in the 1950s by RJ. E WilUams, 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. Braywas using electron spin resonance (ESR) to investigate the kinetics ofxanthine 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, Na 2 {Mo v 2 0 4 {SCH2CH(NH2)COO}2}.5H2Q However, the dimeric anion turned out to be apoor 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 Myfirsttask 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 chem-
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 [M0S2). 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.
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istry of molybdenum—gravimetrically as lead molybdate, PbMo0 4 , or the 8-hydroxyquinoline complex, {Mo VI 0 2 (C9H6NO)2}; colorimetrically as the purple, diamagnetic thiocyanate, {Mo v 2 0 3 (NCS)6}2~, or the emerald green toluene3,4-dithiolate, { M o ^ H ^ } ; volumetrically via reduction to Mo(V) or Mo(III)—also proV% vided insights. Molybdenum is H ^ extraordinarilyversatile: It forms compounds with most inorganic and organic ligands and has oxidation states from (-ID to (VI) and coordination numbers from 4 to 8. Therein lies its challenge and excitement. The chemistry of molybdenum in its higher oxidation states (IV to VI) is dominated by oxospecies—molybdates {MoVI04]2~; polyand heteropolymolybdates; and, in complexes, ^ Mo VI 0 2 ,Mo v O J Mo v 2 0 3 , I M o v 2 0 4 , M O I V O , and ^
Mo IV 0 2 as central cations. ^ The oxide M0O3, the | molybdenum blues, the ° polymolybdates, and the £ remarkable molybdenum I 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. £/
