Molybdenum, oxidation states

One possible reaction scheme, an elaboration of the so-called Chatt cycle, results in the nitrogenase cycle shown in Figure 6.16.51 The cycle shown operates between molybdenum oxidation states Mo(0) and Mo(IV), but others have been proposed that operate between Mo(II) and Mo(VI) levels. 

Molybdenum metalworking, 17 10-11 Molybdenum mill products, 17 9-10 Molybdenum-nickel alloys, 17 102 Molybdenum ore, U.S. exports of, 17 3t Molybdenum oxidation states, 17 20 Molybdenum oxides, 17 38 Molybdenum pentachloride, 17 23 Molybdenum-rhenium alloys, 17 15 Molybdenum sulfide, poisons in representative reactions, 5 258t Molybdenum-sulfur complexes, molecular, 17 37... 

The second example refers to oxomolybdenum dithiolate complex [Md O(qdt)2], where qdt stands for quinoxaline-2,3-dithiolate. This complex also manifests thermal valence isomerism. Helton et al. (2001) also explain it with the change of the molybdenum oxidation state ... 

There are many other such examples, illustrated by the results of Figures 8 (Bare, S.R., unpublished data) and 9 (Ressler et al., 2002). The latter example is interesting in that instead of the edge position as a monitor of the average molybdenum oxidation state, the absolute energy position of a feature in the XANES region was used (indicated by an arrow in Figure 9). 

Recently, Brill and co-workers (43, 44) have isolated mutant strains of Azotobacter vinelandii which produce an inactive nitrogenase component. This component can be reactivated by treatment with the neutralized acid-hydrolysis products of other nitrogenases (which themselves become inactive on such a treatment) but not apparently with any other molybdenum enzymes. This may either reflect a difference between the cofactor in nitrogenase and other molybdenum enzymes or may be caused by the reconstitution conditions used which may not have been sufficiently varied to allow for different molybdenum oxidation states to be attained. In any event, the chemical characterization and authentication of the molybdenum cofactor should reveal some of the intimate details of the molybdenum site. 

The yariation in catalytic function with increasing oxygen coyerage is in excellent agreement with surface characterization by molybdenum core leyel shifts and surface polarizability measurements. Clearly the physical measurements were performed on the actual catalytically actiye sites, and the nature of the actiye sites has been established. The catalytic results combined with the spectroscopic data also serve to calibrate the minimum molybdenum oxidation state required for acid catalyzed C-C bond breaking and formation of a secondary carbenium ion, namely. Mo(IV). 

Subsequent Mo-pterin model investigations addressed a seeond provocative aspeet of Rajagopalan s proposed Moeo strueture. The 1982 proposal for Moco paired an oxidized Mo center with a redueed tetrahydropterin - a juxtaposition of the highest molybdenum oxidation state with the most reduced form of pterin - that seemed incompatible and implied possible redox reactions between the metal and the organie eofactor. This seetion deseribes studies direeted at exploring whether molybdenum and pterin redox reactions might occur. These studies were conducted from 1989 to 1999 and inelude reactivity studies of oxidized molybdenum(vi) with redueed pterins, and studies of redueed molybdenum(iv) with oxidized pterins and pteridines. 

Chevrel phases are frequently considered to be reduced molybdenum sulfides because of the low molybdenum oxidation state relative to M0S2. These compounds are also stable under HDS conditions. The low oxidation states are maintained and no decomposition or transformation to other sulfides has been detected for most compounds (61). Chevrel phases usually are obtained through solid-state syntheses at high temperatures (1000-1300°C) by direct combination of the elements or by direct insertion of ternary metal into the binary MogSg... 

Higher molybdenum oxidation states in chloride melts have been but little studied. The formation of Mo(IV) chloro-species in ionic media was reported in low-temperature ionic liquids nearly three decades ago [16] and... 

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