Molybdenum polynuclear species

Although no polynuclear molybdenum(VI) species containing less than seven metal atoms is known in aqueous solution,215 this does not preclude the existence of such species in organic extracts, the observed amine metal ratio also being consistent with the extraction of Mo4Ot32 or Mo207 species. 

The research of Shilov also seems to have real commercial potential. He has discovered several new systems, some of which are catalytic and function in protic solvents such as methanol. His most recent system uses sodium amalgam or electrochemical reduction of dinitrogen mediated by molybdenum, probably in the form of a molybdenum(III) complex. Phosphines enhance the reaction, and so does lethicin, which coats the amalgam surface. The reaction rate at room temperature and atmospheric pressure is greater than 0.3 mol N2 reduced per catalytic center per second, and the turnover number is several hundreds per molybdenum. " No firm conclusions have yet been drawn concerning the mechanism of dinitrogen reduction, though it is believed that polynuclear species are... 

In an equilibrium study of the adsorption of molybdenum(VI) from aqueous solution onto activated carbon it has been found that the data are best explained in terms of an absorption model comprising the three species [HMo207] , Mo03(H20)3, and [HMo04] of which the dimer predominates by far (80). Computer treatment of potentiomet-ric (81) and spectrophotometric data (82) also indicated the possible existence of [HMo207] as a minor species in aqueous solution at low molybdenum concentration ( 2 X 104 Af). However, as its assumed stability region overlaps with several other polynuclear ions more direct evidence is needed before its existence can be accepted with certainty. 

DR. HAIGHT We are now about to tackle the problem of distinguishing between the reactivities of mononuclear and polynuclear molybdenum species. We added mononuclear molybdenum- tin(II) complexes to water and to perchlorate and our evidence right now is that it becomes a cluster very fast when you do this. 

Metal-oxygen and metal-metal distances have been established for many oxo- and hydroxo-bridged dimers and polynuclear cations, for example (57)220 and several of type (58).217 Early work was on solids, but recently X-ray diffraction studies have established the geometries of a few such species in solution. EXAFS has proved valuable in establishing the geometry of the various di- and tri-nuclear forms of the various oxidation states of molybdenum in aqueous solution.23... 

It is also remarkable that no polynuclear MoVI species containing fewer than seven molybdenum atoms is observed in solution. In other words, in addition to the equilibria 18-C-l and 18-C-2, the system is described by... 

Another type of polynuclear anion may be obtained by refluxing chromium or molybdenum hexacarbonyl with sodium borohydride in tetrahydrofuran to give the sodium salts of the black trinuclear [M3(CO)i4] (M = Cr, Mo) anions (66). These species have not yet been investigated in detail. 

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