Molybdenum forming

Much of what has been said so far in this chapter applies equally well to complexes of second- and third-row transition metals. However, there are some general differences that result from the fact that atoms and ions of the second- and third-row metals are larger in size than those of first-row metals. For example, because of their larger size (when in the same oxidation state as a first-row ion), ions of metals in the second and third rows form many more complexes in which they have a coordination number greater than 6. Whereas chromium usually has a coordination number of 6, molybdenum forms [Mo(CN)8]4 and other complexes in which the coordination number is 8. Other complexes of second- and third-row metals exhibit coordination numbers of 7 and 9. 

In the diagram shown in Figure 6.9, a sulfur atom has been inserted at the cluster s center in agreement with current thinking.24 The M center is attached to protein side chains at only two locations, cysteine 275 (at Fel) and histidine 442 (at Mo), and is unusual in having three-coordinate irons rather than the normal four-coordinate tetrahedral iron coordination sphere.29b The molybdenum forms... 

In keeping with its 4d%5s electron configuration, molybdenum forms many compounds in which its oxidation state is 6+. to an even greater extent than chromium. Also, like chromium, it forms compounds in which II is divalent and those in which it is trivalenl unlike chromium, il forms a number of pentavalenl compounds, and a few more tetravalent compounds, especially complexes. 

Molybdenum forms many other complexes. Of particular interest are the octacyano complexes, containing eight cyanide ions. CN. coordinated to a single tetravalent of pentavalenl molybdenum ion. MolCN)) and Mo(CN)MOg . the latter being exceptionally stable, and both form oclacyanomolybdic acids H, Mo(CN)R 3H-0 and R((Mo[Pg.1039]

Uike chromium, molybdenum forms a number of cyclopeniadicuyl Compounds, many of which are carbonyls, e.g.. CsHsMo(CO)-NO. CsHs MotCOtjX (where X may be Cl. Br. I, H. or CvH-). 

Complexes of molybdenum form the largest set of any of the transition metals. Many of these compounds display spectroscopic properties, isomerism and structural features that illustrate the coordination chemistry of polypyrazolylborate complexes. In addition, this system illustrates that trispyrazolylborate ligands are more bulky than their formal analogue C5H5. 

Diphenyl ditellurium and bis[cyclopentadicnyltricarbonyl molybdenum] formed cyclopen-tadienyltricarbonyl(benzenetellurolato)molybdenum in benzene at 25" when irradiated with light from a 250 W infrared lamp. At higher temperatures, dinuclear and polynuclear benzentellurolato bridged complexes were produced3. 

Molybdenum forms compounds with oxidation numbers +2, +3, +4, +5, and + 6. Of these molybdates are the most important (with oxidation number +6). Molybdates are the salts of molybdic acid, H2Mo04. This acid tends to polymerize with the splitting off of molecules of water. Thus, the commercial... 

Molybdenum forms hundreds of compounds containing Mo—Mo quadruple bonds. The most generally useful entries to this chemistry are shown in the following equations ... 

PolyoxometaUate ions containing molybdenum form an enormous range of compounds which have been reviewed extensively2 (see also Polyoxometalates). They may be subdivided into (i) isopolyanions containing only Mo and (ii) heteropolyanions where other metal and/or nonmetal atoms may be present in the core structure. 

With copper, molybdenum forms a greyish-red hard alloy, of density 7-984. 

Oxybromide.—Molybdenum forms an oxybromide, MoOaBr.j, which may be obtained (a) by the interaction of bromine-vapour and heated molybdenum dioxide (6) by heating together potassium bromide, molybdenum trioxide, and boric or phosphoric acid or (c) by interaction in presence of oxygen of molybdenum trioxide and certain... 

The same ligand [C2S2(CF3)2] coordinated to molybdenum forms pseudo-octahedral (the actual structure is intermediate between octahedral and trigonal prismatic) complexes of the type [Mo C2S2(CF3)2 3] , which have been employed as acceptors [44]. Two CT complexes with different stoichiometries with [FeCpf] have been described. The 1 1 material shows in the solid state a ID-DADA type structure and turns out to be ferromagnetic (0 = - - 8.4 K, but no bulk ferromag-... 

IR spectroscopy shows that CO adsorption on Mo3N/y-Al203 catalysts gives vCO bands at 2045 and 2200 cm 1 due to linearly-adsorbed CO on Mo and N sites respectively.97 Similar data for CO on MoP/y-Al203 includes vCO at 2037 cm 1 from linearly-coordinated CO,98 while for CO adsorption on silica-supported Mo2C catalysts they are at 2089 and 2032 cm-1.99 CO adsorption on silica-supported molybdenum forms a Mo(II) carbonyl, with vCO near 2170 cm-1.100... 

Figure 16 The biosynthesis of Moco and bis-MGD. Shown is a scheme of the biosynthetic pathway for Moco biosynthesis in bacteria and eukaryotes. The proteins involved in the reactions are colored in red for bacterial proteins and blue for human proteins. In bacteria, Moco (54) can be further modified by the attachment of, for example, GMP, forming MGD, and two equivalents of MGDare bound to molybdenum, forming the so-called bis-MGD cofactor (56). Further, Moco can be modified by the replacement of one 0x0 ligand by a sulfido ligand, forming the monooxo Moco (55). The three molybdenum containing enzyme families are divided into the xanthine oxidase, sulfite oxidase, and DMSO reductase families according to their active site structures.

Molybdenum forms a tris(acetylacetonate), an air-sensitive purple-brown solid prepared by heating Mo(CO)6 or K3MoCl6 in acetylacetone. 

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

The bonding in Mo2(NMe2)e can be qualitatively described as follows. Each molybdenum forms four a bonds involving approximate sp ... 

Molybdenum forms compounds with oxidation numbers -1-2, 4-3, 4-4,... 

Molybdenum has a special affinity for sulfur and the close energy match of the Mo 4d and S 3p orbitals often leads to fascinating but unpredictable chemistry. " Molybdenum forms many solid-state sulfides, including the principal ore molybdenite (M0S2), MO2S3, M02SS, non-stoichiometric sulfides and Chevrel phases. These compounds enjoy increasing use as lubricants, electronic materials and catalysts. " ... 

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