Halides molybdenum structures

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

The CSD revealed only one example of the fluoride-bridged dimeric Zn2F2, analogous to the commonly observed unit for the other halides. The compound characterized by Roesky and co-workers is the only structure known with fluoride-bridged zinc atoms.646 Maggard et al. show a bridging fluoride between a molybdenum(V) and the zinc center in a coordination... 

Figure 4.32. The key structural unit [Mo6Cl8]4+ is shown (Cl atom black), which is found in molybdenum (and similarly in tungsten) halides. The edges of the Cl8 cube (around the Mo6 octahedron) have not been indicated emphasis has been given to the interatomic connections.

Binary Systems and Related Compounds.—Halides. The thermodynamics of gas-phase equilibria in the W-F2 and W-F2-H2 systems at high temperatures have been described.The Raman spectrum of solid MoF exhibits Mo—F stretching bands at 746, 722, and 690 cm These results suggest that the compound has a similar structure to NbF4, with each molybdenum co-ordinated to six fluorine atoms.The Raman spectrum of crystalline M0F5 has also been reported and interpreted in terms of the crystal structure.The electronic spectrum of liquid M0F5 has been determined and shown to be consistent with a trigonal-bipyramidal molecular unit. ... 

The molecular complexity of molybdenum (IV) alkoxides is determined by the size and ramification of the alkyl group — the polymeric (R = Me), dimeric (CN = 5, two bridging OR-groups, R = Pr ) with a double metal-metal bond, and even monomers (R = Bu ). The first representatives of W(OR)4 homologous series are tetramers with the [Ti4(OMe)16]-type structure. The derivatives of ramified or bulky alcohols are known only as mixed-ligand complexes (such as dimeric solvates of alkoxide halids with alcohols, various alkoxide hidrides and monomeric complexes with phenantroline, see Table 12.19). 

The homoleptic derivatives of Mo and W(VI) are rather scarcely studied. The only structurally characterized complex, W(OMe)5, possesses the molecular structure analogous to those of alkoxide halids, i.e. a dimer built up of two edge-sharing octahedra. The structure of monooxo homometallic derivatives is unknown and their individuality appears questionable. The only dioxocom-plex of molybdenum(V) isolated as pyridin solvate demonstrates the [Ti(OMe)w]-type structure (Table 12.19). 

In addition to the bimetallic complexes of rhenium and alkaline metals formed as byproducts in the exchange reactions of rhenium halids with alkali alkoxides (such as, for example, LiReO(OPr )5 xLiCl(THF)2 [519]) there has been recently prepared a number ofbimetallic complexes ofrhenium and molybdenum, rhenium and tungsten, and rhenium and niobium [904, 1451]. The latter are formed either due to the formation of a metal-metal bond, arising due to combination of a free electron pair on rhenium (V) and a vacant orbital of molybdenum (VI) atom or via insertion of molybdenum or tungsten atoms into the molecular structure characteristic of rhenium (V and VI) oxoalkox-ides. The formation of the compounds with variable composition becomes possible in the latter case. 

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