Molybdenum halide clusters

The hexanuclear molybdenum halide clusters (Mo6Xs)X4, i.e. (Mo6X 8)CP2CF 4/2, are known to react with some transition metal halide compounds forming two different types of cluster polymeric clusters with bridging outer halide ligands (X ) and discrete clusters with terminal halide ligands (X ). 

Thus far, dimensional reduction has not been applied in this way to any of the other parent solids listed in Table I. Of particular interest are systems that straddle the divide between the edge-bridged and face-centered octahedral geometries. Will reduced niobium-molybdenum halide mixtures simply disproportionate or will they lead to new mixed-metal clusters If the latter, then where does the crossover in cluster geometries occur An important factor in applying dimensional reduction is recognizing the variability in the stable electron counts within a particular cluster system. An extreme case occurs... 

In the kinetic interpretation of ECL data presented above the closest approach of reactants has been assumed. It seems to be fulfilled in the case of organic ECL systems (because of the Coulombic attraction between oppositely charged ions). In contrast, in ECL systems involving identically charged transition-metal complexes, Coulombic repulsion may lead to an increase in the electron transfer distance. The molybdenum(II) halide cluster ion MoeCliJ [197-199] is the one of the best-understood examples. In electrochemical reactions MogCliJ is reversibly reduced or oxidized to stable MosCl and Mo6Cl["4 respectively ... 

Reduction of [Mo CIg] and [Mo ClgX ] anions (X = Cl, Br, or I) by certain tertiary phosphines gave complexes of stoicheiometry MOgClgX3(PR3)3, which are formulated to have an ionic structure [MOgCl8(PR3)g] [Mo Clg]X in which the cation is a derivative of the previously unknown [Mo Clg] cluster. The use of photoelectron spectroscopy to determine structures of metal halide clusters has been reviewed. " A detailed study of the force fields for MojXg anions (X = Cr or Br) has indicated that 6 bonds do not play a significant part in the M—M bond strength of molybdenum halides. ... 

Near quantitative yields are achieved for the molybdenum halides, but substantially lower yields (50%) are obtained in the synthesis of the tungsten clusters, which require higher temperatures and the use of Vycor reaction vessels. A higher yield low-temperature synthesis was devised by McCarley and co-workers for W6C112 using iron as the reducing agent (14). 

Apart from the low-valent carbonyl clusters, there are also a number of middle- and high-oxidation-state clusters, such as (RO)jMo=Mo(OR)3 and CUResReCl " (13.1), which contain metal-metal multiple bonds.The halide cluster Mo6Clg" (13.2) contains an octahedron of molybdenum atoms with eight chloride ions bridging each of the faces. There are also a number of naked metal clusters of the posttransition elements, such as Sn. ... 

Ligand Exchange in Metal Halide Clusters. Rhenium, molybdenum, tantalum and niobium halides are high-valence metal clusters that formally behave as Lewis acids towards halide ions forming anionic complexes that retain the structure of the original cluster. 

Table 4.9. Emission Data for Molybdenum and Ibngsten Halide Clusters...

Jackson, J. A. Turro, C. Newsham, M. D. Nocera, D. G., Oxygen quenching of electronically excited hexanuclear molybdenum and tungsten halide clusters, J. Phys. Chem. 94 (1990) 4500-4507. 

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... 

Solid-state cluster chemistry is dominated by octahedral (M 5L8)L6 and (MsLi2)L units which are the focus of this paper. These two cluster types are different in the way the metal octahedral core is surrounded by the ligands. In (MsLg)L6-type clusters (Fig. 6.1a), typical for molybdenum and rhenium halides, chalcogenides, and chalcohalides, eight innei hgands (L ) cap the octahedron faces and six outer ligands (L ) are located in the apical positions [9]. For metals with a smaller number of valence electrons, the (M6L i2)L -type clusters... 

The octahedral metal clusters that have long been familiar features of the lower halide chemistry of niobium, tantalum, molybdenum, and tungsten represent a category of cluster different from those so far considered in that their metal-metal bonding is best treated as involving four AO s on each metal 49, 133,144,165,178). 

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

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