Tungsten complexes characteristics

A number of allylic chlorides have been reported to yield photoproducts characteristic of the alkene moiety and not of the C—X bond123. Allyl and crotyl halides undergo photochemical reactions with chromium, molybdenum and tungsten complexes, which involve C—X bond cleavage133. 

Se chemical shifts span a wide range of ca. 3300 ppm the extremes are marked by selenoaldehydes, some molybdenum selenides, and cationic heterocycles (up to S = 2434) at the high-frequency end and bridging selenium (p) in tungsten complexes (3 = —900) at the low-frequency end. A detailed discussion of electronic influences on Se chemical shifts and its variation in different classes of compounds has been published. In the following, some general characteristic tendencies are collected. 

AlTUD-1, a new mesoporous, Bronsted acidic aluminosilicate with ideal characteristics for catalyst immobilization was used for the noncovalent anchoring of hydride tungsten complex [WH2(ti -OOCCH3) (Ph2PCH2CH2PPh2)2][BPh4]. The immobilization was carried out by an adsorption process in liquid phase. The new materials were characterized by several techniques spectroscopic methods (ICP-AES, FT-IR and UVA is), X-ray techniques (XPS and XRD), isothermal nitrogen adsorption and elemental analysis. 

Many molybdenum and tungsten complexes have been synthesized to date, mimicking oxygen, sulfur and selenium transfer reactions. Studying their kinetic characteristics can be undertaken via several alternative routes and procedures. These methodologies and their respective results are presented in the following parts of this chapter. 

Tungsten carbide has a complex crystal structure with three phases Wq (subcarbide), the monocarbide WC (also called a-WC), and P-WCj.x, which is unstable and forms only above 1530°C. The monocarbide WC is the most important phase and the one reported here. Its characteristics and properties are summarized in Table 9.9. 

For a number of years, phenolic substances were dosed by colorimetric techniques, based on redox reactions usually known as Folin Ciocalteau methods, even if a number of adjustments were developed to fit different matrix characteristics. The Folin Cioalteau reagent is a mixture of phosphomolybdic and phosphotingstic acids, with molybdenum in the 6+ oxidation state and, when the reaction takes place, it is reduced to form a complex called molybdenum blue and tungsten blue. In this complex, the mean oxidation state is between 5 and 6 and the formed complex is blue so it can be read spectrophotometrically at 750 nm. 

Pentacarbonyl(diphenylmethylene)tungsten(0) is a moderately air-stable soild that is readily soluble in most organic solvents. The resulting solutions are air and light sensitive and decomposed thermally at about 50°. The infrared spectrum of a heptane solution shows bands in the metal carbonyl region at 2070 (m), 1971 (s), and 1963 (s) cm"1, characteristic of a group VI pentacar-bonyl species. The proton NMR spectrum in CS2 or acetone-d6 shows a complex multiplet at 5 7.2 relative to internal tetramethylsilane. 

X-ray structural analysis of 2,2-dimethyl-3-phenyl-l-methylenecyclopropane tungsten pentacarbonyl reveals an octahedral complex with characteristic W—C bond distance of 238 pm. The typical bond distances within the organic ligand are 138 (complexed C=C), 148 (proximal C—C), 154 (distal C—C) pm, compared e.g. with 140, 148 and 154 pm, respectively, for the Feist s ester iron complex analogue (see above). 

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. 

FT-ICR, see Fourier-transform ion cyclotron resonance Fullerene[60], germanium-germanium addition, 10, 748 Fullerenes with cobalt, 7, 51 on cobalt Cp rings, 7, 73 inside metallodendrimers, 12, 401 microwave applications, 1, 334 Pd rc-complexes, 8, 348 Ru—Os complexes, 6, 830 with tungsten carbonyls, 5, 687 )2-Fullerenes, with platinum, 8, 634 Fulvalene actinide complex, synthesis, 4, 232 Fulvalene chromium carbonyls, synthesis and characteristics, 5, 264... 

Figure 9-4 A summary of the protonation chemistry of dinitrogen complexes of molybdenum and tungsten based on a wide range of chemical, electrochemical, and kinetic researches. Species characteristic of all these stages except MVI have been isolated and characterized. In a typical case M = M(Ph2PCH2CH2PPh2)2, X = Q, and M = Mo or W (redrawn from R. R. Eady and G. J. Leigh, J. Chem. Soc., Dalton Trans. 1994, 2739).

The XRD pattern of the new complex in the range of 5°<20<8O° is shown in Figure 4(a). The pattern resembles that of ammonium tungsten silicate. This indicates that they have the same crystal structure. There are three characteristic peaks in the range of 8°-10°. 17°-20° and 25°-30° suggesting that this new complex maintains the Keggin structure. 

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