Tungsten hydrides

To illustrate further aspects of the interplay between covalent and coordinate bonding, let us consider the successive coordinative additions of ammonia molecules (ammine ligands, NH3) to tungsten hydrides in the series H6 2n W(NH3) , n = 1-3,... 

Tungsten surface organometallic chemistry has also been studied because a silica grafted tungsten hydride had been previously shown to exhibit some activity in alkane metathesis reaction (see below) [75, 76]. 

Preparation of Tungsten Hydrides on Silica, Silica-Alumina and Alumina... 

Further treatment of the above surface complexes under hydrogen at 150 °C affords surface tungsten hydrides that have been monitored by IR spectroscopy [76, 77]. 

Propane Metathesis Comparison between Supported Tantalum and Tungsten Hydrides... 

Results in terms of TON indicate that silica-alumina or alumina-supported tungsten hydrides give very similar results but are twice as active as (=SiO)2TaH (3) and even more active than tantalum hydride on alumina or tungsten hydride on silica (Figure 3.12). 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

The i>3 Region. This mode will be ir-active for a linear or bent M-H-M array. Accordingly, low temperature spectra for all 12 combinations of Et4N+ and PPN+ with hydrido- and deuterio-bridged Cr, Mo, and W anions were examined. For spectra recorded at 8°K, the H.D isotope shift allows positive identification of the mode in the four chromium salts and two tungsten hydrides... 

Inverse KIEs, which have kulkn < 1, are associated with reactions in which the transition state has a greater force constant than the reactants, as shown in Figure 8.6. Typically, this situation occurs if the transition state is late (product-like). A compelling demonstration of this phenomenon was provided in a study of hydride transfer from a series of tungsten hydrides to substituted trityl (trityl = triphenyl-methyl) cations.85 The isotope effect kWH/kWn decreased steadily from 1.8 to 0.47 as the rate constant decreased. The trend in wh wd was interpreted as arising from steadily increasing force constants of isotopically sensitive modes of the transition state. 

An interesting series of molybdenum and tungsten hydride complexes with sterically hindered thiolate ligands has recently been established due to the interest in their possible relevance to hydrodesulfurization catalysis and to the active site of the enzyme nitrogenase. The complexes [MH(SAr)3(PMe2Ph)2l (M = Mo, W SAr = TIPT, TMT) were prepared as summarized in Eqs. (8) and (9). 

Hydrogen atom transfer also occurs, as represented in one instance by the reaction of CpW(CO>3 with CpMo(CO)3-H, a process driven by the higher bond energy of the tungsten hydride. The atom transfer process is... 

Under the same conditions, the analogous tungsten hydride reacts with fumaronitrile to form succinonitrile (95 %) and a mixture of fumaronitrile and maleonitrile complexed with tungstenocene (Eq. 60). 

Since many organometallics behave as Lewis bases due to electron-rich metals, protonation is a common reaction. For example, the tungsten hydride 3 undergoes reversible protonation at the metal, forming the water-soluble cationic hydride 4 (Eq. 5). In nickelocene 5, a 20e complex, protonation occurs at the jr-bonded cyclo-pentadienyl ligand the intermediate 6 has a stable, isolable counterpart in the fully methylated derivative. Consecutive loss of cyclopentadiene forms the cation 7, which adds to unchanged nickelocene forming the tripledecker sandwich 8 (Scheme 3). 

In contrast to groups 4 and 5 transition metal complexes supported on silica, the treatment by H2 of W surface organometallic derivatives, namely, [(=SiO)W( C Bu)(CH2 Bu)2] and [(=SiO)2W( C Bu)(CH2 Bu)], mainly lead to sintering (re-formation of silanols and detection of W particles by transmission electron microscopy (TEM)) and the hydride is obtained only in minute amounts. On the other hand, [(AlsO)W( C Bu)(CH2 Bu)2] reacts with H2 to give tungsten hydride species [(AlsO)2W(H) .] with no evidence of sintering, as evidenced by the presence of W-H and Al-H bands in the IR spectrum (Scheme 46). ... 

Scheme 46 Proposed formation of tungsten hydride supported on aiumina.

Lappert and Poland (1969) were the first authors who reported on the discovery and characterization of mononuclear alkyldiazenido complexes by reaction of (tri-methylsilyl)diazomethane with tricarbonyl(cyclopentadienyl)molybdenum hydride and the analogous tungsten hydride. Scheme (10-18) demonstrates that these reactions are not simple substitutions of a carbonyl by a diazenido ligand, but that they are insertions of the diazoalkane into the M-H bond (see also Lappert and Lorberth, 1967). 

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