Transition metal homogeneity ranges

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Summary of literature data on methane decomposition catalysts and preferred temperature range. Catalysts 1 = nickel, 2 = iron, 3 = carbon, and 4 = other transition metals (Co, Pd, Pt, Cr, Ru, Mo, W). The dotted line arbitrarily separates heterogeneous (catalytic) and homogeneous (noncatalytic, gas phase) temperature regimes of the methane decomposition reaction. 

Late transition-metal hydroamination is the method of choice for the atom economical and functional group-tolerant construction of C—N bonds, and in this context Ir plays a central role (indeed, homogenous transition-metal-catalyzed OHA was discovered with Rh and Ir). However, there is a strong need for the development of better OHA catalyst systems that are applicable to a wider range of substrates and conditions. The characteristics of current Ir based catalyst systems to function via N—H bond activation, though, is a potential handicap to achieve this goal, since it implies highly reactive Ir intermediates that are prone... 

Revised proposals for the phase diagrams of the Group 5 transition metal-carbon systems are given in Figure 4.3(a-c). In our opinion they are preferable with respect to the composition and decomposition temperature of the t, phases (within the limits given in Table 4.1) as well as the homogeneity ranges of the fi and 6 phases. 

A wide range of catalysts have been reported for dehydrocoupling of OH-containing compounds with silanes. These include acids and bases and both homogeneous and heterogeneous transition-metal catalysts. Such reactions can be useful in organic synthesis (protection of OH groups), in... 

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