Early transition metals, alternate

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

For the cationic complexes [(f/5-C5H5)Mo(CO)2(i/4-diene)]+, an alternative movement for the diene ligands has been considered, with a metallacyclo-pentene transition state (117). Such transition states or intermediates are well-known for f/4-diene complexes of the early transition metals (118). In the... 

The unfavorable electronegativity issue refers specifically to the early transition metal elements, especially the group III (Sc, Y, actinides, and lanthinides) elements. The more electropositive elements form an equilibrium with Grignard reagents, giving the other alternative reactions a chance to occur, resulting in poor yields of the desired product. 

The high oxophilicity of early transition metal catalysts (titanium, zirconium, or chromium) causes them to be poisoned by most functionalized olefins, particularly the commercially available polar comonomers. However, there are examples of copolymerizations with special substrates or with very high levels of a Lewis acid incorporated into the polymerization system to protect the polar functionality through complexation. " Alternative routes to polar copolymers involving metathesis of cyclic olefins and functionalization of the resulting unsaturated polymer or metathesis of polar cycloolefins followed by hydrogenation to remove the resulting unsaturation have been published.The cost of these multistep... 

Organolanthanide catalysts provide a new alternative to traditional, late transition metal-based catalysts for selective hydrosilylation of alkenes and alkynes (for recent reviews, see (105)). Mechanistically, the transformation is characteristic of early transition metal-catalyzed reaction. However, it is worth... 

Early transition metal carbides are catalyticaUy active for a variety of reactions. Since they are refractory and of high surface area, they could also be promising alternatives to oxides as supports for heterogeneous catalysts. Only a few investigations have used them for this purpose. And in these limited investigations, the metals were loaded onto intentionally passivated Mo C powders due to their high pyrophoricity. 

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