Metal complexes alkoxy hydrides

Both early- and late-metal complexes undergo insertions. The likelihood of insertion, though, depends on the nature of X, A, and B. When X = H, as in metal hydrides such as B-H, Zr-H, and Pd-H, insertion is very facile. Insertions of C=C 77 bonds into M-C and M-H bonds are very important, as will be seen soon. On the other hand, insertions where X = halogen and alkoxy are much rarer. The A=B 77 bond may be C=C, C=N, C=0, or any other. 

The catalytic cycle of the reaction is depicted in Scheme 20.6 [31]. After the initial ligand exchange, the ketone (10) is coordinated to the metal ion of 11 (a), yielding complex 12. A direct hydride transfer from the alkoxide to the ketone takes place via a six-membered transition state (b) in which one alkoxy group is oxidized (13). The acetone (14) and the newly formed alcohol (15) are released... 

We then studied group 5 metals, especially tantalum-for which the laboratory already had great experience. Because of the studied reaction, alkyl or hydride-type compounds such as those developed in the laboratory could not be employed. Consequently, we became interested in alkoxo-type derivatives, either synthesized by reaction of the grafted complex with an alcohol or obtained by direct synthesis starting from an alkoxy-tantalum compound grafted on silica. In all cases, resulting complexes have been characterized by surface organometallic chemistry techniques, especially EXAFS and solid-state NMR (ID and 2D with C-labeled compounds). Indeed various compounds bonded by one, two or three surface bonds have been prepared and characterized. 

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

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