Metal alkoxides ligand coordination

The alkoxides and aryloxides, particularly of yttrium have excited recent interest. This is because of their potential use in the production of electronic and ceramic materials,in particular high temperature superconductors, by the deposition of pure oxides (metallo-organic chemical vapour deposition, MOCVD). They are moisture sensitive but mostly polymeric and involatile and so attempts have been made to inhibit polymerization and produce the required volatility by using bulky alkoxide ligands. M(OR)3, R = 2,6-di-terr-butyl-4-methylphenoxide, are indeed 3-coordinate (pyramidal) monomers but still not sufficiently volatile. More success has been achieved with fluorinated alkoxides, prepared by reacting the parent alcohols with the metal tris-(bis-trimethylsilylamides) ... 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

Microbial resistance to established organic antibiotics is a potentially serious problem and provides an impetus for the development of novel antimicrobial metal compounds. The potency of Ag(I) ions is well known—but how does Ag(I) kill a bacterium Much current attention is focused on Bi(III) on account of its ability to kill Helicobacter pylori, an organism which prevents ulcers from healing. Bis-muth(III) chemistry has many unusual features a variable coordination number, strong bonds to alkoxide ligands, the stereochemical role of its 6s2 lone pair, facile formation of polymers, and dual hard and soft character. 

REACTIONS OF METAL ALKOXIDES AND ARYLOXIDES 15.3.5.1 Ligand Coordination... 

In some cases the rate of substitution of alkoxide ligands drops oif dramatically before total substitution has occurred. This clearly reflects both the electronic and steric saturation of the metal coordination sphere by the bidentate acac ligands. Hence for titanium it is difficult to substitute the last alkoxide, 9 while for Nb and Ta the reaction stops at the eight-coordinate tris-substitution products (equation 64).240,241... 

A large variety of organometallic compounds, e.g., metal alkoxides and metal carboxylates, has been studied as initiators or catalysts in order to achieve effective polymer synthesis [35]. Many reactions catalyzed by metal complexes are highly specific and, by careful selection of metal and ligands, reactions can be generated to form a desired polymer structure [36, 37]. The covalent metal alkoxides with freep or d orbitals react as coordination initiators and not as anionic or cationic inititors [38]. Fig. 1 summarizes some of the most frequently used initiators and catalysts. 

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