Metal compounds, coordinatively

IV. Organosilicon Metal Compounds Coordination Chemistry and Catalysis... 

Due to the presence of an electron-withdrawing group on the dipolarophile, these processes are classified as type 1 reactions. The process involves the transference of charge from the dipole to the dipolarophile. When catalyzed by metallic compounds, coordination of the dipolarophile is highly desired. Usually, coordination of a nitrone to the Lewis acid is more feasible than coordination of a carbonyl compound. For this reason, alkenes that enable a bidentate coordination to the Lewis acid, such as 3-alkenoyl-oxazolidinones (Scheme 5), have been frequently employed as a model system to smdy the metal-catalyzed 1,3-dipolar cycloaddition... 

Fehlhammer WP, Stolzetiberg H. Isocyanide- and heteroaUene-bridged metal complexes. III. Reactions of Pd(77 -CS2)(PPh3)2 with metal compounds. Coordination isomerism in metallodithiocarboxylato metal complexes. Inorg Chim Acta 1980 44(0) L151-2. 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

In most of the nonmetaHurgical uses of molybdenum compounds the metal is coordinated by oxygen or sulfur ligands. Molybdenum nitrides, carbides, and sihcides are, however, coming under increasing study for various appHcations. Roughly 75% of all molybdenum compounds are used as catalysts in the petroleum and chemicals industries. 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

Isothiazole and its 4- and 5-methyl derivatives react with chromium and tungsten hexacar-bonyls under the influence of light to give pentacarbonylmetal compounds, the metal being coordinated via the nitrogen atom (72JOM(44)325, 75MI41702). 

A range of shiny metallic compounds featuring trigonal planar anions SnX3 (X = As, Sb, Bi) have been characterized with composition M6(SnX3)Oo.5 (M = Rb, Cs) the Sn and X atoms in SnX3 (isostructural with C03 ) are coordinated by trigonal prisms of 6M+, and the ions occupy octahedral sites in the M+ lattice. 

The simplest of the ir-bondcd Re-C compounds is the green, paramagnetic, crystalline, therm ly unstable ReMen, w ich, after WMe, was only the second hexamethyl transition metal compound to be synthe zed 11976). It reacts with LiMe to give the unstable, pyrophoric, Lii[ReMe(,, which has a square-antiprismatic structure, and incorporation of oxygen into the coordination sphere greatly H reases the stability, wit e,ss Re CMe, which is thermally stable up to 200 C, and Re "0[Pg.1068]

Simple metal compounds are poorly soluble in non-coordinating ILs, but the solubility of metal ions in an IL can be increased by addition of lipophilic ligands. LLowever, enhancement of lipophilicity also increases the tendency for the metal complex to leach into less polar organic phases. 

In chelation complexes (sometimes called inner complexes when uncharged) the central metal ion coordinates with a polyfunctional organic base to form a stable ring compound, e.g. copper(II) acetylacetonate or iron(III) cupferrate ... 

At the present time the concept of catalytic (or ionic-coordination ) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 

The effect of inner orbital splitting on the thermodynamic properties of transition metal compounds and coordination complexes. P. George and D. S. McClure, Prog. Inorg. Chem., 1959,1, 381-463 (36). 

Transition metal polysulphides coordination compounds with purely inorganic chelate ligands. M. Draganjac andT. B. Rauchfuss, Angew. Chem., Int. Ed. Engl., 1985, 24, 742 (129). 

Compounds, coordination and organo-metallic, electronic and magnetic... 

The interactions in such compounds are now better understood, and the term complex now has a more specific meaning. Not all transition-metal compounds are complexes, but many are. The terms complex and coordination compound are now used almost interchangably. 

The thermodynamic stability of coordination compounds is relatively easy to determine, and provides us with a valuable pool of data from which we may assess the importance of ligand-field and other effects upon the overall properties of transition-metal compounds. The bulk of this chapter will be concerned with the thermodynamic stability of transition-metal compounds, but we will briefly consider kinetic factors at the close. 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

In the presence of N,N,N, N",N"-pentamethyldiethylenetriamine ( = PMDETA), monomeric lithium complexes of bulky formamidinate ligands can be isolated. The compounds (Scheme 12) comprise a Li(PMDETA) center coordinated by a bulky formamidinate in either the E-syn- or E-anti-isomeric form. Two of the structures display coordination of the pendant amidinate imine, and can therefore be considered the first examples of if. r -C = N,N metal amidinate coordination. ... 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Draganjac M, Rauchfuss TB (1985) Transition metal polysulfides Coordination compounds with purely inorganic chelate ligands. Angew Chem Int Ed Engl 24 742-757 DuBois MR (1989) Catalytic applications of transition metal complexes. Chem Rev 89 1-9 Ansari MA, Ibers JA (1990) Soluble selenides and tellurides. Coord Chem Rev 100 223-266... 

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