Main group element oxides

Numerous main group element oxides that have been prepared by CVD techniques have been described in previous chapters. Thus, below is summarized information relating to a few metal oxides which were not discussed. 

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The binary representation is applicable to various other oxide materials. However, an important distinction can be made between borates and other main group element oxide systems, such as aluminates and silicates. In the latter systems cations predominantly reside at sites created by the demands of rigid anionic oxide frameworks. Although some degree of structural control may be obtained by varying cations or by use of template synthesis, the oxide frameworks of these systems tend to be relatively inflexible in comparison with... 

Metal oxides display a variety of unique physical and chemical properties and are employed in numerous technological applications (Table 7-1). Chemical vapor deposition has been used widely for the preparation of metal oxide thin films [10, 13]. The following section summarizes the preparation of several transition metal and main group element oxides by CVD. 

Iron carbonyl clusters incorporating Se or Te have been synthesized by reacting methanolic KOH solutions of Fe(CO)s with the main group element oxides (Scheme 81). Both complexes [E Fe(CO)4)3] lose CO at room temperature in THF solution to form the closed tetrahedral clusters, but in the case of selenium, the loss is so rapid that isolation of the pure complex without Fe-Fe bonds is very difficult. 

R)C2B4H ions and appropriate main group element haUdes, have stmctures containing central main group elements iu the 4+ oxidation states similar to the bis-dicarboUide sUicon sandwich compound. TThe stmcture of the sUicon sandwich compound commo- ](GH.])fi fZ, fri fii is shown iu Figure 27. 

The product of the second reaction is sodium aluminate, which contains the alumi-nate ion, Al(OH)4. Other main-group elements that form amphoteric oxides are shown in Fig. 10.7. The acidic, amphoteric, or basic character of the oxides of the d-block metals depends on their oxidation state (Fig. 10.8 also see Chapter 16). 

Table 63 Ionic radii for main group elements according to Shannon [69], based on r(Oz ) = 140 pm. Numbers with signs oxidation states. All values refer to coordination number 6 (except c.n. 4...

The number of known polycationic compounds of main group elements is far less than that of polyanionic compounds. Examples include the chalcogen cations S]+, S]+, Se]o and Teg that are obtained when the elements react with Lewis acids under oxidizing conditions. The ions S]1, Sc]1 and Te] have a square structure that can be assumed to have a 6n electron system. 

Syntheses of aryl organometallics other than polyhalogenoaryls by thermal decarboxylation are comparatively rare. There are several reasons for this. For transition elements, the thermal stability of simple aryls is often low, especially by comparison with polyhalogenoaryl derivatives, thereby excluding syntheses at elevated temperatures. Electron-withdrawing substituents frequently aid thermal decarboxylation (Section III,A-D), and their absence inhibits major mechanistic paths to both transition metal and main group element derivatives, e.g., SEi (carbanionic) and oxidative addition (Section II). In thermal decomposition of... 

Initially, EC-ALE was developed on the principle that reductive UPD of a metal and oxidative UPD of a main group element were required to form a working cycle. This would then limit compounds that could be formed to those containing a chal-cogenide or a pnictide, as reduced forms of some of these elements were reasonably stable in aqueous solutions. Recently, it has been shown that reductive UPD of both... 

Tin belongs to the long period elements from Rb to Xe and is a main group element because the 4d shell is filled with electrons. Since the valence electrons are 5s2p2, tin occurs in two valences. Whereas valence 2 is formally always positive, valence 4 has amphoteric properties possessing the formal oxidation states +4 or —4, according to the covalently bound substituents and to the reaction partner. 

Main-group elements X such as monovalent F, divalent O, and trivalent N are expected to form families of transition-metal compounds MX (M—F fluorides, M=0 oxides, M=N nitrides) that are analogous to the corresponding p-block compounds. In this section we wish to compare the geometries and NBO descriptors of transition-metal halides, oxides, and nitrides briefly with the isovalent hydrocarbon species (that is, we compare fluorides with hydrides or alkyls, oxides with alkylidenes, and nitrides with alkylidynes). However, these substitutions also bring in other important electronic variations whose effects will now be considered. 

The oxidative carbonylation of amines to give ureas is at present one of the most attractive ways for synthesizing this very important class of carbonyl compounds via a phosgene-free approach. Ureas find extensive application as agrochemicals, dyes, antioxidants, resin precursors, synthetic intermediates (also for the production of carbamates and isocyanates), and HIV-inhibitors. Many transition metals (incuding Au [244], Co [248,253-255], Cu [242], Mn [249,256-258], Ni [259], Rh [246,247,260-262], Ru [224,260,263] and especially Pd [219,225,226,264-276], and, more recently, W [277-283]) as well as main-group elements (such as sulfur [284-286] and selenium [287— 292]) have been reported to promote the oxidative carbonylation of amines, usually under catalytic conditions. In some cases, carbamates and/or oxamides are formed as byproducts, thus lowering the selectivity of the process. 

Reductive elimination is simply the reverse reaction of oxidative addition the formal valence state of the metal is reduced by two (or one in a bimetallic reaction), and the total electron count of the complex is reduced by two. While oxidative addition can also be observed for main group elements, this reaction is more typical of the transition elements in particular the electronegative, noble metals. In a catalytic cycle the two reactions always occur pair-wise. In one step the oxidative addition occurs, followed for example by insertion reactions, and then the cycle is completed by a reductive elimination of the product. 

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