Metalloids defined

Metalloid peroxides behave as covalent organic compounds and most ate insensitive to friction and impact but can decompose violentiy if heated rapidly. Most soHd metalloid peroxides have weU-defined melting points and the mote stable Hquid members can be distilled (Table 3). Some... 

Regardless of their possible metallic properties, metal-rich Zintl system or phases are defined here as cation-rich compounds exhibiting anionic moieties of metal or metalloid elements whose structures can be generally understood by applying the classical or modern electron counting rules for molecules. 

In this book series, macromolecules containing metal and metal-like elements are defined as large structures where the metal and metalloid atoms are (largely) covalently bonded into the macromolecular network within or pendant to the polymer backbone. 

A metal atom cluster as defined by Cotton [1] is still a very broad term, because non-metal atoms can also be part of the cluster core. In this chapter mainly two types of metal atom clusters are presented the polyborane analogous polyhedral and the metalloid clusters E Rr of group 13 elements E. The structures and bonding of the polyhedral clusters with n < r are similar to those in the well-known polyboranes. 

Adduct formation results in well-defined species. Generally speaking, phosphorus compounds act as Lewis bases [exceptions being penta-valent phosphorus halides as reviewed by Webster <1966 106)] for other examples in which the relevant Lewis acids are metalloid derivatives see references 1966, 107 and 1969, 186. Adducts involving boron have recently been reviewed elsewhere(1969 94 andl02) and are by far the most numerous and use has been made of phosphorus, boron, proton and fluorine resonances, in some cases at varying temperature. 

A cross-coupling reaction can be partially defined by equation (1), where Nu is a carbon (or heteroatom) nucleophile see Nucleophile), R X is an electrophilic substrate, X is a halogen or other appropriate leaving group, and M is a metal or metalloid. At first glance, it would appear that simple nucleophihc substitution reactions should fall under this definition. However, what makes the cross-coupling chemistry special is its ability to perform transformations that cannot be accomplished with simple substitution chemistry. 

The most logical organization of the mechanistic and stereochemical features of the allylmetal addition reaction is by metal. These two most important components of the reaction are inexorably bound and critically dependent on the nature of the metal. In turn, the metal also carries with it the types of ligands and activators that are often also integral to a discussion of the mechanism and moreover influential in the stereochemical outcome of the process. Thus, each subsection, defined by metal or metalloid class, will contain a discussion of the current structure of understanding of the reaction mechanism followed by the stereochemical consequences at all levels of stereoselection outlined above. 

Various empirical and chemical models of metal adsorption were presented and discussed. Empirical model parameters are only valid for the experimental conditions under which they were determined. Surface complexation models are chemical models that provide a molecular description of metal and metalloid adsorption reactions using an equilibrium approach. Four such models, the constant capacitance model, the diffuse layer model, the triple layer model, and the CD-MUSIC model, were described. Characteristics common to all the models are equilibrium constant expressions, mass and charge balances, and surface activity coefficient electrostatic potential terms. Various conventions for defining the standard state activity coefficients for the surface species have been... 

The various metallic glasses reported in the literature [4.9] fall into a few well-defined categories (i) late transition metal + metalloid (ii) early transition metal + late transition metal or group IB metal (iii) earth alkali metal + group IB metal (iv) early transition metal + alkali metal and, (v) Actinide + early transition metal. In catalysis research, exclusively metallic glasses of categories (i) and (ii) have been used so far. Table 4.1 lists glassy metals which have been used in catalytic studies. Note that metal-zirconium alloys and Ni, Fe, and mixed Ni-Fe alloys with P and/or B as metalloid have been used most frequently. 

Speciation is defined as the process of identifying and quantifying the different defined species, forms or phases present in a material or the description of the amounts and types of these species, forms or phases present . In some cases, it is possible to identify, by using single or sequential extractions, operationally defined determinations which identify groups of metals without clear identification. In this situation, it is possible to refer to, for example, ethylenediaminete-traacetic acid (EDTA)-extractable trace metals. The reasons why speciation is important is that metals and metalloids can be present in many forms, some of which are toxic. 

Coordination chemistry is the study of coordination compounds or, as they are often defined, coordination complexes. These entities are distinguished by the involvement, in terms of simple bonding concepts, of one or more coordinate (or dative) covalent bonds, which differ from the traditional covalent bond mainly in the way that we envisage they are formed. Although we are most likely to meet coordination complexes as compounds featuring a metal ion or set of metal ions at their core (and indeed this is where we will overwhelmingly meet examples herein), this is not strictly a requirement, as metalloids may also form such compounds. One of the simplest examples of formation of a coordination compound comes from a now venerable observation - when BF3 gas is passed into a liquid trialkylamine, the two react exothermally to generate a solid which contains... 

This division, based purely upon physical properties, which, in many cases, are ill-defined, has become insu cient. Several elements formerly classed under the at ve niles with the metals, resemble the metalloids in their chemical characters much more closely than they do any of the metals indeed, by the characters mentioned above, it is impossible to draw any line of demarcation which shall separate the elements distinctly into two groups. 

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