Borderline elements

This chapter is a review of the CVD of non-metallic elements and covers boron, silicon, and germanium. Silicon and germanium are borderline elements with metalloid characteristics. Both are important semiconductor materials, particularly silicon, which forms the backbone of the largest business in the world the electronic industry. All three materials are deposited by CVD on an industrial scale and a wide variety of CVD reactions are available. 

With the exception of nitrogen, all of the Group Vb elements are expected to form pentacoordinate compounds in their 5+ oxidation state, and this is, indeed, the case with some of the halides, alkoxides, etc. It was not until the pioneering work of Georg Wittig and his collaborators, however, that the first examples of pentaorganyls of these borderline elements between metals and metalloids were reported (95-99, 102, 104). In this early investigation, a complete set of the pentaphenyls could be obtained and characterized (95-99, 102), but apart from the pentamethyl-antimony case, all attempts for the preparation of pentaalkyl derivatives failed (104). 

Later workers have correlated the classification of elements in class (a) and class (b) with Pearson s principle of hard and soft acids and bases (HSAB principle) (see Hard Soft Acids and Bases) on the basis that class (a) metal ions are hard acids and class (b) are soft acids. Borderline elements in the Ahrland-Chart-Davies classification tend to be harder in the higher oxidation states and softer in their lower oxidation states. 

FIGURE 6-n Location of Class (b) Metals in the Periodic Table. Those in the outlined region are always class (b) acceptors. Others indicated hy their symbols are borderline elements, whose behavior depends on their oxidation state and the donor. The remainder (blank) are class (a) acceptors. (Adapted with permission from S. Ahrland, J. Chatt, and N. R. Davies, Q. Rev. Chem. 

In addition to these two classes there are borderline elements which can show both characters. Since their character depends on the donor atoms, it may be interesting to study the stability of the complexes of the borderline elements and look for factors which effect the strength of metal-donor bonds. From this point of view the stability constants of some metal complexes formed with the ligands containing O, S, or Se as the donor atom were studied and their affinities towards bivalent metals were compared (2),... 

The elements that form network solids lie on the right side of the periodic table, bordering the elements that form molecular crystals on one side and those that form metals on the other. Thus they are intermediate between the metals and the nonmetals. In this borderline region classifications are sometimes difficult. Whereas one property may suggest one classification, another property may lead to a different conclusion. Figure 17-3 shows some elements that form solids that are neither wholly metallic nor wholly molecular crystals. 

Aluminum forms a complete series of AlYX compounds (Y = S, Se, Te X = Cl, Br, I). Furthermore, a number of compounds with Se(IV) and Te(IV) are known, such as TeClJ AlClj (197) and SeClJ AICI4 (364), which are not considered here. A borderline case consists in the reduced phases found in the systems (TeCl4-(-4AlCl3)-Te and (SeCl4 + 4A1C13)-Se (88), e.g., Te (AlCl4-)2 (89), Te + (Al Clf)2 (89), Te + (AlCli-) (88), Se5+ (AlClj) (88), and Se + (AlClj)2 (87), which contain cyclic pol3dellurium and polyselenium cations. For a detailed review of homopolyatomic ions of the posttransition elements, see (86). 

The physiologically active elements are anatomical or histological elements, which are the real and simplest carriers of the simplest known vital properties. So Bernard draws a clear borderline between chemistry and histology. Life is bound to histology as to its proximate condition. 

Jellium is a good model for sp metals. This group of metals comprises, amongst others, the elements Hg, Cd, Zn, Tl, In, Ga and Pb, all of which are important as electrode materials in aqueous solutions. They possess wide conduction bands with delocalized electrons, which form a quasi-free-electron gas. The jellium model cannot be applied to transition metals, which have narrow d bands with a localized character. The sd metals Cu, Ag and Au are borderline cases. Cu and Ag have been successfully treated by a modified version of jellium [3], because their d orbitals are sufficiently low in energy. This is not possible for gold, whose characteristic color is caused by a d band near the Fermi level. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

There are no sharp borderlines between some of these areas of mental performance (elements of attention, perception and some motor function are involved in all forms of performance), and allocation to a specific area is debatable for man tests. When studies are planned in practice this will be considered by compiling a battery of tests containing some redundancy while still retaining parameters from various areas of performance. 

Figure 2.3 Elements displaying soft acid or soft base behavior as donor or acceptor atoms in their compounds (heavy shading). Elements shown in light shading are borderline soft carbon is soft in CN, CO, and related donor molecules. Unshaded elements are hard. Heavy line encloses bases.

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