Groups metallic behavior

Because atoms get larger down a group, metallic behavior (such as... 

Because atoms get larger down a group, metallic behavior (such as ability to form cations and basicity of oxides) inaeases, and this trend becomes especially apparent in Groups 3A(13) to 6A(16). 

Sihcon is a Group 14 (IV) element of the Periodic Table. This column iacludes C, Si, Ge, Sn, and Pb and displays a remarkable transition from iasulatiag to metallic behavior with increasing atomic weight. Carbon, ia the form of diamond, is a transparent iasulator, whereas tin and lead are metals ia fact, they are superconductors. SiUcon and germanium are semiconductors, ie, they look metaUic, so that a poHshed siUcon wafer is a reasonable gray-toned mirror, but they conduct poorly. Traditionally, semiconductors have been defined as materials whose resistance rises with decreasiag temperature, unlike metals whose resistance falls. 

Many factors influence the chemical behavior of an alkoxide, including leaving group, metal ion, solvent and temperature. Electrophile geometry can also promote one type of alkoxide behavior over another. 

Many liquid alloys, in particular, the alkali-group IV alloys, exhibit (Zintl) anion clustering and show strong effects of compound formation. A typical example of such Zintl systems are sodium-tin alloys. In the solid NaSn crystal the Zintl anions Sn appear [1]. An interesting question is the stability of these anions in the liquid. Furthermore, the electrical conductivity of these alloys shows a strong dependence on composition [2] For the limiting (sodium-rich or tin-rich) cases a metallic (small) conductivity appears, but for the nearly equimolar compositions a semi-metallic behavior - with a considerably smaller conductivity - is observed. 

In their electrochemical surface properties, a number of metals (lead, tin, cadmium, and others) resemble mercury, whereas other metals of the platinum group resemble platinum itself. Within each of these groups, trends in the behavior observed coincide qualitatively, sometimes even semiquantitatively. Some of the differences between mercury and other. y- or p-metals are due to their solid state. Among the platinum group metals, palladium is exceptional, since strong bulk absorption of hydrogen is observed here in addition to surface adsorption, an effect that makes it difficult to study the surface itself. 

This chapter will mainly use examples of coordination phenomena involving transition metals, but where necessary and useful examples may include the coordination behavior of main group metals. There may also be occasion to give examples involving nonmetal systems. The ions used as examples will be both positive and negative ions such as the simple bare metal ions M+ or M , cluster metal ions M and M , and other metal containing ions M E (where E can be another metal, element, or ligand). 

A large number of papers has been devoted to the influence of substituents upon the reactivity of benzene nucleus. Extensive studies concerning various benzene derivatives and catalysts from the platinum group metals have been published by H. A. Smith and his co-workers (for a summary see 36). The most consistent sets of data on alkylbenzenes are available from him and other groups of authors. Table VI summarizes the influence of the structure of a single alkyl group Table VII (94, 95, 97-103) summarizes the influence of the number and position of the methyl groups. Both series show very similar behavior on all metal catalysts, a decrease in rate with the size... 

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

A comprehensive report which focussed on the La2 xSrxCu O4-x/2+S ser es was published (139) in 1983 by this research group. In this broad review they reported the magnetic and electrical transport properties of these mixed-valent copper oxides in the temperature range 120-650 K. They concluded that the original semiconducting behavior in La2Cu04 transformed to semi-metallic behavior as the Cu3+ content increased with Sr-substitution. No experiments were conducted below 50 K, and therefore superconductivity was not observed. Three series of compounds, with 0.00 < x < 1.20 were... 

The behavior of the copper group metals toward RX is rather different from that of nickel and palladium. Organometallic products have been isolated in only a few cases so far. Silver atoms react with i-C3F7I,... 

The molecular derivatives of platinum group metals are usually rather well soluble in organic solvents and volatile in vacuum. At normal pressure they demonstrate very low thermal stability and easily decompose producing fine metal powders. This decomposition occurs more easily for the derivatives of branched radicals as it is based on a P-hydrogen elimination process. An important feature of the chemical behavior of these alkoxide complexes is their rather high stability to hydrolysis. Some derivatives can even form outer sphere hydrates when reacted with water in organic solvents. This stability to hydrolysis can at least partially be due to the kinetic inertness of the complexes of this group. 

Tellurium (m.p. 450 °C, density 6.25 g cm-3) is more metallic in its appearance, but it is not a good electrical conductor as are most metals. Polonium, on the other hand, is typically metallic in its electrical properties. Selenium and tellurium are best regarded as semiconductors, and sulfur is nonmetallic in behavior (an insulator). Thus, the usual trend from nonmetallic to metallic behavior is shown in going down Group VIA of the periodic table. All of these elements differ substantially from oxygen in their chemical properties. 

Even for the heaviest member of the group, astatine, there is little evidence for any unambiguously metallic behavior. 

The marked dependence of 7( P—M— P) on stereochemistry for complexes of the platinum group metals has been used in NMR spectroscopy for several years 109,115). For methyl or tert-h xty tertiary phosphines, when the two phosphines are mutually trans then a triplet pattern results, but if the two phosphines are mutually cis, then a doublet pattern results as a consequence of the spectrum being of the AA X X type and the dependence of V( P—M— P) on stereochemistry. Exactly the same behavior is found in NMR spectroscopy with the advantage that the technique is far more versatile and is not normally troubled by resolution problems. Thus for cr-[RhCl3(CO)(PBu"2Ph)2] triplet patterns have been observed for six of the eight different carbon atoms in the tertiary phosphine ligand 164). When —M—is... 

Molybdenum and tungsten are rendered passive more readily in acid than in alkaline solution this is the reverse of the behavior exhibited by chromium and the iron-group metals. Although oxidizing agents generally favor passivity, such is not the case with a tin anode in this instance, too, chloride ions do not have the inhibiting effect they have in other cases. It is apparent, therefore, that each metal requires its own specific conditions in order that it may be rendered passive. 

Kondo, Y and Kuboto, M., Precipitation Behavior of Platinum Group Metals from Simulated High Level Liquid Waste in Sequential Denitration Process, J. Nucl. Sci. Technol., 29(2), (1992), ppl40-l48. 

Wei, Y.Z.,et al., Adsorption and Elution Behavior of Platinum Group Metals In Nitric Acid Medium , Proceeding of IEX 96, 174, Royal Society of Chemistry (July, 1996). 

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