Metallic-like properties

Iodine is a bluish-black, lustrous solid, volatizing at ordinary temperatures into a blue-violet gas with an irritating odor it forms compounds with many elements, but is less active than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form beautiful purple solutions. It is only slightly soluble in water. 

Based on the results of our band-structure calculations we assume that the metal-like properties of lanthanum nitridoborates are related by B-B interactions between adjacent BNx units in structures. 

The discovery of the metal-like properties of conducting polymers has once again focused attention on the oxidation and reduction characteristics of aromatic systems. It turns out that most of these conducting materials consist of chainlike connected carbocyclic or heterocyclic aromatics [94-97]. 

The exciting discovery of the metal-like properties and superconducting behaviour of the non-metallic polymer poly (sulfur nitride) (or polythiazyl), (SN), in 1973 sparked much activity in sulfur-nitrogen chemistry.This interest continues as a result of the prediction that molecular chains incorporating thiazyl units could serve as molecular wires in the development of nanoscale technology. 

Consider the set of model compound reactions shown in reactions (48), (49), and (50). These are prototypes reactions which, if applied at the high polymer level, might yield polymers of the type depicted (idealistically) in 3.67 or 3.68. Synthesis of species corresponding exactly to these structures has not been accomplished, but polymers that bear metal atoms linked to phosphorus every five or so repeating units have been prepared by the chemistry shown in reactions (51) and (52).116 The aim of this type of work is to optimize the synthetic procedures to increase the loading of metal atoms to the point where metallic-like properties become evident. 

Clusters of Ag atoms have metal-like properties at 55 atoms, but they are not metals. The gap between HOMO and LUMO being 0.2 eV is still 10 times larger than kT expected for a bulk metal. The work function is calculated to be 1 eV larger than that of the bulk, and the cohesive energy is only about one-third that expected for bulk. The electrons behave like free electrons. The trend in each of these quantities with size is in the direction from single atom properties toward bulk metal properties. Ousters of Pd have characteristics that differ from bulk metal properties in much the same way as do Ag clusters. One exception is the calculated number of unoccupied d states per atom of small clusters, which is very close to bulk values of 0.6. 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

The poorly characterized i(dddt)21 Ag,Brv product, which probably is a polymeric silver bromide containing species, behaves like a metal down to 4.3 K (167). The Pd analogue, [Pd(dddt)2]Ag1 54Br3 50, is metallic down to 1.3 K (169). Its crystal structure consists of layers of [Pd(dddt)2] moieties alternating with layers of silver bromide complex anions. A noticeable feature lies in the existence of a uniform stacking of Pd(dddt)2 in the conduction layers, instead of the stacking of dyads usually encountered in Pd complexes (9, 160, 175-177). Several other dddt Pd compounds exhibiting metal-like properties have been reported but poorly characterized (see Table III). 

Of the group-IB and -IIB elements, only Cu forms compounds with Si, e.g., CujSi, Cu4Si, Cu,5Si4, CujSi and CuSi which have metal-like properties (Table 1,... 

The chalcogens are one of the most interesting families in the periodic table. The first member, oxygen, is a gas with very un-metal-like properties. The next two members of the family, sulfur and selenium, are solids, with increasingly metallic properties. Tellurium, near the bottom of the family, looks and behaves very much like most metals. The slow change of properties, from less metal-like to more metal-like, occurs in all families in the periodic table. But the change is seldom as dramatic as it is in the chalcogens. 

Tellurium is a grayish-white solid with a shiny surface. It has a melting point of 841.6°F (449.8°C) and a boiling point of 1,814°F (989.9°C). Its density is 6.24 grams per cubic centimeter. It is relatively soft. Although it has many metal-like properties, it breaks apart rather easily and does not conduct an electric current very well. 

In this compound and several other structurally analogous organic and inorganic compounds, the conductivity is inversely dependent on temperature over a limited temperature region, and other characteristically metal-like properties are observed. Included in this group are polymeric sulfur nitride, (SN)a , and a group of partially oxidized, square planar platinum and iridium complexes that are collectively referred to as KCP in Figure 1 both are discussed elsewhere in this volume. 

In 1973 Labes et ah 8,9) synthesized crystalline bundles of impure (SN)a. flbers. Although the S N atomic ratio was 1 1, the material contained 5.48% impurity (4.93% 0, 0.42% H, and 0.13% C). However, metallic-like conductivity was observed in directions parallel to the (SN), flbers, and this increased sharply with decrease in temperature. Six different samples had conductivities at room temperature of 10, 89, 230, 640, 1470, and 1730 ohm" cm" Since the electrical conductivity of an anisotropic substance can be affected enormously by even traces of impurities, we decided that it was most important to attempt to synthesize analytically pure crystals of (SN). and to examine the physical and chemical properties of the material. Only in this way would it be possible to determine whether the metallic-like properties reported for (SN). (8, 9) were characteristic of the pure material. 

Some oxides of the second and third row transition elements also exhibit metallic-like properties, for example, ReOi and ReOs. In these compounds the metal electron energy levels of interest are the 5d, rather than the 3d as in the case of the first row transition metals. 

This behavior is schematically represented in Figure 9.5. The slope of the rise of the current in the accumulation region in Figure 9.5, often differs from the 59 mV expected according to Eqs. (9.8) and (9.10). Otherwise, in the accumulation region the semiconductor electrodes approach metal-like properties and the classical theory of electron transfer must be applied. 

The metal-like property of these polymers is based on their chemical nature, which consists of chains of conjugated double bonds. If these polymers are oxidized, they become electrical conducting. In the neutral state they can have properties like an inorganic semiconductor. This has now become as important as the metal-like conductivity. One example is the development of an organic field effect transistor (OFET). 

Figure 11.19 High frequency part of capacitance and resistance of a polypyrrole film as function of the potential. The film was prepared by anodic oxidation in a perchlorate electrolyte. Additionally, the cyclic voltammogram is shown. The film has metal-like properties at positive potentials (E> OV) and neutral state properties at negative potentials (E < -0.5 V).

Polyphenylene sulfides (PPS), partially crystalline polymers, are produced by the reaction of p-dichlorobenzene and sodium sulfide. This polymer has metallic-like properties and responds well to reinforcement. PPS possesses good creep and good moisture resistance and a low coefficient of thermal expansion. 

The periodic table of elements is divided into horizontal rows and vertical colunuis. Elements in a particular column have similar chemical behaviom. Looking at the periodic table, the metals are in Row 2 (lithium, beryllium), Row 3 (sodimn, magnesium, aluminium), Row 4 (potassium, K through to gallium, Ga), Row 5 (rubidimn through to tin), Row 6 (caesium to bismuth) and Row 7 (francium to actinium). There are two special series of metals from atomic number 58-71 and 89-103. The first are the rare earth metals and the second the radioactive metals (those beyond 92 do not occur naturalfy). Nos 90 and 92 occur naturally and are used for atomic power. The rest of the elements in the table ate non-metals. Some have some metal-like properties and are called metalloids, e.g. nos 5, 14, 32, 33, 51, 52, 84 and 85. 

Near the end of this decade the experience gained from field application developments with nylon and other plastics revealed a need for polymers offering more metal-like properties, especially higher strength and stiffness and less sensitivity to moisture than nylon. 

Figure 2.3 A schematic diagram illustrating the allowed energy levels (stationary states) for electrons bound to a free Arsenic atom (left), and those for electrons bound to Arsenic when present within the pure elemental solid (right). This bonding results in the formation of MOs as defined by the interaction of the valence electrons (4s and 4p levels). As metallic bonding occurs, pure Arsenic is defined as a metalloid (a nonmetal displaying metal-like properties). Reprinted with permission from van der Heide (2012) Copyright 2012 John Wiley and Sons.

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