Group 14 IVA

Element Ionization Energy (kJ mol ) Electron Affinity (kJ mol ) Melting Point CO Boiling Point CO Electro- negativity 

Source See Table 8-3. Notes Sublimes. Approximate value. 

Therefore, by measuring the remaining amount of one can determine to what extent this isotope has decayed and, in turn, the time elapsed since death. Often called 

What fraction of rernains in a sample that is 50,000 years old  

This is 50,000/5730 = 8.73 half-lives. For first-order reactions (such as radioactive decay), the initial amount decreases by j during each half-life, so the fraction remaining is = 2.36 X 10 1 

For these three elements, for the first time in this Chapter, combinations of a nonmetal with chalcogens and halogens have to be discussed. As regards their structure and properties, most of these do not belong to the field of this review as described in the introductory Chapter. For those interested in the ternary carbon compounds that have the composition CSX2 (X = Cl, Br, I), information may be found in an issue of Gmelin 142). 

Only sulfide chlorides of silicon have thus far been described (e.g., 143). Among these, a nonvolatile compound, Si2S3Cl2, was mentioned 351), but no details were given. Areas of vitrification have been found in the systems Si-S-Br 185), Si-S-I (185), Si-Se-Br (185), and Si- 

Se-I (100, 185). These chalcogenide halide glasses belong to the so-called semiconducting glasses that have attracted much attention in solid-state physics, because of their physical properties (see Section XII,B) they are, however, not treated in this review. 

The first chalcogenide halides of tin were reported in 1963 (24). Although numerous publications since then have been devoted to this subject, the existence and true composition of some the compounds described here still seem questionable. It is, therefore, advisable to start with a discussion of the systems SnY-SnXj (Y = S, Se, Te X = Cl, Br, I). 

Sn2Sl2, was said (385) to exist in an a and a )8 form. The crystal structures of both forms have been elucidated (119, 239, 240, 388). Apart from questions that still remain concerning the true relationship of a-and )8-Sn2Sl2, a publication by Fenner (121) on the synthesis and structure of a new ternary phase, Sn4SIg, contradicts both of the phase diagrams already mentioned. 

The carbon family consists of carbon, silicon, germanium, tin, and lead. All these form oxides which look like CO (e.g., SiO, PbO ). They also form monoxides. As medical enthusiasts, you probably have heard of carbon monoxide, and its harmful effects. CO is a colorless and odorless gas, and it has even higher affinity for hemoglobin than oxygen in the red blood cells. 

A dititanium compound with two bridging ylide ligands has been prepared [equation (2)], and characterized by an A-ray crystal structure determination. 

The cyclometallation of heterocyclic amines by dicyclopentadienyltitanium has been reported. The reaction sequence is shown in equation (3) the cyclo-metallated product was obtained in 25 % yield. 

Fachinetti, C. Biran, C. Floriani, A. Chiesi Villa, and C. Guastini, J. Amer. Chem. Soc., 1978, 100, 1921. 

Scharf, D. Neugebauer, U. Schubert, and H. Schmidbaur, Angew. Chem. Intermt. Edn., 1978,17, 601. 

Benzyltitanium compounds of cyclic silylamides (2) and (3 X=NMe or CHg) were prepared from the corresponding halides and PhCH2MgCl. Pyrolysis experiments showed (2) to be stable to ca. 140 X decomposing via an ionic mechanism, in contrast with [Ti(CH8Ph)4], which is said to decompose by a radical mechanism. The crystal structure of (2) has been determined. 

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HOj and various organic derivatives Group IVA Tv. 1,2-Ethanediol Terephthalic 184)... 

Hydrogen reduction has a major advantage in that the reaction generally takes place at lower temperature than the equivalent decomposition reaction. It is used extensively in the deposition of transition metals from their halides, particularly the metals of Groups Va, (vanadium, niobium, and tantalum) and Via (chromium, molybdenum, and tungsten). The halide reduction of Group IVa metals (titanium, zirconium, and hafnium) is more difficult because their halides are more stable. 

Metallo-organics are compounds in which the atom of an element is bound to one or more carbon atoms of an organic hydrocarbon group. Many of the elements used in MOCVD are the metals of groups Ila, Ilb, Illb, IVb, Vb, and VIb, which are non-transitional. The metallo-organics thus complement the halides and carbonyls, which are the precursors for the deposition of transition metals (Groups IVa, Va, and Via) and their compounds. 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

Structure. The structure of the refractory-metal carbides increases in complexity with increasing group number. Thus the carbides of Group IVa are characterized by a single cubic monocarbide. In those of Group Va, a M2C phase exists as well as the monocarbide. The carbides of Group Via are far more complex and have several compositions. 

Although boron forms borides with many elements, only the borides of the transition metals have been investigated extensively for their CVD characteristics. Boron forms stable borides with the transition metals, and the most refractory of these and those with the greatest potential interest are the borides of the elements of Groups IVa (Ti, Zr, Hf), Va (V, Nb, Ta) and, to a lesser degree. Via (Cr, Mo, W) (see Table... 

Borides of Group IVa. UB2, ZrB2, and HfB2 are readily deposited by the hydrogen reduction of the metal halide, usually the chloride. Atypical reaction is as follows ... 

The Group IVa borides can also be deposited with diborane as a boron source in a pressure range ofafewTorrto 1 atm as follows ... 

Borides of Group Va. The borides of Group Va, Nb2, and TaB2, are more difficult to deposit than those of Group IVa, since the incorporation of free metal in the deposit is difficult to avoid. However, relatively pure deposits can be obtained by the co-reduction of the bromides at high temperatures (1500°C) and low pressure, or by the coreduction of the chlorides if the molar gas mixture is preheated to 700-800°C just before entering the reactor.t ] The incorporation of free... 

As in the case of Group IVA, combinations with these typical non-metals will not be treated in detail insofar as nitrogen compounds are concerned. Of interest with respect to this review are the highly conducting compounds that are obtained by the room-temperature reaction of tetrasulfur tetranitride with halogens, e.g., (SNBrg4)j. (1, 366,423). 

The compressibility of group-IVA and -VIA transition-metal boride powders is measured by the dimensions and weights of the blanks, by measuring the stroke of the punches with a cathetometer, or alternatively by electrical conductivity (based upon the metallic conductivity of most borides). The process of densiheation by pressing is defined by ... 

Table 3. Hot-Pressing Characteristics of Group-IVA Metal Borides... 

Titanium monosulfide, TiS, assumes two forms, both of which are of the NiAs type. In 384, the packing of sulfur is of the ABAC type, with alternate layers of metal sites being fully occupied but the intermediate sites half filled. A series of intermediate phases Ti2+xS4 (0.2 < x < 1) also occurs. The trisulfide TiS3 is best represented as TiS (S2) . The trichalcogenides of Group IVA elements are typified... 

This view is supported by the types of compounds that can be prepared. Group IVa metals in the tetravalent state have no d electrons and tetra-valent vanadium has one. Compounds with a large number of d electrons, e.g., nickel, do not form benzyl compounds readily and attempts to synthesize Ni (benzyl) 2 have not succeeded. 

In contrast to the zirconium compounds the chromium halides are less active than pure Cr(allyl)3 Cr (allyl) Cl2 seems to be completely inactive. The only other element known in which halide substitution of the corresponding allyl compound results in activation is hafnium, suggesting the effect may be confined to group IVa. 

FIGURE 6.8 Boiling points of hydrides of groups IVA,VA, VIA, and VIIA. 

Polyatomic species containing atoms from group IVA are produced by reducing the elements in liquid ammonia that contains some dissolved sodium. In accord with the hard-soft interaction principle (see Chapter 9), isolation of species containing large anions is best accomplished when a large cation of... 

Alkyls of group IVA elements are essentially unreactive in air, but group IIIA alkyls are extremely reactive. Provide an explanation for this great difference in behavior. 

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