Solids binary halides

The present review will be confined to the solid, binary halides, and principal attention will be focused on developments which have occurred since the subject was last reviewed (SI). For an interesting and authoritative review of the entire subject of the actinide elements, the recent account of G. T. Seaborg (74) can be warmly recommended. 

The immense number of chemical compounds formed by the halogens provides chemists with an extraordinary database from which numerous chemical and physical phenomena can be correlated with respect to various periodic trends. From databases like Inorganic Crystal Structure Data (ICSD, http //www.fiz-karlsruhe.de ) and International Centre for Diffraction Data (ICDD, http //www.icdd.com) with 67 000 and 25 000 entries, respectively, one can easily make out that halides are one of the dominant classes of compounds besides oxides. Even within the subset of inorganic solids, there is tremendous diversity of composition, stracture, and properties and to summarize this would create its own encyclopedia. Therefore, the discussion in this article is limited primarily to binary halides, their structures, and some of their properties, except halides of elements which are nonmetals. Binary actinide hahdes are discnssed elsewhere see Actinides Inorganic Coordination Chemistry). Complex hahdes (sohd phases containing two or more kinds of metal ions), ... 

Lead forms binary halides in both oxidation states, although the tetrahalides, PbX4, are generally much less stable than their bivalent analogs, PbX2. Lead(TV) fluoride is the only halide of lead which is stable under ordinary conditions. It is a crystalline solid with a melting point >600 °C and obviously... 

In the series of the binary halides of selenium and tellurium, the crystal structure determinations of tellurium tetrafluoride (100) and of tellurium tetrachloride on twinned crystals (65, 66) were the key to understanding the various and partly contradictory spectroscopic and other macroscopic properties (e.g., 66,161,168,169,219,220, 412), as well as the synthetic potential of the compounds. In contrast to the monomeric molecular i//-tbp gas phase structures with C2v symmetry (417), the solid state structures of both are polynuclear. As the prototype of the chlorides and bromides of selenium and tellurium, crystalline tellurium(IV) chloride has a cubane-like tetrameric structure with approximate Td symmetry (Fig. 1). Within the distorted TeCla+a octa-hedra the bonds to the triply bridging chlorine ligands are much longer than to the terminal chlorines. The bonding system can be described either covalently as Te4Cli6 molecules, or, in an ionic approximation, as [(TeCl Cn4] with a certain degree of stereochemical activity of the lone pairs toward the center of the voluminous cubane center (65, 66). 

Although the binary halides in the solid state all have giant structures (all but the fluoride (WO3 structure) having the EeCl3 (Bil3) structure), they exhibit molecular structures in the gas phase and many complexes are known. 

Table 19. Summary of mass spectrometric Knudsen effusion studies of binary halides since about the year 1980. (The gaseous species or solid compounds are underlined if enthalpies of dissociation or formation are given)... 

The only binary halide of vanadium(V) is VF5 (equation 21.13) it is a volatile white solid which is readily hydrolysed and is a powerful fluorinating agent. In the gas phase, VF5 exists as trigonal bipyramidal molecules but the solid has a polymeric structure (21.4). The salts K[VFg] and [Xe2Fii][VF5] are made by reacting VF5 with KF or XeFg (at 250 K) respectively. 

The only binary halide of Mn(lV) is MnF4, prepared from the elements. It is an unstable blue solid which decomposes at ambient temperatures (equation 21.50). Crystalline MnF4 is dimorphic. The building blocks in ot-MnF4 are tetramers like those in VF4 and p-CrF4 (21.14). However, in these three metal fluorides, the assembly of the tetramers differs and in ot-MnF4, they are linked to give a three-dimensional network. 

The only binary halide of Mn(III) is the red-purple MnF3 which is made by the action of F2 on Mn(II) halides at 520 K. It is thermally stable but is immediately hydrolysed by water. The solid state structure of Mnp3 is related to those of TiF3, VF3, CrF3, Fep3 and C0F3 but is Jahn-Teller distorted (high-spin fi " ) there are, however, three... 

Binary halides are best made by action of HF, HCl, Bt2 or I2 on hot Zn ZnF2 is also prepared by thermal decomposition of Zn(BF4)2- The vapours of the halides contain linear molecules. Solid ZnF2 adopts a rutile structure (Figure 6.21) and has a high lattice energy and melting... 

A large number of mixed metal or complex fluorides have been characterized for the rare earths. In part, these systems have been extensively studied because of their interesting optical, electrical and magnetic properties and because of their potential as host materials for ions of spectroscopic interest. The M(I)X-RfllDXj, the M(I)X-R(IV)X4 and the M(II)X2-R(III)X, (M(I)-aIkali metal, M(II) alkali earth or divalent rare earth) systems have been most extensively investigated however, a limited number of M(III)X3-R(III)X3 and M(IV)X4-R(III)X3 systems have been described. The mixed metal halides have interesting complex vapor species and several of the solid phases have structures which are closely related to those of binary halides. Phase equilibria of mixed metal systems have been determined primarily by thermal and X-ray diffraction analyses. The preparative procedures and many properties of the mixed halides have been reviewed by Brown (1968) and the mixed fluorides have been reviewed by tsanova (1971). 

The (compositionally) simplest mineral class comprises the native elements, that is, those elements, either metals or nonmetals that occur naturally in the native state, uncombined with others. Native gold, silver, and copper, for example, are metals that naturally occur in a ductile and malleable condition, while carbon - in the form of either graphite or diamond -and sulfur are examples of nonmetallic native elements. Next in compositional complexity are the binary minerals composed of two elements a metal or nonmetallic element combined with oxygen in the oxides, with a halogen - either fluorine, chlorine bromine, or iodine - in the halides, or sulfur, in the sulfides. The oxide minerals, for example, are solids that occur either in a somewhat hard, dense, and compact form in mineral ores and in rocks, or as relatively soft, unconsolidated sediments that melt at moderate to... 

Binary Systems and Related Compounds.—Halides. The thermodynamics of gas-phase equilibria in the W-F2 and W-F2-H2 systems at high temperatures have been described.The Raman spectrum of solid MoF exhibits Mo—F stretching bands at 746, 722, and 690 cm These results suggest that the compound has a similar structure to NbF4, with each molybdenum co-ordinated to six fluorine atoms.The Raman spectrum of crystalline M0F5 has also been reported and interpreted in terms of the crystal structure.The electronic spectrum of liquid M0F5 has been determined and shown to be consistent with a trigonal-bipyramidal molecular unit. ... 

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