Ionic halides, binary

Binary ionic or covalent hydride reduction of organic halides is important in or-... 

In the solid state, the binary ionic halides exist as crystals made up of an ordered array of halide anions and metal cations (M"+). The arrangement of the crystalline array is determined by the lattice energy of the crystal, and the relative numbers (stoichiometry) and sizes of the anions and cations which are present. Because of the small size of the F anion, fluorides often differ in structure from other halides of the same metal. 

In many binary, ionic crystals (M" X ), e.g. in oxides or halides with sufficient electronegativity difference, the valence band (VB) can be assigned to the X orbitals and the conduction band (CB) to the M orbitals (see Section 2.2), so that, in a localized picture, the following simplified presentation reproduces the reaction in absolute notation ... 

Early in their work on molten salt electrolytes for thermal batteries, the Air Force Academy researchers surveyed the aluminium electroplating literature for electrolyte baths that might be suitable for a battery with an aluminium metal anode and chlorine cathode. They found a 1948 patent describing ionically conductive mixtures of AICI3 and 1-ethylpyridinium halides, mainly bromides [6]. Subsequently, the salt 1-butylpyridinium chloride/AlCl3 (another complicated pseudo-binary)... 

Although naturally occurring compounds of transition metals are restricted in scope, a wide variety of compounds can be synthesized in the laboratory. Representative compounds appear in Table 20-2. These compounds fall into three general categories There are many binary halides and oxides in a range of oxidation numbers. Ionic compounds containing transition metal cations and polyatomic oxoanions also are common these include nitrates, carbonates, sulfates, phosphates, and perchlorates. Finally, there are numerous ionic compounds in which the transition metal is part of an oxoanion. 

Their unique characteristics are a result of their outer shells having seven electrons, and thus requiring only one electron to become complete. This -1 oxidation state makes them extremely reactive with both metals and some nonmetal elements that form negative ions, and they may form either ionic or covalent bonds. They can also form compounds with each other these binary compounds of the halogens are called halides. ... 

A large number of binary AB compounds formed by elements of groups IIIA and VA or IIA and VIA (the so-called III-V and II-VI compounds) also fcrystallize in diamond-like structures. Among the I-VII compounds, copper (I) halides and Agl crystallize in this structure. Unlike in diamond, the bonds in such binary compounds are not entirely covalent because of the difference in electronegativity between the constituent atoms. This can be understood in terms of the fractional ionic character or ionicity of bonds in these crystals. 

Binary compounds formed between metals and group 6 or group 7 elements usually occur in the form of ionic crystals rather than as isolated molecules. The most typical example is, of course, given by the alkali halides, studied by Lowdin in his classic treatise from 1948 [1], Another important class of ionic crystals, with somewhat different properties, are the metal oxides, which play a central role in many contexts in chemistry and physics. To mention only one example, their catalytic properties have long been recognized and subject to extensive study, and have given rise to numerous applications of enormous practical importance. 

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

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