P.A. Cox - Inorganic chemistry (793955), страница 27
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For example, the solids with empirical formulae NaO, KO2, LiS, CaC2 and NaN3 contain the ionsandrespectively. Combination of an electropositive metal with a p-block element of intermediateelectronegativity gives so-called Zintl compounds. Some contain discrete polyatomic units such as Ge4 tetrahedra inKGe; in others there are continuous bonded networks such as Si chains in CaSi, or layers in CaSi2.
Often thesestructures can be understood by isoelectronic analogy with the nonmetallic elements (see Topics C1 and D2): thus Ge44− (in KGe) has the same valence electron count as P ; Si2− (in CaSi) is similarly isoelectronic to S, and Si− (in CaSi ) to42P. Although this analogy is useful the ionic formulation may be misleading, as the solids are often metallic in appearanceand are semiconductors.116D5—MORE COMPLEX SOLIDSThe term metal-metal bonding is used when such homoelement bonding involves the more electropositiveelement of a binary pair.
Again, it may sometimes be present when an unusual oxidation state is found. For example,HgCl contains molecular Hg2Cl2 units with Hg-Hg bonds, and GaS also has Ga-Ga bonds (see Topics G4 and G5).Metal-rich compounds are formed by early transition metals, with formulae such as Sc2Cl3 and ZrCl, and structuresshowing extensive metal-metal bonding. They are especially common with elements of the 4d and 5d series andsometimes may not be suspected from the stoichiometry.
An example is MoCl2, which contains the cluster [Mo6Cl8]4+with a metal-metal bonded Mo octahedron (see Topic H5). Metal-metal bonding often gives rise to anomalous magneticor other properties, but the surest criterion is a structural one, with metal-metal distances comparable with or shorterthan those found in the metallic element.Ternary structuresTernary structures are ones with three elements present, examples being CaCO3 and CaTiO3. Oxides are thecommonest examples of such structures and exemplify some of the important principles (see Topic F7). Twofundamentally different structural features are possible, as follows.• Complex oxides are compounds containing complex ions, which appear as discrete structural units.
For example,calcium carbonate has a structure based on rocksalt with the different sites occupied by Ca2+ andions.• Mixed oxides are exemplified by CaTiO3, which, although often called ‘calcium titanate’, does not have discretetitanate ions. The perovskite structure (Fig. 1. shows a corner-sharing network of TiO6 octahedra (essentially theReO3 structure; see Topic D3, Figs 1 and 3) with Ca2+ occupying the large central site coordinated by 12 oxygenions.This division is not absolute, however, and the varied structures of silicates provide examples of intermediate cases.ZrSiO4 (zircon) has discrete ionsbut silicates such as CaSiO3 do not contain individualunits but areformed from tetrahedral SiO4 groups sharing corners to make rings or infinite chains (see Topic D3, Fig.
3). Furthersharing of corners can make two- and three-dimensional networks. The different structures of carbonates and silicatesreflect some typical and very important differences in bonding preference between periods 2 and 3 in the p block (seeTopics F1 and F4).Complex oxides are normally found when a nonmetal is present, with oxoanions such as nitratecarbonatephosphateor sulfatebut are also sometimes formed by metals in high oxidation states (e.g.permanganatein KMnO4). When a compound contains two metallic elements the mixed oxide form is morenormal, but it is important to note that the compound formula itself provides very little guide to the structure (compareCaCO3 and CaSiO3 above). A similar structural variety is found with complex halides.
For example, the K2NiF4structure is based on layers of corner-sharing NiF6 octahedra with no discrete complex ions, whereas K2PtCl4 containsindividual square planar ions [PtCl4]2−. These differences reflect the bonding preferences of NiII and PtII (see Topics H4and H5).SECTION D—STRUCTURE AND BONDING IN SOLIDS117Fig. 1. Unit cell of the perovskite structure of CaTiO3.Microporous solidsZeolites are aluminosilicate solids based on a framework of corner-sharing SiO4 and AlO4 tetrahedra. Theseframeworks contain pores and channels of molecular dimensions, which in natural minerals (or after laboratorysynthesis, usually by hydrothermal methods, see Topic B6) contain species such as water and hydrated ions. Removal ofthese species (e.g. by careful heating under vacuum) leads to microporous materials with empty channels and pores.It is possible to make synthetic zeolites of composition SiO2 with no aluminum, but when AlIII is present the frameworkformula is [AlxSi1−xO2]x− and the charge must be compensated by extra-framework cations.
In as-prepared zeolites thesemay be alkali cations,or organic amines, but when the pore materials are removed they are replaced by H+,which forms strong Brønsted acid sites within the pores.The structure of the zeolite faujasite is shown in Figure 2. In this conventional representation the frameworkstructure is shown without depicting atoms directly. Each line represents an Si—O—Si or Si—O—Al connection.Four lines meet at tetrahedral vertices representing the positions of the four-coordinate Si or Al atoms. Space-fillingmodels of this zeolite show that the pores can accommodate molecules up to about 750 pm in diameter.In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening by replacementof Ca2+ with Na+ or another ion (see Topic J4).
When dehydrated they have important catalytic applications, promotedby the Brønsted acid sites, and by the large area of ‘internal surface’. They are used for the cracking of petroleum andfor the isomerization of hydrocarbons, where limited pore size exerts a ‘shape selectivity’, which allows one desirableproduct to be formed in high yield (see Topic J5).Intercalation and insertion compoundsAlkali metals and bromine react with graphite to form solids known as intercalation compounds, where the foreignatoms are inserted between the intact graphite layers (see Topic D2). Many other layered solids, for exampledichalcogenides such as TaS2, which have structures similar to CdI2 (Topic D3), will also form intercalationcompounds. The inserted species may be alkali metals, or electron donor molecules such as amines or organometalliccompounds.
Sometimes compounds of definite composition may be formed, such as KC6 or C8Br, but in other casesintercalated phases may be nonstoichiometric, such as LixTiS2 (0<x<1). Most intercalation reactions involve electrontransfer between the guest and the host, and modify the electronic properties.Fig. 2. Representation of the structure of faujasite (see text).118D5—MORE COMPLEX SOLIDSThe term insertion compound is used for solids where atoms or ions enter a three-dimensional framework withoutdisrupting its essential structure. Many oxide bronzes are of this type, based on transition metal oxides with insertedalkali or other electropositive metals. For example, the sodium tungsten bronzes are of composition NaxWO3,where x can range from zero up to about 0.9. Their structures are based on the ReO3 framework (see Topic D3) withNa occupying the large vacant site.
The structure therefore resembles that of perovskite (Fig. 1. except that the siteoccupied by Ca in CaTiO3 is only partially occupied in NaxWO3. As with intercalation, electron transfer is alsoinvolved, and NaxWO3 has a metallic appearance and good electronic conductivity whereas pure WO3 is a pale yellowinsulator (see Topic D7).As explained in Topic B6 the synthesis of solids often requires high temperatures, because of the slow diffusion ofatoms. In intercalation compounds and some insertion compounds however, diffusion of guest species is more facile,and such compounds can often be made prepared under fairly mild conditions, sometimes known as chimie douce(‘gentle chemistry’). Intercalation compounds of graphite can be made directly by exposure of the solid to Br2 or toalkali metal vapors.
Insertion compounds of lithium (which is small and diffuses quickly in many hosts) can be preparedby reaction with n-butyl lithium. For example:Insertion compounds of hydrogen such as HxMoO3 can also be prepared, either by direct reaction of the host with H2 inthe presence of a platinum catalyst, or by reduction with metallic zinc in aqueous acid. The structural features aredifferent from those containing alkali metals however. One would not expect the very small H+ ion to occupy anintersitial site in the same way as a metal cation, but rather to form a covalent bond with oxygen. Techniques such as IRspectroscopy (see Topic B7) do indeed show the presence of OH groups, so that the compound above should beformulated as MoO3−x(OH)x.Section D—Structure and bonding in solidsD6LATTICE ENERGIESKey NotesThe Born-Haber cycleTheoretical estimatesApplicationsRelated topicsThe lattice energy of an ionic compound is the energy required toseparate the solid into gas-phase ions.
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