P.A. Cox - Inorganic chemistry (793955), страница 46
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The Zn compounds are best regarded as polymeric, whereas those of Cd areprototypes of the important CdCl2 and CdI2 layer structures. Both sets of compounds are soluble in water, butsolutions of Cd halides contain a variety of complex ions [CdXn] in equilibrium. Hg halides have varying coordination, withtwo close neighbors in HgCl2 making this compound essentially molecular, the others being more polymeric. Solubilityin water is low but increases markedly with rise in temperature, giving undissociated HgX2 molecules.Among the oxides and sulfides, only CdO adopts the octahedral rocksalt structure found with group 2 elements,although the solid is normally very deficient in oxygen and the electrons not used in bonding give rise to metallicproperties.
ZnO and ZnS are prototypes of the tetrahedrally coordinated wurtzite and zinc blende (or sphalerite)structures; in fact, ZnS can adopt either structure, as can CdS and CdSe. HgO and HgS have chain structures with lineartwo-coordination of Hg. Many of these compounds are colored and show electronic properties characteristic of smallbandgaps and nonstoichiometry (see Topic D7).Table 1.
Overall equilibrium constants (log10β4) for the formation of some [ML4] complexesLower oxidation statesThe +1 oxidation state is fairly stable for mercury, and invariably involves the dimeric [Hg-Hg]2+ ion. Evidence for thiscomes from solid-state structures, and in solution from many sources:200G4—GROUP 12:ZINC, CADMIUM, AND MERCURY• HgI species are diamagnetic whereas Hg+ would have an unpaired electron;• Raman spectra of solutions show a band from the Hg-Hg stretching vibration similar to that seen in solids;• Equilibrium studies (e.g. by electrochemistry) are consistent withwith an equilibrium constantat 25°C; the equilibrium expression involving Hg+ would havea different form.is marginally stable in aqueous solution, but the disproportionation equilibrium can be upset byUncomplexedany ligand for which the HgII compound is more stable.
Thus addition of sulfide, cyanide and many other ligands causesdisproportionation. In solid compounds theion always has two ligands strongly bonded. For example, Hg2Cl2 haslinear Cl-Hg-Hg-Cl molecules, and salts with noncomplexing anions such as nitrate contain the hydrated ion [H2O-HgHg-H2O]2+.Oxidation of Hg with AsF5 gives species containing linearandions, culminating in a metallic compoundHg0.33AsF6, which contains linear chains of mercury atoms.Zn and Cd analogs ofare much less stable, principally because the larger lattice energies obtained with the smallerM2+ ions tend to force disproportionation (see Topic D6).andcan both be identified spectroscopicallywhen the elements react with melts of the corresponding chloride.
Adding AlCl3 gives the solid compoundbut no solid zinc (I) compounds have been prepared.Organometallic compoundsThe elements form compounds R2M and RMX, where R is an alkyl or aryl group and X a halide. M-C bond strengthsare in the order Zn>Cd>Hg but nevertheless the mercury compounds are the most easily formed; for example, fromThe Hg compounds are also the least reactive towards air or water, partly because the competing Hg-O bond is so muchweaker than with Zn or Cd.
They are useful for preparing organometallic compounds of other elements. Water-solubleions can be obtained, such as [CH3Hg]+, which has been used as a prototype ‘soft’ acid in the hard and soft acid and base(HSAB) classification (see Topic C9). All organomercury compounds are extremely toxic, as they pass through cellmembranes much more easily than inorganic forms.Section G—Chemistry of non-transition metalsG5GROUP 13: ALUMINUM TO THALLIUMKey NotesThe elementsMIII aqueous chemistryMIII compoundsLower oxidation statesRelated topicAluminum is the commonest metallic element on Earth, occurringwidely in aluminosilicate minerals and in deposits of the hydroxidebauxite.
It is very electropositive and potentially very reactive, butforms a stable oxide film. Gallium, indium and thallium are rarer andless electropositive.Al3+ is amphoteric and complexes strongly with hard ligands. Ga3+ andIn3+ are similar, but Tl3+ is a strong oxidizing agent and shows softcomplexing properties.AlIII is octahedral in fluorides and in most oxides (including manycomplex oxides), and tetrahedral with larger or less electronegativeatoms (and sometimes also in oxides).
Heavier halides form moleculardimers Al2X6. The tetrahedral [AlH4]− complex and dimeric alkyls arealso formed. The heavier elements of the group form less stablecompounds, especially TlIII.Stability of MI increases down the group. Tl+ is strongly basic andshows some resemblance to group 1 cations in its solution and solidstate chemistry. Mixed valence and metal-metal bonded compounds ofGa, In and Tl are also known.Introduction to non-transition metals (G1)The elementsThe elements aluminum, gallium, indium and thallium have valence electron configurations (ns)2(np)1 and for the lighterelements their chemistry is dominated by the +3 oxidation state.
The group trends are very different, however, fromthose in groups 1 and 2. The Al3+ ion has a large charge/radius ratio and is strongly polarizing, so that significantdeviations from simple ionic behavior are often observed. The filling of the d shells (and 4f in period 6) leads todecreased electropositive character for Ga, In and Tl similar to that shown in group 12 (see Topics G1 and G4). Thereis also a progressive stabilization of lower oxidation states down the group.Aluminum is the commonest metallic element in the Earth’s crust, being a constituent of almost all silicate minerals(see Topic J2). Weathering leaves deposits of the very insoluble aluminum minerals AlO(OH) and Al(OH)3, knowntogether as bauxite, which forms the principal source of the element.
The metal is extracted by electrolysis of fusedcryolite Na3[AlF6]. Although reactive when clean, the metal easily forms a very resistant oxide film, which allowswidespread applications as a lightweight construction material and in cooking and other vessels.202SECTION G—CHEMISTRY OF NON-TRANSITION METALSFig. 1.
Frost diagram showing the oxidation states of Al, Ga, In, Tl in aqueous solution at pH=0.Ga, In and Tl are much less common elements, obtained in small amounts from sulfide minerals of other elementsand used only in specialized applications. The metals are less reactive than aluminum; Fig. 1 shows a Frost diagram inwhich the much larger negative slope (negative electrode potential; see Topic E5) of Al is apparent. Thalliumcompounds are extremely toxic but do not normally pose an environmental hazard because they are little used.MIII aqueous chemistryAl3+ is amphoteric and will dissolve in acidic and alkaline solutions (see Topic E2). The [Al(H2O)6]3+ ion is formed atlow pH but undergoes increasing protolysis as the pH increases above four, and polymeric species such as [Al13O4(OH)7+24(H2O)12] can be identified.
The very insoluble Al(OH)3 is formed at neutral pH but redissolves above pH 10:Al3+ shows typically ‘hard’ complexing behavior and has a particularly strong affinity for negative charged and/orchelating ligands, such as oxalateand EDTA (see Topic E3).Ga3+ is similar to Al3+ but In3+ is more basic. Tl3+ differs as it is a strong oxidizing agent, readily forming Tl+ (see belowand Fig.
1). It also shows strong class b or ‘soft’ complexing behavior, although not so marked as that of Hg2+ (seeTopic G4).MIII compoundsAll aluminum halides can be made by direct reaction, but AlF3 is best produced by reaction with anhydrous HF. Ithas a structure based on corner-sharing AlF6 octahedra (similar to ReO3; see Topic D3). Solid AlCl3 has a polymericlayer structure, but in the gas phase or nonpolar solvents is molecular and dimeric Al2Cl6 (see Topic C9, Structure 1).The bromine and iodide have the molecular dimeric form in the solid state. Aluminum halides are strong Lewis acids(see Topic C9) and AlCl3 is frequently used as an acid catalyst, for example, in organic Friedel-Crafts reactions.G5—GROUP 13: ALUMINUM TO THALLIUM203Complex halides containing the ions [AlF6]3− and [AlCl4]− are easily formed and can be useful for the preparation ofcompounds containing unusual cations such as(see Topic G4).The most stable form of the oxide Al2O3 is α-alumina with the corundum structure where Al3+ ions occupy twothirds of the octahedral holes in a hexagonal close-packed oxide lattice.
Another form γ-Al2O3 has a defect spinelstructure (see below). So-called β-alumina is in fact a mixed oxide of aluminum of approximate formula NaAl11O17with a disordered arrangement of Na+ ions, and is a good ionic conductor (see Topic D7).Aluminum forms many mixed oxides of which the aluminosilicates are major constituents of minerals (seeTopic J2).
In these compounds aluminum sometimes replaces a portion of the silicon present as corner-sharing SiO4groups (see, e.g. zeolites, Topic D5). The mixed oxide mineral spinel MgAl2O4 gives its name to an importantstructure type. One-half of the octahedral holes and one-eighth of the tetrahedral holes are filled in a cubic close-packedarray of oxide ions. In the normal spinel form adopted by MgAl2O4 the divalent Mg2+ ion is in tetrahedral sites andthe trivalent Al3+ is octahedral. (See Topic H4 for other examples.) In the defect spinel structure of γ-Al2O3 a fractionof the cation sites are occupied at random.Halides and oxides of Ga and In are fairly similar to those of Al, but have less negative enthalpies of formation and(with In) a tendency to higher coordination.
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