P.A. Cox - Inorganic chemistry (793955), страница 44
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In some respects, lithium differs slightly from the rest ofthe series. The solubilities and the thermal stabilities of its compounds follow patterns that are more similar to those ofgroup 2 elements than to those of the rest of group 1. This diagonal relationship can be understood from the smallsize of the Li+ cation, which leads to trends in lattice energies and solvation energies more like those of the highercharged ions in group 2.Only sodium and potassium are moderately abundant on Earth, and are major elements of life (see Topic J3).
Theyoccur in many silicates, but weathering reactions at the Earth’s surface lead to the dissolution of the very solublecations, which are common in sea water and are eventually deposited in halide minerals such as NaCl and KCl (seeTopic J2). Li, Rb and Cs are of lower abundance, and obtained from silicate minerals.
Francium is radioactive. Itslongest-lived isotope 223Fr has a half-life of only 22 min and occurs in exceedingly small amounts in uranium minerals(see Topics A1, I2).G2—GROUP 1: ALKAI METALS193Table 1. Properties of alkali metals: melting and boiling points, atomization and ionization enthalpies, ionic radii and standard electrode potentialsThe elements are soft low-melting metals and are very strong reducing agents, reacting violently with many substances.Their major applications are as compounds (especially sodium chloride, hydroxide and carbonate) but the elements canbe made by electrolysis of fused halides, and sodium metal is used in industrial processes such as the production ofmetallic Ti (see Topic J4).Solution chemistryAqueous chemistry is entirely dominated by the M+ ions.
The M+/M electrode potentials are all extremely negative(see Table 1), that of Li being slightly more so than the others because of the large solvation energy as a result of itssmall size. The higher solvation of lithium can be seen in the ionic mobilities determined from the ionicconductivities of dissolved salts. It might be expected that the smallest ion would be the most mobile, but in fact Li+ isthe least mobile and it appears that the smallest ‘bare’ ion becomes the largest on solvation.The M+ ions have only weak complexing tendencies, but these can be enhanced by suitably sized macrocyclicligands (see Topic E3).
Ligands with different cavity sizes can be used to discriminate between alkali ions.The metallic elements dissolve in liquid ammonia (see Topic F5) and related amines (e.g. ethylamine C2H5NH2)to give solutions which contain solvated electrons in addition to cations. In some solvents there is evidence forequilibria involving alkali anions M−. The solutions are useful reducing agents for the preparation of unusually lowoxidation states (e.g. [Ni0(CN)4]4−) including anionic compounds of the alkali elements themselves (see below).Solid compoundsThe alkali metals react with many other elements directly to make binary solids.
The alkali halides are often regardedas the most ‘typical’ ionic solids (see Topics D3–D6). Their lattice energies agree closely with calculations althoughtheir structures do not all conform to the simple radius ratio rules, as all have the rocksalt (NaCl) structure at normaltemperature and pressure, except CsCl, CsBr and CsI, which have the eight-coordinate CsCl structure. The alkalihalides are all moderately soluble in water, LiF being the least so. (The influence of ionic radius on solubility is discussedin Topic E4.)The elements also form hydrides by direct interaction between the elements.
LiH is the most stable and is a usefulprecursor for other hydrides (see Topics B6, F2). Lithium also reacts with N2 to form the nitride Li3N.The elements form oxides M2O, which have the antifluorite structure for Li-Rb. Cs2O has the very unusual antiCdI2 structure with adjacent layers of Cs− (see Topic D4). All compounds are very basic and react with water and CO2to produce hydroxides and carbonates, respectively. Except for Li, however, the simple oxides are not the normalion, and sodium theproducts of burning the elements in air. K, Rb and Cs form superoxides MO2 containing theperoxide Na2O2 with. The relative stability of these compounds with large cations of low charge can be194SECTION G—CHEMISTRY OF NON-TRANSITION METALSunderstood by lattice energy arguments (see Topics D6 and F7).
Rb and Cs also form suboxides when oxygen supplyis very deficient, for example, Rb9O2 (1) and Cs11O3; the structure of the former compound is based on two facesharing octahedra with direct Rb-Rb bonding giving distances shorter than in the metallic element.Hydroxides MOH are very important compounds for all the alkali metals, being easily formed by reaction of oxides withwater (or atmospheric moisture), and soluble in water giving classic strong base behavior (see Topic E2). Compoundsof oxoacids are commonly encountered, such as carbonate, nitrate, sulfate, etc.
As these anions are fairly large, lithiumcompounds tend to be the most soluble in the series (see Topic E4). Many of these compounds crystallize in a variety ofhydrated forms (e.g. Na2CO3.nH2O with n=1, 7 or 10).The combination of the reducing power of alkali metal-ammonia solutions with the strong complexing power ofmacrocyclic ligands allows compounds to be made containing unusual anions, such as [Sn9]4− (see Topics C7 and G6).Among the unexpected products of such reactions are alkalide and electride salts.
An example of an alkalide is [Na(2.2.2.crypt)]+Na−, where crypt is the cryptand ligand 2. The crystal structure shows that the Na− ion is larger than I−. Inelectrides such as [Cs(18-crown-6)2]+e− there is a ‘bare’ electron trapped in a cavity in the lattice.Organometallic compoundsLithium is exceptional in forming molecular alkyls with oligomeric structures, for example, the tetrameric Li4(CH3)4(3). Bonding in the ‘cubane’-like framework is provided by delocalized electrons. These compounds may be preparedby direct reaction between Li metal and alkyl halides and are useful reagents for preparing organometallic compoundsof other elements, and as alternatives to Grignard reagents in organic synthesis (see Topics B6, G3).
Organometalliccompounds of the other elements form solids with somewhat more ionic character.Section G—Chemistry of non-transition metalsG3GROUP 2: ALKALINE EARTHSKey NotesThe elementsSolution and coordination chemistrySolid compoundsOrganometallic compountsRelated topicBeryllium is a rare element; the others form manyminerals such as carbonates and sulfates. All elementsare highly electropositive and reactive, with chemistrydominated by the +2 oxidation state.Be2+ is amphoteric, the other M2+ aqua ions basic.They form complexes with electronegative (andespecially chelating) ligands, stability generallydeclining down the group.Be is normally four-coordinate and its compounds aremore polymeric than ionic.
The other elements formionic oxides and halides with coordination numbersranging from six to eight. Thermal stability ofoxoanion salts increases with cation size.Beryllium alkyls are polymeric. Magnesium formsGrignard reagents, which are useful in organic andorganometallic synthesis.Introduction to non-transition metals (G1)The elementsThe elements known commonly as alkaline earths have atoms with the (ns)2 configuration and almost always have the+2 oxidation state in their compounds.
Molecules such as MgH can be detected at high temperatures in the gas phase,the instability of the +1 state under normal conditions being due to the much greater lattice energies obtained with M2+(see Topic D6). Some data illustrating the factors underlying group trends are discussed in Topic G1.
Beryllium is distinct,as the very small and polarizing Be2+ ion forms compounds with more covalent character than with the other elements,where a high degree of ionic character is normal. Be shows some similarities both with its diagonal neighbor aluminum,and with the group 12 element zinc (see Topics G4 and G5).Calcium and magnesium are very abundant elements, being common in silicate minerals and occurring in major depositsof CaCO3, CaMg(CO3)2 (dolomite) and MgKCl3.3H2O (carnallite).
Calcium fluoride and phosphate minerals are themajor sources of the elements F and P, respectively (see Topic J2). The moderately abundant heavier elements arefound principally as sulfates SrSO4 and BaSO4, whereas beryllium is rather rare and occurs in beryl Be3Al2Si6O18.Radium is radioactive, its longest-lived isotope 226Ra having a half-life of 1600 years and being found in uranium196SECTION G—CHEMISTRY OF NON-TRANSITION METALSminerals (see Topics A1, I2). Calcium and magnesium are major elements in life but beryllium and its compounds arevery toxic (see Topic J3).The metallic elements are all potentially very reactive towards air, water and most elements, but Be and Mg formpassivating oxide films.
Elemental magnesium is manufactured in large quantities either by electrolysis of molten MgCl2or by reduction of MgO, and is used in lightweight alloys and as a reducing agent. The other elements are used mainlyas compounds.Solution and coordination chemistryThe properties of the M2+ aqueous ions show trends expected from their increasing size down the group. Be2+ (like Al3+) is amphoteric (see Topic E2). The insoluble hydroxide dissolves in both acid solution:and in alkaline conditions:The simple aqua cation is present only in strongly acidic conditions. As the pH increases, successive protolysis andpolymerization reactions first give soluble species with Be-OH-Be bridges, and then the solid hydroxide. The other M2+ions are basic.
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