P.A. Cox - Inorganic chemistry (793955), страница 47
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TlIII is more strongly oxidizing; for example, there is no TlIII iodide, andthe compound of stoichiometry TlI3 in fact contains Tl1 with the linear tri-iodide ion I3− (see Topic F9).Al, Ga and In form tetrahedrally coordinated solids with elements of group 15, which are part of the series of III–Vsemiconductors (i.e. groups 13–15, III–V in old nomenclature; see Topic A4). The mixed compounds galliumaluminum phosphide Ga1−xAlxP and the arsenide Ga1−xAlxAs are used for light-emitting diode (LED) displays andsemiconductor lasers.Aluminum hydride AlH3 has a structure similar to that of AlF3. The tetra-hydroaluminate ion [AlH4]− is a powerfulreducing and hydride transfer agent, generally used in the form of lithium aluminum hydride’ LiAlH4 made by reactionof LiH with AlCl3.
Stability of hydrides decreases down the group but [GaH4]− is fairly stable and the unstabledigallane molecule Ga2H6 has been identified with a structure like that of diborane (see Topic C1, Structure 16).Organoaluminum compounds are dimeric but the bonding is different from that of halides as the bridgingmethyl groups in Al2(CH3)6 (1) must be held by three-center two-electron bonds similar to those in diborane (seeTopic C7). Organometallic compounds of Ga, In and Tl are less stable than for Al and do not dimerize.Lower oxidation statesGas-phase molecules such as AlH, AlCl and AlO are known at high temperatures and low pressures but, as in group 2,disproportionation occurs under normal conditions because of the much higher lattice or solvation energies associated withM3+ (see Topics D6 and G1).
As these energies decrease with ion size down the group, the tendency todisproportionation also declines, and lower oxidation states become commoner. Figure 1 shows the possibility offorming In+ and Tl+, the former prone to disproportionation, the latter much more stable. The increasing stability ofions with the (ns)2 configuration in lower periods is often called the inert-pair effect. It is particularly marked inperiod 6 because of the high ionization energies of these elements (see PbII, Topic G6) but it is important to rememberthat it depends not on ionization energies alone but on a balance of different energy trends.204SECTION G—CHEMISTRY OF NON-TRANSITION METALSLike K+, which has a very similar size, Tl+ is very basic in solution, and forms some compounds with similarstructures to those of alkali metals (e.g. TlCl has the CsCl structure). It has a greater affinity for soft ligands, however,and sometimes its solid structures show an irregular coordination suggesting the influence of a lone-pair of electrons aswith SnII (see Topic G6).Ga and In form +1 compounds with large low-charged anions, and also some in which the oxidation state isapparently +2 (or sometimes even fractional).
The gas-phase M2+ ions have the (ns)1 configuration with one unpairedelectron, and in chemical situations always either disproportionate or form metal-metal bonds. The former possibilityleads to mixed valence compounds such as ‘GaCl2’ (in fact, Ga+[GaIIICl4]−). The alternative gives ions [M-M]4+(isoelectronic to), although they are never found on their own but are always strongly bonded to ligands, as in[Ga2Cl6]2− (2). (Note the difference between this structure and that of Ga2Cl6 (like Al2Cl6), where there are noelectrons available for direct Ga-Ga bonding.)All elements of the group form Zintl compounds with electropositive metals (see Topic D5). Continuous networksof covalently bonded atoms are generally found, rather than the clusters common with group 14.
For example, NaAland NaTl have tetrahedral diamond-like networks of Al or Tl, which can be understood on the basis that Al− and Tl−have the same valence electron count as carbon.Section G—Chemistry of non-transition metalsG6GROUP 14: TIN AND LEADKey NotesThe elementsMIV chemistryMII chemistryOther compoundsRelated topicsFound in the minerals SnO2 and PbS, the elements are commoner than otherheavy metals. They have rather low electropositive character. Leadcompounds are very toxic.Many SnIV compounds are known, some with molecular structures.
PbIV isstrongly oxidizing and binary compounds are limited to oxide and fluoride,although complex ions and covalent compounds are known. There is nosimple MIV aqueous chemistry for either element.Most SnII compounds have structures influenced by the pair of nonbondingelectrons. PbII compounds more often have regular ionic structures.Aqueous Sn2+ is amphoteric; Pb2+ forms strong complexes.Organometallic compounds are known in both MII and MIV states.Polyatomic anions can be made.Carbon, silicon and germaniumIntroduction to non-transition(F4)metals (G1)The elementsTin and lead show some resemblance to the lighter elements in group 14, especially Ge (see Topic F4).
Although theyare distinctly more metallic in their chemical and physical characteristics, simple cationic chemistry is the exceptionrather than the rule. As with group 13 (see Topic G5), two oxidation states MII and MIV are found, the MII formbecoming more stable for lead.Both elements have rather low abundance, but are commoner than other heavy metals. They occur in the mineralscassiterite SnO2 and galena PbS. They each have several stable isotopes, Sn more than any other element (10). SomePb isotopes are derived from the radioactive decay of uranium and thorium (see Topics A1, I2). The isotopiccomposition of Pb (and thus its atomic mass) varies detectably according to the source, and such variations have beenused to estimate the age of rocks and of the Earth.The elements are readily produced by reduction of their ores and are soft, low-melting, somewhat unreactive metals.Tin is used for plating, and both elements in low-melting alloys (e.g.
solder) and as many compounds. Applications oflead, however, are declining as its compounds are very toxic (see Topics J3 and J6). A continuing major use is in leadacid batteries, which depend on two reactions involving the Pb0, PbII and PbIV states:206SECTION G—CHEMISTRY OF NON-TRANSITION METALSOccurring at different electrodes, these give a cell potential of 2 V, larger than can be obtained easily from any otherpair of electrode reactions in aqueous solution.MIV chemistryMany binary SnIV compounds are known.
SnO2 has the rutile structure, and SnX2 with X=S, Se, Te the CdI2 layerstructure (see Topic D3). SnF4 has a layer structure constructed from corner-sharing octahedra, but other tetrahalidesform tetrahedral molecules. The halides are good Lewis acids, especially SnF4, which forms complexes such as [SnF6]2−.The PbIV state is strongly oxidizing and only oxides and fluorides form stable binary compounds.
PbO2 and PbF4 havethe same structures as with tin, and mixed-valency oxides such as Pb3O4 (containing PbIV and PbII) are known. OtherPbIV compounds include salts containing the [PbCl6]2− ion as well as some molecular covalent compounds, such thetetraacetate Pb(CH3CO2)4 and organometallic compounds (see below).Neither element shows any simple aqueous chemistry in the MIV state, as the oxides MO2 are insoluble in water at allpH values. Reaction of SnO2 in molten KOH gives the octahedral hydroxoanion [Sn(OH)6]2−, in contrast to the normaltetrahedral silicates and germanates, but in parallel with isoelectronic compounds such as Te(OH)6 also found in period5 (see Topics F1 and F8).
Other ‘stannates’ are mixed oxides without discrete oxoanions (e.g. CaSnO3 with theperovskite structure; see Topic D5).MII chemistryThe structural chemistry of SnII and PbII compounds is extremely complex and varied. The M2+ ions have the (ns)2configuration and hence a nonbonding electron pair which can have a stereochemical influence analogous to that inmolecules (see Topic C2). Thus the Sn coordination found in SnO (1) shows tin with four oxygen neighbors on one sideand a ‘vacant’ coordination site apparently occupied by the lone-pair. SnII sulfide and halides have polymeric structureswith similar stereochemical features, but PbII compounds appear to be more ionic, and less influenced by thenonbonding electrons.
One form of PbO has the same structure as SnO, but the structures of many other compoundsare similar to those found with the larger M2+ ions in group 2 (see Topic G3), examples being PbS (rocksalt) and PbF2(fluorite). Solubility patterns of some PbII salts also parallel those found in group 2 (e.g. insoluble sulfate and carbonate)but differences appear with softer anions: thus PbS is insoluble in water, the heavier halides insoluble in cold water butmore soluble in hot.The aqueous M2+ ions are fairly acidic, Sn2+ especially so and shows typical amphoteric behavior, undergoing strongprotolysis to form polymeric hydroxo species, which dissolve in alkali to form the pyramidal [Sn(OH)3]−.
Pb2+forms complexes with a class b pattern of stability analogous to that of Cd2+ (see Topics E3 and G4) although it does notcomplex with NH3 in aqueous solution.G6—GROUP 14: TIN AND LEAD207Other compoundsMIIorganometallic compounds are found with cyclopentadienyl. Sn(C5H5)2 has a ‘bent sandwich’ structure 2,where the stereochemical influence of the lone-pair is apparent (compare ferrocene, Topic H10, Structure 3). MIVorganometallic compounds with M-C σ bonding are extremely varied and include simple tetraalkyls MR4 andcompounds with Sn-Sn bonds similar to those of Si and Ge (see Topic F4). Tetraethyl lead has been widely used as agasoline additive to improve combustion but is being phased out because of the toxic hazard associated with all leadcompounds.Reaction of alloys such as NaSnx with macrocyclic ligands in amine solvents gives compounds containing anionic clusterssuch as [Sn5]2−, [Sn9]4− and [Pb5]2−.
These have multicenter metal-metal bonding, which can often be rationalized byWade’s rules (see Topic C7, Structures 4 and 7).Section H—Chemistry of transition metalsH1INTRODUCTION TO TRANSITION METALSKey NotesScopeVertical trendsHorizontal trendsElectron configurationsRelated topicsTransition elements form groups 3–11 in the d block. They havedistinct chemical characteristics resulting from the progressive fillingof the d shells. These include the occurrence of variable oxidationstates, and compounds with structures and physical propertiesresulting from partially filled d orbitals.Elements of the 3d series are chemically very different from those inthe 4d and 5d series, showing weaker metallic and covalent bonding,stronger oxidizing properties in high oxidation states, and theoccurrence of many more compounds with unpaired electrons.Electropositive character declines towards the right of each series.Elements become less reactive and their compounds show a tendencytowards ‘softer’ behavior.
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