P.A. Cox - Inorganic chemistry (793955), страница 39
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They form oxoacids, of which phosphoric acidis the most important.These include many sulfides, phosphonitrilic compounds with ringand chain structures, and compounds with metals, which aregenerally of low ionic character.Introduction to nonmetalsNitrogen (F5)(F1)The elementsThe heavier elements in the same group (15) as nitrogen are occasionally known as ‘pnictogens’ and their compounds withmetals as ‘pnictides’. Although the elements form some compounds similar to those of nitrogen, there are verypronounced differences, as is found in other nonmetal groups (see Topics F1 and F5).Phosphorus is moderately abundant in the Earth’s crust as the phosphate ion; the major mineral source is apatite Ca5(PO4)3(F,Cl,OH), the notation (F,Cl,OH) being used to show that F−, Cl− and OH− can be present in varyingproportions.
Arsenic and antimony are much rarer. They occur in minerals such as realgar As4S4 and stibnite Sb2S3, butare mostly obtained as byproducts from the processing of sulfide ores of other elements. Elemental P is obtained byreduction of calcium phosphate. The complex reaction approximates to:Most phosphates are used more directly without conversion to the element.Phosphorus has many allotropes. It is most commonly encountered as white phosphorus, which containstetrahedral P4 molecules with Td symmetry (1). Other forms, which are more stable thermodynamically but kineticallyharder to make, contain polymeric networks with three-coordinate P. White phosphorus is highly reactive and toxic.
ItF6—PHOSPHORUS, ARSENIC AND ANTIBODY169will combine directly with most elements, glows in air at room temperature as a result of slow oxidation, and combustsspontaneously at a temperature above 35°C. Arsenic can also form As4 molecules, but the common solid forms of thiselement and Sb are polymeric with three-coordination. They are markedly less reactive than phosphorus.Enormous quantities of phosphates are used, in fertilizers, food products, detergents and other household products. Forfertilizer applications apatite is converted by the action of acid to the much more soluble compound Ca(H2PO4)2,known as ‘superphosphate’ (see Topic J4).Hydrides and organic derivativesThe hydrides phosphine PH3, arsine AsH3 and stibine SbH3 can be prepared by hydrolysis of metal phosphides, orby reduction of molecular compounds such as PCl3.
The molecules have a pyramidal (C3v) structure but with bondangles less than in NH3 (see Topic C6). They are very toxic gases, with decreasing thermal stability P>As>Sb. Unlikeammonia they are not basic in water. The hydrazine analog diphosphane P2H4 and a few other catenated compoundswith P-P bonds can be made, although their stability is low.Organic derivatives include alkyl and aryl phosphines such as triphenyl phosphine (C6H5)3P.
As with the hydridesthese compounds are much less basic than the corresponding nitrogen compounds towards acceptors such as H+, but aregood ligands for transition metals in low oxidation states, as they have π-acceptor properties (see Topic H9). Cyclicpolyarsanes such as (AsPh)6 (where Ph is a phenyl group, C2H5) with As—As bonds are readily made, and with verybulky organic groups it is possible to prepare compounds with E=E double bonds, for example,(compare C, Si and Ge; Topic F4).
Unlike with nitrogen, the five-coordinate compounds Ph5E are known. The P andAs compounds have the normal trigonal bipyramidal geometry (Topic C2) but Ph5Sb is unexpectedly square pyramidal(2).HalidesPhosphorus forms the binary compounds P2X4 (with a P—P bond), PX3 and PX5 with all halogens. With As and Sb acomplete set of EX3 compounds is known, but the only EV halides stable under normal conditions are AsF5, SbF5 andSbCl5.
AsCl5 has been identified from the UV irradiation of PCl3 in liquid Cl2 but decomposes above −50°C. Mostknown halides can be obtained by direct reaction of the elements in appropriate proportions, but P and F together formonly PF5 and the trihalide can be prepared by reacting PCl3 with ZnF2 or HgF2. The molecular substances have the170F6—PHOSPHORUS, ARSENIC AND ANTIBODYexpected structures, pyramidal (C3v) for EX3 and trigonal bipyramidal (D3h) for EX5 (see Topic C2).
However, somehave a marked tendency to undergo halide transfer, and in the solid state PCl5 and PBr5 form the ionic structures [PCl4]+[PCl ]− and [PBr ]+Br−, respectively. Presumably it is the lattice energy associated with an ionic solid that stabilizes64these forms. Many halide complexes are known. AsF5 and SbF5 are Lewis acids with a very strong affinity for F−, giving[AsF6]− or fluoride bridged species such as [Sb2Fn]− (3).Oxohalides EOX3 form tetrahedral molecules with E=P, but polymeric structures with As and Sb.
POCl3 is animportant intermediate in the manufacture of organophosphorus compounds, used, for example, as insecticides.Oxides and oxoacidsP4O6 (4) and P4O10 (5) can be obtained by direct reaction of the elements, the PV compound ‘phosphorus pentoxide’being the normal product when phosphorus burns in air. Under carefully controlled conditions intermediate oxides P4On(n=7, 8, 9) can be made. The oxides of As and Sb have polymeric structures, and include a mixed valency compoundSb2O4 with SbIII in pyramidal coordination and octahedral SbV.P4O10 is an extremely powerful dehydrating agent, reacting with water to form phosphoric acid H3PO4. This is aweak tribasic acid with successive acidity constants exemplifying Pauling’s rules (Topic E2): pK1=2.15, pK2=7.20 andpK3= 12.37.
Neutral solutions contain about equal concentrations ofandand are widely used asbuffers. A wide variety of metal orthophosphates, containing ions with each possible stage of deprotonation, areknown. Further addition of P4O10 to concentrated phosphoric acid results in the formation polyphosphates with P-OP linkages as in silicates. These linkages are kinetically stable in aqueous solution and are important in biology (seeTopic J3). Metaphosphates such as KPO3 have infinite chains of corner-sharing octahedra as in the isoelectronicmetasilicates such as CaSiO3 (see Topic D5).The PIII oxoacid phosphorous acid H3PO3 does not have the structure P(OH)3 that its formula suggests, but istetrahedral with a PH bond: HPO(OH)2. It is thus diprotic with a similar pK1 to phosphoric acid.
The trend is continuedwith hypophosphorous acid H2PO(OH). Both acids are strong reducing agents.Arsenic acid H3AsO4 is similar to phosphoric acid but is a relatively strong oxidizing agent. SbV oxo compounds havedifferent structures and are based on the octahedral [Sb(OH)6]− ion. Aqueous AsIII and SbIII species are hard tocharacterize; they are much more weakly acidic than phosphorous acid and are probably derived from As(OH)3 and SbF6—PHOSPHORUS, ARSENIC AND ANTIBODY171(OH)3. The corresponding salts tend to have polymeric structures, for example, NaAsO2 with oxygen linked [−As(O−)−O]∞ chains isoelectronic with SeO2.Other compoundsThe sulfides of As and Sb are found in nature.
As2S3 and Sb2S3 with the stoichiometries expected for AsIII and SbIII havepolymeric structures. Compounds such as As4S4 (6) and P4Sn (n=3−10) are molecules based on P4 or As4 tetrahedrawith bridging −S− groups inserted; some of the phosphorus compounds also have terminal P=S groups similar to P=Oin 5.Phosphazines are compounds containing repeated -PX2N- units. For example, the reactiongives rings and chains with a distribution of n values. The (PX2N) unit has the same number of valence electrons as(Me2SiO), which forms silicone polymers (see Topic F1, Table 1.
and Topic F4). In the valence structure as drawn in 7 Pand N carry formal charges, but there is probably some P=N double bonding.Binary compounds with metals are generally of low ionic character. Many of those with transition metals have theNiAs and related structures (see Topics D3 and D4) and show metallic properties. Some compounds appear to containpolyanionic species (e.g.
P24− isoelectronic with S22− in Sr2P2, and P73− in Na3P7), although the bonding is certainly notfully ionic.Section F—Chemistry of nonmetalsF7OXYGENKey NotesThe elementOxidesPeroxides andsuperoxidesPositive oxidation statesRelated topicsOxygen compounds are extremely abundant on Earth. The elementexists as dioxygen O2 (which has two unpaired electrons) and the lessstable allotrope ozone O3. The strongly oxidizing properties of O2are moderated by the strength of the double bond.Nonmetallic elements form molecular or covalent polymericstructures and have acid properties, giving oxoacids with water.Many oxides of metallic elements have ionic structures and are basic.Intermediate bonding types and chemical properties are common, forexample, with metals in high oxidation states.Ionic peroxides and superoxides containandrespectively.Hydrogen peroxide and other peroxo compounds contain O—Obonds, which are weak.Salts containing [O2]+ and some oxygen fluorides are known.Electronegativity and bondIntroduction to nonmetalstype (B1)(F1)Chemical periodicity (B2)Sulfur,seleniumandtellurium (F8)The elementOxygen is the second most electronegative element after fluorine, and forms thermodynamically stable compounds withnearly all elements.
It rivals fluorine in the ability to stabilize the highest known oxidation states of many elements,examples where there is no corresponding fluoride beingand OsVIIIO4. Oxidation reactions with O2 are oftenslow because of the strength of the O=O double bond (490 kJ mol−1).Oxygen is the most abundant element on Earth, making around 46% of the Earth’s crust by mass. The commonestminerals are complex oxides such as silicates and carbonates.
Oxygen is also a constituent of water, and of nearly allbiological molecules. Atmospheric O2 comes almost entirely from photosynthesis by green plants, and is not found on otherknown planets. Reactions involving dioxygen, both in photosynthesis and in respiration by air-breathing animals, areimportant in biological chemistry (see Topic J3).Oxygen can be extracted from the atmosphere by liquefaction and fractional distillation. The liquid boils at −183°C(90 K) and is dangerous when mixed with combustible materials. The compressed gas is used in metallurgy (e.g. steelmaking) and the liquid as an oxidizer for rocket propulsion.F7—OXYGEN173Oxygen has two allotropes, the normal dioxygen O2 form and ozone O3 (1) formed by subjecting O2 to an electricdischarge. Ozone is a trace constituent of the atmosphere, where it plays an important role as an absorber of UVradiation.As predicted by molecular orbital theory (see Topic C4) dioxygen has two unpaired electrons and some of its chemistryshows diradical characteristics; in particular, it reacts readily with other radicals.
Singlet oxygen is an excited state inwhich the two electrons in the π antibonding orbitals have paired spins. It is produced in some chemical reactions andhas different chemical reactivity.OxidesOxygen forms binary compounds with nearly all elements. Most may be obtained by direct reaction, although othermethods (such as the thermal decomposition of carbonates or hydroxides) are sometimes more convenient (seeTopic B6). Oxides may be broadly classified as molecular, polymeric or ionic (see Topics B1 and B2). Covalentoxides are formed with nonmetals, and may contain terminal (E=O) or bridging (E-O-E) oxygen.
Especially strongdouble bonds are formed with C, N and S. Bridging is more common with heavier elements and leads to the formationof many polymeric structures such as SiO2 (see Topics F1 and F4).Water H2O is the most abundant molecular substance on Earth. It is highly polar, with physical propertiesdominated by hydrogen bonding, and an excellent solvent for ionic substances and reactions (see Topics C10 and E1–E5). Many hydrated salts are known (e.g. CuSO4.5H2O), which contain water bound by coordination to metal ionsand/or hydrogen bonding to anions.
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