P.A. Cox - Inorganic chemistry (793955), страница 42
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ClO2 is used as a bleaching agent.Except for fluorine the elements have an extensive oxoacid chemistry Figure 1 shows Frost diagrams with theoxidation states found in acid and alkaline solution (see Topic E5). The sharp trend in oxidizing power of the elements(X2/X− potential) can be seen. As expected from Pauling’s rules (see Topic E2) the hypohalous acids X(OH) andchlorous acid ClO(OH) are weak acids, but the halic acids XO2(OH) and especially perchloric acid ClO3(OH)and perbromic acid are strong. Periodic acid is exceptional, as, although periodates containing the tetrahedralion are known, the predominant form in water is the octahedral IO(OH)5, which, as expected, is a weak acid.The redox behavior is strongly pH dependent but is also influenced by kinetic factors.
From the pH=14 diagram inFig. 1 it can be seen that Cl2 and Br2 disproportionate in alkaline solution. The thermodynamically expected products182SECTION F—CHEMISTRY OF NONMETALSFig. 1. Frost diagrams for the halogens in aqueous solution at pH=0 (a) and pH=14 (b). X represents any halogen, except F for positive oxidationstates.are X− andbut the hypochlorite ion ClO− is formed in cold conditions, and further disproportionation occurs onheating.The perhalic acids and their anions are strong oxidizing agents, especiallywhich is not thermodynamicallystable in aqueous solution. They do, however, have considerable kinetic stability. Perchlorates of organic ororganometallic cations are very dangerous as they may appear stable, but can explode unpredictably with extremeforce.Interhalogen and polyhalogen compoundsBinary compounds known as interhalogen compounds with stoichiometry XYn are found between every pair ofhalogens F-I.
For neutral molecules n is an odd number and when n>1 the terminal atom Y is always the lighterelement. The maximum n found with a given pair increases with the difference in period number, some examples beingIBr, ICl3, BrF5 and IF7. Most interhalogen compounds are obtained by direct reaction. They are strongly oxidizing andthe fluorides are good fluorinating agents.F9—HALOGENS183Many interhalogen and polyhalogen anions and cations are also known, some forming easily. For example, aqueoussolutions containing I− dissolve I2 to form .
In liquid BrF3 the following equilibrium occurs:In accordance with the solvent-system concept (see Topic E1), fluoride donors such as NaF act as bases in thismedium (giving Na+ and),and fluoride acceptors such as SbF5 act as acids (givingand).Other cationic species can be prepared by strong oxidation of the elements (e.g.
with AsF5) in a suitablenonaqueous solvent. Examples includeandwhich are also known in solid salts with anions such as.Most species have the structures predicted by the VSEPR model (see Topic C2). Listed according to the stericnumber (SN) below, the geometries and point groups areSection F—Chemistry of nonmetalsF10NOBLE GASESKey NotesThe elementsXenon compoundsCompounds of other noble gasesRelated topicNoble gases occur as uncombined atoms in the atmosphere,and are uncommon except for argon.
Helium has anexceptionally low boiling point and does not solidify exceptunder pressure.Xenon forms some binary fluorides and oxides, as well asfluoride complexes and oxoanions. All are very reactivecompounds.The only binary compound of krypton is a very unstabledifluoride. Some other molecules have been prepared atvery low temperatures.Introduction to nonmetals (F1)The elementsWith their closed-shell electron configurations the noble gas elements of group 18 were long regarded as chemicallyinert. However, in 1962 Bartlett noted that the ionization energy of xenon was similar to that of O2, and by reactionwith PtF6 attempted to prepare the compound analogous to [O2]+[PtF6]− (see Topic F7).
He obtained a complexproduct containing the ion [XeF]+ (with a valence structure 1 isoelectronic to dihalogen molecules) rather than theexpected Xe+. Many compounds of xenon are now known, mostly with F and O, and few of krypton.The gases are not generally abundant on Earth, although argon (formed by the radioactive decay of 40K) makes upabout 1 mol % of the atmosphere, and helium (formed by radioactive decay of uranium and thorium; see Topics A1 andI2) occurs in natural gas. Radon is radioactive, 222Rn with a half-life of 3.8 days also being formed by radioactive decayfrom 238U.
The boiling points of the elements show the trend expected from van der Waals’ forces (Topic C10), that ofhelium (4.2 K) being the lowest of any substance. Helium is also unique as it does not solidify except under pressure;the remaining elements form monatomic solids with close-packed structures (see Topic D2).
Liquid helium is used formaintaining very low temperatures (e.g. for superconducting magnets), argon as an inert gas in some metallurgicalprocesses, and all the elements in gas discharge tubes.Xenon compoundsThe binary fluorides XeF2, XeF4 and XeF6 are thermodynamically stable and can be prepared by direct reaction underappropriate conditions. They are reactive fluorinating agents. The bonding can be described by three-center molecularorbital pictures or by resonance structures (e.g. 2; see Topic C6) in which no valence-shell expansion is required. TheF10—NOBLE GASES185structures of XeF2 (linear) and XeF4 (square-planar D4h) are those expected in the VSEPR model (see Topic C2) butthat of gas-phase XeF6 has proved elusive. It is believed that (as predicted for a molecule with a lone-pair) the shape isnot a regular octahedron, but that fluxional processes lead to a rapid interchange between different distortedconfigurations.
In the solid structure, some association between molecules occurs and the geometry around Xe isdistorted, as expected in the VSEPR theory.Compounds that appear to contain the [XeF]+ (1) and bent [Xe2F3]+ ions are known although the former is alwaysstrongly coordinated to a counterion such as. Complex anions includeandthe first of whichhas a unique pentagonal planar structure with D5h symmetry (3), as expected from VSEPR.Oxohalides such as XeOF4 are known.
Hydrolysis of XeF6 gives XeO3, which disproportionates in alkaline solution:Salts containing the octahedral XeVIII perxenate ionare known, and by the action of acid the tetrahedralxenon tetroxide XeO4 is formed.All xenon-oxygen compounds are very strongly oxidizing and thermodynamically unstable; some such as XeO3 aredangerously explosive.Recently there has been a renewal of interest in xenon chemistry, with the preparation of many novel compoundswith Xe-O, Xe-N and Xe-C bonds.
Strongly electron withdrawing groups are required on N and C, an example beingthe compound (C6F5)2Xe which like XeF2 has linear coordination about Xe and is made as follows:More remarkably, it has been found that xenon can act as a ligand, and a gold complex containing the square planar ion[AuXe4]2+ ion has been prepared.Compounds of other noble gasesNo krypton compounds appear to be thermodynamically stable, but KrF2 can be made from the elements in an electricdischarge at very low temperatures, and a few compounds of the cationic species [KrF]+ and [Kr2F3]+ are also known. Asthe ionization energy of Kr is higher than that of Xe, the lower stability of krypton compounds is expected from thebonding models shown in structures 1 and 2, where Xe carries a formal positive charge.Reactions performed at very low temperatures have succeeding in making a variety of molecules that have apparentlyvery low barriers to decomposition and so are not even kinetically stable at room temperature.
Typical is thetriatomic molecule HArF, in which it appears that the H-Ar bond is covalent but that the Ar-F bond has a high degree of186SECTION F—CHEMISTRY OF NONMETALSionic character: thus formulation [HAr]+ F− may be appropriate. It is predicted to be unstable with respect to Ar+HF byaround 570 kJ mol−1, and the activation barrier to decomposition may be only 25 kJ mol−1.Section G—Chemistry of non-transition metalsG1INTRODUCTION TO NON-TRANSITION METALSKey NotesScopePositive ionsGroup trendsNon-cationic chemistryRelated topicsNon-transition metals include groups 1 and 2 of the s-block elements,group 12, and p-block elements in lower periods.
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