P.A. Cox - Inorganic chemistry (793955), страница 50
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These become increasingly strongly oxidizing, and lower(e.g.states are more stable for later elements. The M3+/M2+ couple shows atrend related to ionization energies and ligand field stabilization energy.High pH stabilizes some high oxidation states and reduces the tendency ofothers to disproportionate. The species present change according to theacidic, basic or amphoteric character of the metal ions.Early elements show hard complexing behavior; later ones have anincreasing affinity for ligands such as NH3.
Complexing can alter redoxpotentials, and stabilizes some states such as CoIII and CuI.Complex formation (E3)Introduction to transitionElectrode potentials (E5)metals (H1)3d series: solid compounds(H4)Oxidation statesFigure 1 shows a Frost diagram for the elements Sc-Zn with the electrode potentials for aqueous species appropriate toacid solution (pH=0). Lines with positive (negative) slopes on this diagram indicate potentials that are more (less)strongly oxidizing with respect to the standard couple (H+/H2). The diagram displays clearly the major trend to lesselectropositive character across the series.
(See Topic E5 for the construction and interpretation of these diagrams.)The negative slopes for the M2+/M or M3+/M couples show that metals early in the series are strong reducingagents. This tendency decreases across the series, and copper has a positive Cu2+/Cu slope, showing that copper metaldoes not react with acids to give hydrogen. It will, however, dissolve in strongly oxidizing acids such as HNO3 or in thepresence of some complexing agents.Higher oxidation states are also much more accessible for elements early in the series. As far as Mn (group 7),elements can attain the group oxidation state (corresponding to the formal d0 electron configuration; seeTopic H1).
This becomes more oxidizing along the series TiIV<VV<CrVI<MnVII, and permanganate MnO4− is usedwidely as a strong oxidizing agent in acid solution.M2+ ions are stable for all elements except Sc and Ti, where these oxidation states are too strongly reducing to existin water. M3+ ions are formed by elements up to Co, although for Mn and Co the uncomplexed ions are very stronglyoxidizing in acid solution. The ionization energy (IE) trend is the most important factor controlling the change in redoxstability, and the discontinuity in the trend of third IE values after Mn (see Topic H1, Fig.
2) is reflected in a similar218SECTION H—CHEMISTRY OF TRANSITION METALSFig. 1. Frost diagram for elements of the 3d series in aqueous solution at pH=0.break in the M3+/M2+ redox potentials, Fe3+ being less strongly oxidizing than Mn3+. Changing solvation energies alsohave an effect, and these are in turn influenced by ligand field stabilization energies (LFSE, see Topic H2). The Cr3+/Cr2+ couple is more reducing than expected from its third IE as a consequence of the large reduction of LFSE between Cr3+(d3) and Cr2+ (d4).Also seen in the diagram is that some intermediate oxidation states are prone to disproportionation.
Thus MnVIundergoes the following reaction in acid solution:Mn3+ and Cu+ also disproportionate although all these reactions can be influenced by pH or complexing.Effect of pHIn a half-cell reaction such asincreasing pH (and hence decreasing H+ concentration) will favor the left-hand side and so lower the redoxpotential.
Thus some high oxidation states such asare more accessible in alkaline than in acid solution. ChangingpH can also alter the tendency to disproportionate. For example, MnIII, which is unstable in alkaline solution, isnevertheless readily formed as Mn(OH)3 by air oxidation of Mn(OH)2. MnVI also resists disproportionation in alkalinesolution (see Topic E5, Fig. 2. for data in alkaline solution).The species present may change with pH in a way that depends on the oxidation state. Low oxidation states (+2) arealways cationic and as pH increases an insoluble hydroxide is eventually precipitated.
As the oxidation state increases sodoes the acidic character of the hydrated cation (see Topic E2). Thus M3+ ions undergo protolysis even at pH values aslow as 1 or 2; deprotonation can be a first step in the formation of oxygen-bridged dimers as withH3—3D SERIES: AQUEOUS IONSThese may undergo further polymerization before precipitating as Fe(OH)3.High oxidation states (+6, +7) are acidic and always present as anionic speciesdimerization occurs at low pH:219although withWith intermediate oxidation states more complex amphoteric and polymeric behavior is observed.
Thus VV formshydratedin acid solution below pH 2, and the anionic speciesat high pH. Over an intermediate pH rangecomplex polyvanadates are formed, most prominent being the decavanadate ion [V10O28]6− (normally present inprotonated forms).Complex formationComplexing behavior depends on the oxidation state and position in the series. higher oxidation states tend to formstronger complexes with the ‘hard’ anionic ligands F− and chelating agents such as EDTA (see Topic E3). Thus we havecomplexes such as [TiF6]2−, [VF6]− and [FeF6]3−.
Later elements, especially in low oxidation states, have more affinityfor softer ligands such as heavier halides or ammonia. The stabilities of complexes found with many ligands, especiallyammonia or amines, follow a trend known as the Irving-Williams series:Two contributions to this trend are (a) the general decrease in electropositive character resulting from increasedeffective nuclear charge (see Topic H1) and (b) ligand field stabilization energies (see Topic H2), which increase thestability of complexes with ligands higher in the spectrochemical series than water in all ions except Mn2+ (d5) and Zn2+(d10).Complexing can have a strong effect on redox chemistry, the general rule being that a ligand stabilizes whicheveroxidation state it complexes with most strongly (see Topic E5).
Two important examples are the following.(i) Cu+ forms strong complexes with ligands such as CN− and I− so that the CuI/Cu potential becomes negative andcopper metal will react with acids to form hydrogen; these ligands also stabilize the CuI state againstdisproportionation.(ii) Many ligands (e.g. NH3) complex strongly with Co3+, giving a low-spin d6 state with a large LFSE. The resultingcomplexes such as [Co(NH3)6]3+ are much less strongly oxidizing than aqua Co3+ ion, which itself oxidizes water.Generally speaking, negatively charged ligands complex more strongly with ions of higher oxidation state and so reducethe redox potential, whereas neutral π-acceptor ligands, being electron withdrawing, tend to stabilize cations of lowercharge and so raise the potential.
Ligand field stabilization and other effects cause many complications, however, whichcan upset these simple generalizations.Section H—Chemistry of transition metalsH43d SERIES: SOLID COMPOUNDSKey NotesOxidation statesHalide and oxidestructuresOther binarycompoundsElements: occurenceand extractionRelated topicsCompounds are formed with elements in the group oxidation state upto Mn, higher states being found mostly with oxides and fluorides.Lower oxidation states are more stable for later elements. Manymixed-valency and nonstoichiometric compounds are known.Transition metal ions are most often found in octahedralcoordination. Tetrahedral coordination is commoner for highoxidation states.
Ligand field stabilization energies sometimesinfluence the coordination. Ions with d4 and d9 configurations havedistorted coordination geometries.Sulfides have different structures from oxides (often NiAs or layertypes), and some containions. Compounds with N and C areoften described as interstitial.Elements early in the series occur in oxide minerals that cannot bereduced easily.
Later elements occur in sulfides and are easier toextract.Binary compounds: simpleLattice energies (D6)structures (D3)3d series: aqueous ions (H3)Oxidation statesTable 1 shows some oxides and halides of the 3d series elements, selected to show the range of stable oxidation states.These follow the same trends as found in aqueous chemistry (see Topic H3).
Elements early in the series formcompounds up to the group oxidation state, for example, TiO2, VF5 and CrO3. With increasing group number thehigher oxidation states become increasingly hard to form, and can be found only with oxides and/or fluorides, andsometimes only in ternary but not binary compounds. For example, with VV we can make VF5 and V2O5 but not VCl5.With MnVII the only binary compound is Mn2O7 but this is much less stable than ternary permanganates such asKMnO4.The stabilization of high oxidation states by O and F may be attributed at least partly to their small size, which givesthe large lattice energies necessary according to the ionic model to compensate for ionization energies (see Topic D6).Additional lattice stabilization is possible in ternary structures, as in compounds such as K2FeO4 and K2CoF6 where nobinary compounds with the corresponding oxidation state are stable. It should be recognized that many of thecompounds in high oxidation states are not very ionic, and arguments based on the high bond strengths formed by Oand F to more electropositive elements may be more satisfactory than using the ionic model.SECTION H—CHEMISTRY OF TRANSITION METALS221Table 1.
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