P.A. Cox - Inorganic chemistry (793955), страница 57
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Specialist applications of individual lanthanides depend onthe spectroscopic properties of Ln3+ ions (e.g. Nd in lasers) and on the magnetic properties of some of the elements(e.g. Sm). The ions can be separated by ion-exchange chromatography from aqueous solution, using the variation ofcomplexing properties across the series (see below).Oxidation state +3The Ln3+ state is the most stable under normal conditions for all elements in the series. Halides LnX3 and oxides Ln2O3are known for all elements, as well as an extensive range of oxo salts including mixed and hydrated compounds such asLn2(SO4)3.3Na2(SO4).12H2O. Ionic radii vary from 104 pm (La3+) to 86 pm (Lu3+) and this relatively large size for 3+ions (cf.
Al3+ 53 pm) is associated with correspondingly high coordination numbers in solid compounds. LnF3compounds for earlier elements have nine-coordination, Ln2O3 are seven-coordinate. For later Ln elements thedecrease in radius leads to changes in structure with reduction in coordination.The aqua Ln3+ ions show slight acidity, which increases from La to Lu as the radius decreases but is still much lessthan for Al3+.
Strong complexes are formed with hard oxygen donor ligands, and especially chelating ones such asEDTA (see Topic E3) or β-diketonates (L-L=[RC(O)CHC(O)R]− 1), which give eight-coordinate complexes [Ln(L-L)−4] . Complex strengths generally increase across the series as the radius decreases, and this may be used to separate amixture of Ln3+ ions. For example, in an ion-exchange chromatography column with a complexingL1—LANTHANUM AND THE LANTHANIDES249agent present in aqueous solution, the earlier lanthanides, which are less strongly complexed, are retained preferentiallyon the column and elute more slowly.The organometallic chemistry of lanthanides is much more limited than in the d block (see Topic H10).Compounds such as (C5H5)3Ln and (C5H5)2LnX (X=Cl, H, etc.) have more ionic character than for transitionelements, and compounds with neutral ligands such as CO are not stable.
Some interesting chemistry has, however,been found with compounds such as (C5(CH3)5)2LuH where a bulky ligand is combined with a small lanthanide. Forexample, the methane activation reactionoccurs under mild conditions in solution.Other oxidation statesAccording to the ionic model the relative stability of Ln2+ and Ln3+ compounds is determined by a balance between thethird ionization energy (I3) of the lanthanide, and the difference of lattice (or solvation) energies associated with the twoions (see Topics D6 and G1). The I3 value for lanthanides is small enough that most Ln2+ compounds are unstable withrespect to disproportionation to Ln and Ln3+.
The exceptions are of two kinds. For Sm, Eu and Yb, I3 is large enoughto stabilize a number of compounds such as SmO, EuF2 and YbCl2. The aqueous Ln2+ ions are strongly reducing,especially so for Sm and Yb. On the other hand, compounds with large anions have small lattice energies and sodisproportionation is less favorable. Thus LnS and LnI2 are known for all Ln.
Many of these compounds are metallic inappearance and highly conducting, which suggests an unusual electron configuration as 4f orbitals on one atom cannotoverlap sufficiently with orbitals on other atoms to form bands (see Topic D7).
A formulation such as (Ln3+)(S2−)(e−) issometimes given, implying a (4f)n configuration appropriate to Ln3+ with one electron delocalized in a band (formedprobably from overlapping 5d orbitals). For compounds of Sm, Eu and Yb this peculiarity disappears, and, for example,EuS and YbI2 are not metallic but have ‘normal’ Ln2+ ions.Ln4+ compounds are known only for elements with the lowest I4 values.
Ce4+ is known in aqueous solution and formsmany compounds such as CeO2. Pr4+ and Tb4+ are more strongly oxidizing, giving fluorides LnF4, and being presenttogether with Ln3+ in mixed-valency oxides such as Pr6O11 (which is actually nonstoichiometric).Section I—Lanthanides and actinidesI2ACTINIUM AND THE ACTINIDESKey NotesNuclear propertiesChemical propertiesRelated topicsAll actinide elements of the 5f series are radioactive. Th and U are longlived and occur in minerals that also contain their radioactive decayproducts.
Elements beyond uranium are made artificially, bybombardment with neutrons or with nuclei. Uranium and plutonium areused as nuclear fuels.Early actinides show a variety of oxidation states. The +6 state is commonfor U but becomes progressively more strongly oxidizing. Later actinidesare more similar to lanthanides, with the +3 state being common.The nuclear atom (A1)Lanthanum and the lanthanidesThe periodic table (A4)(I1)Nuclear propertiesFollowing actinium (group 3) are the 14 elements of the actinide series (represented by the symbol An) associatedwith progressive filling of the 5f shell and so analogous to the lanthanides. All are radioactive, their longest-lived isotopesbeing shown in Table 1.
The progressively shorter half-lives reflect the decreasing stability of heavy nuclei, resulting fromthe changing balance between the attractive strong interaction and the repulsive Coulomb forces (see Topic A1). Mostactinide nuclei undergo α decay by emitting 4He, but for heavier elements spontaneous fission into two fragmentsis an increasingly important alternative decay route.Only thorium and uranium have half-lives long enough to survive since the formation of the Earth (see Topic J1).Thorium is found together with lanthanides in the phosphate mineral monazite (LnPO4), and uranium occurs aspitchblende U3O8 and carnotite K2(UO2)2(VO4)2.3H2O. Uranium is principally used as a nuclear fuel, as theisotope 235U undergoes neutron-induced fission, the nucleus splitting into two smaller fragments together withmore neutrons, which can thus initiate a chain reaction.
The energy liberated (about 2×1010 kJ mol−1) is vastlygreater than that obtainable from chemical reactions.232Th, 235U and 238U are the first members of radioactive decay series, forming other radioactive elements withatomic numbers 84–91, which are therefore present in small amounts in thorium and uranium ores.
The 238U series isillustrated in Topic A, Fig. 1. Each series ends with a different stable isotope of lead (208Pb, 207Pb and 206Pb,respectively) and the proportions of these present in natural lead samples varies detectably. This variation can be used togive geological information, including an estimate of the age of the Earth.Transuranium elements beyond U do not occur naturally on Earth but can be made artificially.
The neutronirradiation of 238U in nuclear reactors produces 239U, which rapidly undergoes β decay to 239Np and thence to 239Pu.SECTION I—LANTHANUM AND ACTINIDES251Table 1. Longest-lived isotopes of actinidesFurther neutron irradiation produces heavier actinides in progressively smaller amounts, up to Fm. The remainingelements Md, No and Lr cannot be obtained in this way but have been produced in exceedingly small quantities bybombardment of lighter actinides with nuclei such as 4He and 12C using particle accelerators.
(Note that the longestlived isotopes listed in Table 1 are not necessarily the ones most easily made.) Similar methods have been used to maketransactinide elements with atomic number up to 110, presumably forming part of a 6d transition series. However,the very small quantities made (often a few atoms only) and their very short half-lives make chemical studies almostimpossible.Chemical propertiesUnlike the 4f orbitals in the lanthanides, the 5f orbitals in the earlier actinide elements are more expanded and so can beengaged in chemical bonding.
This leads to a pattern of chemistry more analogous to that found in the d block, with thepossibility of variable oxidation states up to the maximum possible determined by the number of valence electrons(see Topic H1). Most thorium compounds contain ThIV (e.g. ThO2) and with uranium the states from +3 to +6 can beformed. UO2 is frequently nonstoichiometric, and the natural mineral U3O8 probably contains UIV and UVI. Uraniumhexafluoride is made industrially using ClF3 as a fluorinating agent (see Topic F9):Being volatile, it is used to separate the isotopes 235U and 238U for nuclear fuel applications.
Many other UVI compoundscontain the uranyl iona linear unit with bonding involving both 5f and 6d orbitals: examples include themineral carnotite (see above) and Cs2[UO2Cl4] where uranyl is complexed to four chloride ions.The maximum attainable oxidation state in the series is +7, in the mixed oxides Li5AnO6 (An=Np, Pu). Withincreasing atomic number high oxidation states become more strongly oxidizing, as in the d block.
This trend isillustrated in Fig. 1. which shows a Frost diagram with the oxidation states of some actinides found in aqueous solution(see Topic E5). The oxocationsandare characteristic for AnV and AnVI with An=U, Np, Pu and Am,but the slopes of the lines in the diagram show their increasingly strong oxidizing character.
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