D. A. Johnson The Open University Milton Kevnes MK7 6AA.
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Principles of Lanthanide Chemistry
At one time, it was thought that the chemistry of one Ianthanide element was verv similar to that of the next. Along with this notion went the conviction that, within the l a c thanide series, any change in properties did not greatly deviate from a smooth variation with atomic number. Today we know that this is not the case: although many chemical reactions of the lanthanides conform to the traditional belief, there are others in which the elements behave very differently. In this paper, I suggest a way of distinguishing these two classes of reaction. Formulation of the Prlnclple In a recent paper in this Journal (I), Cater used thermodynamic data to establish an important principle of hightemperature lanthanide chemistry. He suggested that
a dramatic variation in properties across the lanthanides does occur in t h w reactions in which the rare earth atoms p a s from a combined to a free state. or vice versa. The comhined state may be in the solid metsl, a solid campound, or a gaseous molecule. c he free state is the state of gaseous atoms. . . .On the other hand, for reactions in which the rare earth atoms are transferred from one combined state to another, only very small periodic effects are observed. Typical examples of the processes considered by Cater were the reversed forms of the reactions, (1) M(g) + O W = MOM M(g) = M(s) 12) in which the free state of the gaseous atoms changes to a comhined state in a gaseous molecule or the solid metal. The variations in AHmBat 298.15 K for these two reactions are shown in the two upper plots in Figure 1.The variations are indeed dramatic. Cater's axiom can be applied very successfully to the high temperature equilibria with which his article was concerned. However, it is not a universal principle of lanthanide chemistry. This is apparent from the two lower plots in Figure 1 which show the changes in AHrn@at 298.15 K for the processes,
MCI2(s')+ 'hClz(g) = MCl& (3) M2+ (g') = M3+(g)+ e-(g) (4) The meaning of the primes against the physical states of MClz and M2+ is explained below. I t is obvious that the variations in AHmBfor reactions (3) and (4) are vew similar to those for reactions (1) and (2). However, in reakion (3), the lanthanide element passes from one combined state to another, and in reaction (41, it is in an uncombined state on both sides of the equation. There is, in fact, a principle which embraces both the examples cited by Cater, and those cited here (2).I t is as follows: the lanthanide elements behave similarly in reactions in which the 4f electrons are conserued. and uerv in . differently ..
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Here and throughout the paper, n is the number of 4f electrons carried by the tripositive ion in its groilnd state. Thus for lanthanum, n = 0, for gadolinium, n = 7 and for ytterbium, n = 13. [Xe]4f"+' is the ground state configuration for all gaseous dipositive ions except La2+(g)and GdZ+(g),so for these twoions, theionization energy has been corrected in Figure 1by the known energy differencebetween the [Xe]4f"5d1ground state, and the lowest level of the (Xel4f"+' configuration.
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Fiaure Varlatlms in AH-a ... at 298.15 K lor reactions 11U4). . . . . Data fw - 1. reactlom (1) and (2) are from relersncs I 0;tor react~ons(31and 14) horn references (2) and ( 10). In p l m 1-3, open and c l d circles rapream estimated and expalmsntal values, rerpsctlvsly. For the status of the dam In olot 4, see ~~~~
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reference(2)
reactions in which the number of 4f electrons changes. If the 4f electrons are conserved, the energy of the reaction varies nearlv smoothlv with atomic number. If the number of 4f ~~-~ ,~ electrons decreases hy one, the uariotion in the energy across the series is tv~ifiedbv the variation in the third ionization energy of the eiements:~his variation (AH,' for reaction (4)) is shown in the lower plot of Figure 1.The prime against the physical state of ~ 2 + - i reaction n (4) indicates that the M2+ ion has a configuration1 of the type [Xe]4fn+l. The prime against the physical state of MClz in reaction (3) has asimilar meaning. All known dichlorides contain M2+ions with [Xel4fn+l configurations (2).Clearly then, reactions (3) and (4) are processes in which the number of 4f electrons decreases by one. Moreover, as Cater's article makes clear, this is also a property of reactions (1) and (2): the gaseous atoms largely have configurations [Xe]4f"+16s2,while in the MO molecule and the solid metal there are n 4f electrons and three valence electrons per metal atom. Furthermore, the parallelism between the two upper plots and the two lower ones would be even closer if the lanthanum, cerium, and gadolinium points in the two upper plots were lowered. But these are just the ooints where the easeous atoms have the confinurations i ~ e ] 4 f " 5 d ' 6 s ~correction . to [Xe]4fn+16s2achieves the neeessarv lowering. Thus if all points in the two upper are .. plots . madeto refer & a process inwhich the f electrons decrease by one, the parallelism between the four plots becomes even more maiked.. As discussed elsewhere (2), among other examples which reveal the characteristic energy variation of Figure 1are ~
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Volume 57, Number 7, July 1980 1 475
MZ+(aq') + HC(aq) = M3+(aq) + '/*HZ (g) MClz (8') = 'IsM(s) + 2hMCh (4
(5)
(6)
(7) 'hM20ds) + 'ILMg) = Mods) M [4f"+16s2](g)= M [4fn5d'6s2](g) (8) If we ignore the minor exceptions of europium and ytterbium for reaction (6), then in all these reactions, the number of 4f electrons decreases hy one. In the case of reaction (7), the maxima occur a t gadolinium and lutetium rather than a t europium and ytterbium.
Unusual Oxldatlon States of the Lanthanides For many years, the relative stabilities of the less-common oxidation states of the lanthanides has been explained in terms of the stahilities of empty, half-filled and filled-shell contigurations. For example, tilestability ot'europium(l1) and ytterbiumUl) halides was explained by the presence of [Xel lp and IXel4f1' a~nfirurations:the existence of samnriumlll~and thul&(i1) halid& by the fact that they are only one electron short of these stable states (3). The principle advocated in this paper gives thermodynamic expression to such explanations. The energies of reactions (3)-(6) show two overall increases from lanthanium to europium and from gadolinium to ytterbium. These increases are separated by a sharp drop between europium and gadolinium, so the maxima occur a t those points where an electron is lost from a 4f" or 4ft4 configuration. However, the new principle. copes with an anomaly that the old generalization could not: and the downward hreak at the 314-shellt,etweend~sprosi~~m erbium accounts for the fact that dysprusium(I11 halidesare well known whereas attempts to makeerhium(11) halides have been unsuccessful (4). The relative stabilities of the +4 oxidation states can be similarly explained by considering reactions such as (7). Because d o t s tvoified hv those in Figure 1 show such laree disconiinuitie;, the distribution of ;;able +2 and + 4 st&s within the lanthanide aeries is very irregular. The existence o f these oxidation states thus provides the most obvious refutation of the view that the lanthanide elements are rhemically very similar. Theory of the Principle As discussed elsewhere (2.5). . . . the variations in Fieure 1are :,recisely what theories of many electron atoms predict for an .I r I I" iunization.'l'he overall increase may be attributed to the increase in nucletlr charge as one moves across the series; the irrecularities are raused bv discontinuities in the chanaes in inter
