Oxidation states lanthanoids

Thus far, we have focused exclusively upon the block metals. For some, the term transition elements defines just these J-block species for others, it includes the rare earth or lanthanoid elements, sometimes called the inner transition elements . In this chapter, we compare the elements with respect to their valence shells. In doing so, we shall underscore concepts which we have already detailed as well as identifying both differences and similarities between certain aspects of main and inner transition-metal chemistry. We make no attempt to review lanthanoid chemistry at large. Instead our point of departure is the most characteristic feature of lanthanoid chemistry the +3 oxidation state. 

Both phenomena attest to the covalency of the chemical bonding in these species. Incidentally, they also highlight the different characters and implications of the spectrochemical and nephelauxetic series. Within either lanthanoid- or (higher oxidation state) J-block species, the ligand orbitals overlap with the metal s functions... 

The members of the actinoids have a somewhat greater tendency to form complexes than those of the lanthanoids. They also show a wider variety of complexes due to their more numerous oxidation states. The cations of the actinoids display coordination numbers which are often greater than 6. Although not many data are available, M02+ and M02" " ions attach 5 or 6 HOH molecules giving the central atom a coordination number of 7 or 8, and the and ions attach 9 or 10 HOH molecules. The coordination numbers with ligands other than HOH are often of these magnitudes, but sometimes may be somewhat smaller. 

E23.3 The unusual oxidation states displayed by two of the lanthanoids, +4 for Ce and +2 for Eu (see Exercise 23.1) were exploited in separation procedures because their charge/radius ratios are very different than those of the... 

Recently, attention has been focused on lanthanoid/crown ether complexes,5 since they may be used for lanthanoid ion separation, for stabilizing unusual oxidation states, and for studying high coordination numbers, Moreover, lan-thanoid(III) ions are increasingly used as spectroscopic probes in systems of biological interest.6... 

Table 24.2 shows that a wide range of oxidation states is exhibited by the earlier actinoids, but from Cm to Lr, the elements resemble the lanthanoids. This follows from the lowering in energy of the 5/ atomic orbitals on crossing the period and the stabilization of the 5/ electrons. 

Lanthanum and the lanthanoids, except Eu, crystallize in one or both of the close-packed structures Eu has a bcc lattice and the value of Vetai given in Table 24.1 can be adjusted to 205 pm for 12-coordination (see Section 5.5). It is important to notice in Table 24.1 that Eu and Yb have much larger metallic radii than the other lanthanoids, implying that Eu and Yb (which have well-defined lower oxidation states) contribute fewer electrons to M—M bonding. This is consistent with the lower values of Eu and Yb, 177 and 152kJmoU respectively, compared with the other lanthanoids (206-430 kJmoU ). The lowest... 

In accordance with the variation observed in their successive ionization potentials. Table 2-1, the (3+) oxidation state is a common characteristic chemical feature of the lanthanoid series. With a few exceptions, typically associated with elements having a relatively low fourth ionization potential (Ce, Pr, Tb), Table 2-1, the (3+) oxidation state exhibits a high stability. In the case of the three elements mentioned above, the (4+) oxidation state is very relevant as well. In particular, higher oxides, i.e. dioxides and mixed-valent (+3/+4) compounds are well known for... 

In 1970, tne first cyclooctatetraenyl compounds of the lanthanoides in the trivalent oxidation state were prepared (lOii). The reactions of LnCla with KaCsHe in THF afford the following complexes, which are stable up to about 160°C, but sensitive towards air and moisture (10U,105,... 

The simplest homoleptic organometallic derivatives of the lanthanoides in the oxidation state Ln " " are the compounds Ln(CH3)3. The driving force for the inner transition elements to achieve high coordination numbers forcasts a high reactivity for these compounds. Thus, for a long time, only indirect evidence could be found for the existence of such derivatives, and the isolation and characterization of compounds belonging to this class was not possible before the last decade. 

In I96U, the blue solution of metallic europium in liquid ammonia was found to react with cyclopentadiene. The sublimation of the crude product at U00 C in vacuo yields an ammonia-free syndwich complex, as the first organometallic compound of a lanthanoide in the oxidation state Ln (183). The ytterbiinn complex has been prepared in a similar manner (15) ... 

ORGMOMETALLIC COMPOUNDS OF THE LANTHANOIDES IN THE TETRAVALENT OXIDATION STATE... 

Typical ligands for the organometallic compounds of the transition metals in low oxidation states are CO and the olefins. Therefore, no experiment has been left undone to synthesize such carbonyl or olefin complexes of the elements of the third subgroup and of the lanthanoides. Whereas lanthanoide carbonyls could be detected only in a matrix until now (213), the first indications have been obtained for the existence of olefin complexes. 

The characteristic oxidation state is and the great similarity in the size of the ions leads to a very close similarity of chemical properties and hence to great difficulties of separation using conventional methods. In addition, cerium can assume a Ce + state and ytterbium a Yb state. Chromatographic and solvent-extraction methods have been specially developed for the lanthanoids. 

Besides the ubiquitous oxidation state Ln +, the higher oxidation state Ln" + is also encountered with some lanthanoids, for example, in the case of the ions Ce" + (f°, orange-yellow), Pr + (f, colorless), Nd" + (f, blue-violet), Tb + (f , colorless), and Dy + (f , orange-yellow) (Table 1). However, all three states Ln + + " + are never encountered for the same element. Thus, the highly important mechanistic steps of oxidative addition and reductive elimination typical for the d-block metals cannot occur with the f-block metals as they would involve M + or transformations, respectively. ... 

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