Tetravalent lanthanides

The oxidation state iv is firmly established only for Ce, Pr, and Tb, though the preparation of complex fluorides containing Nd(iv) and Dy(iv) has been claimed. The only lanthanide ion stable in aqueous solution is Ce , and this is probably always present in complexes, as in the nitrato complex in (NH4)2 [Ce(N03)g] (p. 664), The only known binary solid compounds of these elements in this oxidation state are  

In the lanthanides the oxidation state ii is most stable for Eu, appreciably less so for Yb and Sm (in that order), and extremely unstable for Tm and Nd. It appears that Eu is probably the only lanthanide which forms a monoxide, the compounds previously described as monoxides of Sm and Yb being Sm2GN and Yb20C. Of the many 4f compounds MX which crystallize with the NaCl structure only EuO, SmS, EuS, and YbS contain M(ii) all other compounds MS, for example, the bright yellow CeS (and also LaS) are apparently of the type M S (e). The cell dimensions of the 4f compounds MS decrease steadily with increasing atomic number except those of SmS, EuS, and YbS, the points for which fall far above the 

All dihalides of Eu, Sm, and Yb are known, but less is known of the dihalides of Nd and Tm. Of the structures of the Yb dihalides only that of Ybl2(Cdl2 structure) appears to be known. The structures of Eu and Sm dihalides are summarized in Table 9.15 (p. 353) and the accompanying text, where the similarity to the alkaline-earth compounds is stressed. Compounds MIj formed from the metal and MI3 by La, Ce, Pr, and Gd are not compounds of M(ii), but have metallic properties and may be formulated (e). 

The ionic radius of Yb is close to that of Ca and tliose of Sm and Eu are practically identical to that of Sr . Accordingly many compounds of these three 4f elements are isostructural with the corresponding alkaline-earth compounds, for example, dihalides, EuO and MS with the alkaline-earth sulphides (NaQ structure), EUSO4 and SmS04 with SrS04 (and BaS04). 

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As a general rule the c.t. bands shift to lower energies with increasing oxidation state, whereas Rydberg transitions (such as 4f- -5d transitions) shift to higher energies. It may, therefore, be expected that the lowest absorption bands of the tetravalent lanthanide ions will be due c. t. transitions and those of the divalent lanthanide ions to Af- -5d transitions. 

Let us consider the tetravalent lanthanide ions first. Because this valency is hard to realize, the number of investigations is restricted. Jorgensen and Rittershaus 3) described the diffuse reflection spectra of Pr + and Tb + in Th02 and Y2O3 and Blasse and co-workers those in Zr02 [4). These spectra show strong absorption bands in the visible which were ascribed to c.t. transitions. 

This is arguably the most prolific area of tetravalent lanthanide chemistry, and Ce, Pr, Nd, Tb and Dy are all represented. 

An empirical set of effective ionic radii in oxides and fluorides, taking into account the electronic spin state and coordination of both the cation and anion, have been calculated (114). For six-coordinate Bk(III), the radii values are 0.096 nm, based on a six-coordinate oxide ion radius of 0.140 nm, and 0.110 nm, based on a six-coordinate fluoride ion radius of 0.119 nm. For eight-coordinate Bk(IV), the corresponding values are 0.093 and 0.107 nm, based on the same anion radii (114). Other self-consistent sets of trivalent and tetravalent lanthanide and actinide ionic radii, based on isomorphous series of oxides (145, 157) and fluorides (148, 157), have been published. Based on a crystal radius for Cf(III), the ionic radius of isoelectronic Bk(II) was calculated to be 0.114 nm (158). It is important to note, however, that meaningful comparisons of ionic radii can be made only if the values compared are calculated in like fashion from the same type of compound, both with respect to composition and crystal structure. 

The only C8H8 complexes of a tetravalent lanthanide are Ce(C8H8)2 and some substituted derivatives.35... 

A more extensive set of actinide complexes is formed with tungstates of the Keggin and Dawson structure, An[XWi 1039)2 and AnpC2Wi706i]2 (X = P, Si, B, As An = Th, U, Np, jj ggg ligands form very stable complexes of tetravalent lanthanides and actinides. 

Chapter 2 is concerned with the chemistry of scandium, yttrium, and the lanthanides and is discussed according to the nature of the ligand in which the donor is from Groups 14-17. Divalent and tetravalent lanthanide chemistry is also described. 

The most stable polyoxometalate structure is the icosahedral Silverton structure M" (MoO O /20lj/3)f2 =M Mo 2042 (M = Ce, Th, U) in which the central metal atom forms an MO 12 icosahedron with the interior oxygen atoms. The central metal atom is 12-coordinate and therefore is a large tetravalent lanthanide or actinide with accessible / orbitals. The oxygen atoms in the Silverton structure are of the following types ... 

Tables 9-11 list the predicted thermodynamic functions for the hydration of divalent, trivalent and tetravalent lanthanides as calculated by Bratsch and Lagowski (1985b). Values for yttrium hydration are also included when available. The formation values refer to the reaction Ln, Ln"a while the hydration values relate to the use of eqs. (28)-(30). The standard state ionic entropies given in table 10 are corrected for...

The experimental data that exists confirms that the range of LnlV species is very limited. Given that the potential for the oxidation of water is -1.23 V, it would be expected that all of the tetravalent lanthanide ions should be unstable to reduction by water. 

This chapter gives an overview on the chemistry of tetravalent lanthanide compounds, especially those of tetravalent cerium. Following a brief introduction, it covers the tetrahalides, dioxides, and other lanthanides(IV) salts. Coordination compounds of cerium in the oxidation state +4 include halogeno complexes and complexes of oxo acids, /3-diketonates and related Schiff-base complexes, as well as porphyrinates and related complexes. 

Among the tetravalent lanthanide ions, only Ce" + is readily available in aqueous solution E Ce +/Ce + = +1.44 VU12MH2SO4, 1.61 Vin IMHNO3, 1.70 V in IMHCIO4). The different values for the reduction potentials indicate... 

The synthesis and full characterization of organolan-thanide(IV) complexes remain a very difficult and often unpredictable task. Thus far, organolanthanide chemistry in the oxidation state +4 remains entirely limited to cerium. This can be traced back to the very highly positive normal potentials of the other tetravalent lanthanide ions Nd" +, Tb +, and... 

Dy + (e g., Pr +2.86 V), which make them very strong oxidizing agents. Not even well-defined pseudo-organometalhcs (i.e., alkoxides, amides, and related compounds) of tetravalent lanthanide ions other than 00" + have ever been isolated. Only alkoxides and amides of tetravalent cerium form fairly well-investigated classes of compounds. In contrast, true organocerium(rV) containing Ce-C bonds remain scarce. 

In a patent based on the above results, Fernando et al. (1991) separated the trivalent lanthanide oxides or hydroxides from the tetravalent lanthanides which do not react with carbon dioxide under supereritieal eonditions. The rare earth carbonates from the trivalent oxides or hydroxides of La, Nd, Sm, Eu, Gd, Dy, Ho, Pm, Tm, and Lu can readily be formed after one hour at 40°C and 100 atm with yields of up to 95% of the normal carbonates, instead of the hydroxy carbonates. The oxides or hydroxides of Pr, Tb, Er, Yb, and Ce do not form earbonates under these conditions. 

M. Htilsen, A. Weigand, and M. Dolg, Quasirelativistic energy-consistent 4f-in-core pseudopotentials for tetravalent lanthanide elements, Theor. Chem. Acc., 122, 23-29 (2009). 

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