Compounds of lanthanides

SPECIATION ANALYSIS OF MATERIALS BASED ON INORGANIC COMPOUNDS OF LANTHANIDES... 

It is crucial to discover the relationship between chemical compositions and hydrogen decomposition pressures of the AB5 compounds. Intermetallic compounds of lanthanide and transition metals form an interesting class of structures. The AB5 series crystallize in the hexagonal CaCus (P6/mmm) structure (see Figure 1). Generally, radius ratios (raAb) greater than 1.30 form the CaCus-type... 

There are many compounds of lanthanides exhibiting coordination polyhedra close to a dodecahedron with triangular faces. There remains a great interest in the various environments and ligand combinations for which the dodecahedral arrangement is favourable. 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

The considerable change in size of the Ln3+ ions from La3+ (1.17 A) to Lu3+ (1.00 A) means that homologous compounds of lanthanides with appreciably different radii may differ in structure. Examples are the following ... 

A number of early transition metal compounds, e.g., Ti111 hydrides, have long been known as hydrogenation catalysts. Similarly, metallocene compounds of lanthanides and actinides can be extremely active, as a comparison of the turnover numbers of 1-hexene hydrogenations at 25°C (1 bar H2) show CpfLuH, 120,000 [Ir(COD)-(py)(PCy3)]PF6, 6400 [Rh(COD)(PPh3)2]PF6, 4000 RuHCl(PPh3)3, 3000 ... 

Organometallic compounds of lanthanide metals other than Sm, Eu, and Yb are very rare until now. But the development of this chemistry became possible after the synthesis of divalent precursors of Tm, Dy, and Nd in the late 1990s, namely of their diiodides (see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry) and by using hgands such as phospholyl or arsolyl, which stabilize divalent lanthanide ions. 

Why does the octet rule not work well for compounds of lanthanide and actinide elements Suggest a number other than eight that may be more suitable. 

Fig. 2. Photo-electron spectra of compounds of lanthanides, hafnium and tantalum in the region from I between 0 and 40 eV. The origin of the signals from various shells is indicated the intensities are not on the same scale going from one compound to another, and an arbitrary height of background has been removed. Signals due to Kas photons with 9.8 eV higher energy than the main exciting line are marked 3C3...

The use of the catalysts based on individual compounds of lanthanides of various structures may help to elucidate the nature of active centres. This explains the interest in the problems of synthesis and conversions of these compounds [31-40]. 

Derivatization procedures carried out on crude samples can produce materials with improved sublimation characteristics. This technique has been used to produce volatile compounds of lanthanides and actinides that have then been sublimed prior to analytical determinations. Derivatives have been made using -diketones (hexafluoroacetylacetone or acetylacetone), benzoyltrifluoroacetone, and then-oyltrifluoroacetonates. 

The extent of electrostatic (ionic) and orbital (covalent) considerations in determining the structures of such compounds of lanthanides, actinides and early d-block transition elements is not clear. As with Cp M (M = Ti, Zr, Th, U) the complexes M(CgHg)2 show structural trends apparently related to the size of the metal atom. 

However, binuclear coordination compounds of lanthanides with H2LI702 are... 

Fig. 52. (top) Enthalpies of formation of intermetallic compounds of lanthanides and actinides (Th, U, Pu) with transition elements of the columns headed by Ni, Co and Fe. (bottom) Enthalpies of formation of intermetallic compounds of gadolinium and actinides (Th,U, Pu) with transition elements of the thirst row (Mn — Ni), second row (Ru— Pd) and third row (Re—>Pt). The references of the data are quoted in tables 5 to 20. The data which appear the more reliable have been joined by lines (see the text for discussion). 

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