Rare-earth cations, coordination

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

A Derivatives of 1 1 complexes As a result of the chirality of ai-[P2-W17O61]10- (Figure 5) solutions of [ Ce(o i-P2Wi706i)(H20)4 2]14 contain enantiomeric pairs of monomers in equilibrium with the meso dimer. Addition of chiral amino acids to such solutions causes a doubling of the 31P-NMR resonances as a result of diastereomer formation presumably caused by coordination of the amino acid to the rare-earth cation (Sadakane et al., 2001). No splitting was observed when similar experiments were carried out with complexes of the achiral a2 isomer. Formation constants for the two diastereomers of the complexes with L-proline were estimated as 7.3 1.3 and 9.8 1.4 M-1. The corresponding proline complex of achiral [Ce W C i)]7- has a formation constant of 4.5 0.1 M-1 (Sadakane et al., 2002). 

As this chapter has shown, the oxophilic (hard acid) nature and high- and variable-coordination requirements of the rare-earth cations can frequently be satisfied by... 

Table 18.3.1. Coordination number and geometry of rare-earth cations...

Nai+x[Zr2-xRx(P04)3] with Xmax < 1, where R is a rare earth cation (Fig. 10) [97-102], One can suppose that a similar situation can be observed for actinide(III) compounds (or solid solutions). However, none of them has been reported to the present time. An overview on known structures of actinide and similar lanthanide phosphates described above demonstrates a wide variety of coordinations for tri- and tetravalent actinides. The ThOn, UOn, and some LnOn polyhedra with n changing from 10 to 6 are shown in Fig. 12. 

Laser flash photolysis has also been used to study the photophysics and photochemistry of metallocene-containing cryptands and their complexes with rare earth cations [69]. The metallocene moiety was shown to act as an efficient centre for the radiationless deactivation of the lanthanide excited state. Detailed time-resolved studies permitted the characterisation of the coordination chemistry about Dy within the cryptate and showed, once again, that the functions within the host cryptand primarily responsible for coordination of the guest cation were the amide carbonyl groups. 

Rare-earth silicate compounds form a chapter in the chemistry of large-cation silicates. With RE + ionic radii ranging from 0.85 to 1.28 A, the crystal structures consist of isolated (Si04) tetrahedra or groups of (SigOv) and (SigOio) Rare-earth cations show six- to tenfold oxygen coordination. 

Fig. 28. Oxygen coordination around the rare earth cations in A-type Pra(Si207)...

Empirical values of effective ionic radii of the trivalent rare earths have been obtained from the quasi-linear relation, unit-cell volmne vs. (RE3+ionic radius) , for some isostructural series. The data used to construct Fig. 45 are the values V/Z (A) , as listed in Tables 1, 4 and 14, for unit cell volume per formula unit, together with experimentally confirmed interatomic distances, i.e. the mean values for a given rare-earth coordination. Since the oxygen radius is known to be dependent upon its coordination number, too (Fig. 43 0 = 1.32 A, == 1.34 A, rvQ = 1.36 A), this had to be taken into account in evaluating the experimental values < f(RE—O) > which represent the sum of the ionic radii. Mean coordination munbers of the oxygens siuround-ing the i rare-earth cations are listed in Tables 22/23. Thus,... 

With the smallest of the rare-earth cations, Sc, a third structure type is found for rare-earth trifluorides ScFs crystallizes with the ReOs (AIF3) type of structure (Losch et al. 1982) with octahedral coordination of the cations and all octahedra [ScF ] sharing common comers. 

The chlorides, bromides and iodides of the bivalent rare-earth cations occur in four different structure types that are known from UCI3, PuBr3, AICI3 (YCI3) and FeCU (Bils) with coordination numbers of 9, 8, and 6 for the latter two, respectively. 

An interesting aspect of the pyrochlore structure is the unique coordination found for the rare earth at the A cation site. Although this environment is sometimes viewed as a distorted cube, two oxygens are much closer than the remaining six. These are in fact among the shortest R-O distances ever observed, and such rare earth cations are in the highest electric field gradient ever observed. 

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