The Case of Lanthanides

For lanthanides, the metal radii decrease monotonically with increasing atomic number (Fig. 2), with the three anomalies of Ce, Eu and Yb. 

Let us assume that the metallic valence is three for most lanthanides, and two for Eu and Yb. This is consistent with the chemistry of these elements, since (Fig. 1) three is their most common oxidation number. Trivalency has been explained in the proceeding chapter, by assuming the easy promotion of one 4 f electron to a 5 d level. The conduction quasi-free electrons are therefore of 5d and 6 s origin. We may say that the conduction band has a prevalent (s, d) character. 

The trend of radii vs. Z shown in Fig. 2 finds then an explanation consistent with the picture of ion cores, each containing a 4P non-binding shell. In fact, the 4f shells screen only imperfectly the outer electrons. Adding one nuclear charge when going from Z to Z -f 1, means increasing the central field of the nucleus, without increasing as efficiently 

---

In the case of lanthanide elements, other sandwich-like compounds such as the triple-decker phthalocyanincs have been reported. Yttrium(III) acetate hexahydratc when treated with phthalonitrile at 230 C for 8 hours forms a triple-decker phthalocyanine.202... 

Octasubstituted bis(phthalocyanines) 22 have been reported in the case of lanthanide elements. They contain eight bidentate substituents bound to the macrocycle.159,194... 

The variety of stoichiometries that one comes across in the case of lanthanide complexes is much more than what one usually encounters in the case of cf-block transition metal complexes. This is due to the wider range of coordination numbers that are possible for the lanthanides. The stoichiometry of a lanthanide complex depends on the size of the metal ion, the size of the ligand, the nature of the anion and the synthetic procedure used. 

The amount of unreacted target element that eluted was determined by measuring its radioactivity directly in the case of actinides, and by activation analysis in the case of lanthanides. The distribution of the radioactive neutron capture product was determined by counting both the eluate and the eluted zeolite. All irradiations were done in the Oak Ridge Research reactor in a pneumatic tube facility with a thermal neutron flux of about 4 X 1013 neutrons cm-2 sec-1 or, for a few long irradiations, in a tube adjacent to the reactor core at the fluxes stated in Table VI. 

Particular cases applicable to low spin and high spin Fe(III) will be discussed in Section 5.2. The case of lanthanides will be discussed in Section 2.8.4. 

THE CASE OF LANTHANIDES AND ACTINIDES 2.8.1 Electronic properties of lanthanides... 

The relaxation equations discussed here and in Section 3.4 and 3.5 take a different form in the case of lanthanides and actinides. For these systems, in fact, the J quantum number substitutes the S quantum number and gj substitutes ge. In the absence of chemical exchange phenomena (r 1 r ) the equations for the... 

In this section, lifetime values (and in few cases, data on emission spectra) are collected for a luminescent probe in a given solvent, to which variable amounts of supporting electrolytes have been added. To the best of our knowledge, such experiments have been performed only for aquo ions (never for complexes), in H2O or D2O, but in two cases. Although the term supporting electrolyte implies no chemical reactivity towards the probe, results involving chloride based electrolytes, for which a complexation process is sometimes evoked in the case of lanthanides, are also presented in this section. When discussed at all in the literature, decay spectra are said to be monoexponential. 

In the case of lanthanides, the 4f electrons, which are involved in the photoexcitation process, are strongly shielded. Hence one may assume that ... 

The coordination number, N of lanthanides varies significantly and no intrinsic property of 4/ group atoms predetermines a high propensity for a given N value and symmetry as is common with transition elements like 3d3 Cr(III), 3d6 Co(III), 3d8 Ni(II), 4d6 Rh(III), and 5d6 Ir(III) and Pt(IV). In the case of lanthanides, N takes on values from 3 to 16 as shown by examples given below. 

The coordination number of six which is common for d-transition metals and scandium, is rather rare in the case of lanthanides. Authentic six-coordinate rare earth complexes... 

The above series in general is similar to the ligand nephelauxetic series observed in the case of d-transition metals. The greatest nephelauxetic effect has been observed in sulphides [46], cyclopentadienides [47], and oxides [48] of lanthanides. However, attempts to formulate a common and general nephelauxetic series for the lanthanide series have been futile using aquo ions as reference standards. In the case of lanthanide complexes with the same ligand in aqueous solutions, the absorption band positions of light and heavy lanthanides shift in different directions. This unusual behavior of complexes may be due to the differences in structure of aquo ions and the complexes. 

Besides these discrepancies between crystallographic results and the instantaneous evidence obtained by spectroscopic measurements, there occurs another interesting discrepancy between classical physico-chemical and spectroscopic investigations. A very characteristic case is that of normal anion complexes and the outer-sphere ion-pairs in solution. This distinction is clear-cut in stable complexes (23) such as [Cr(H20)6]" S04 and [Cr(S04)(H20)4 or In the case of lanthanide chlorides, it is well-known that spectroscopic evidence (42, 75) for inner-shell complexes NdCl(H20)x indicates a formation constant hundreds of times smaller than that of the ion pair Nd(H20)g+ Cl . In organic solvents the inner complexes are relatively more abundant (2, 43). In certain cases, one also... 

The experiments crystallized plagioclase in the composition range Ango-An4o, which covers most terrestrial magmatic plagioclases. In the case of lanthanides, Bindeman et al. (1998) find a positive correlation between RT In and Xad-For lanthanum the relationship is... 

The Lewis acid complexes to the lone electron pair of the aldehyde anti to the R group, forcing the R group into an endo position in the transition state. In the case of lanthanide catalysts, the large steric bulk of the lanthanide metal-ligand complex causes the increase in cis selectivity (Scheme 2). 

---