Lanthanoids, characteristics

As one moves through the actinoids, the trend toward lanthanoid characteristics becomes more evident with Am as Figure 18.7 (at 10 M) illustrates. A much larger area in the figure is occupied by the An ion, but resemblance to U, Np, and Pu remains in the upper regions of the E-pH diagram. 

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. 

Part of the absorption spectrum of an aqueous solution of neodymium(iii) -configuration/ - is shown in Fig. 10-4. The situation shown there is quite typical of the whole of the lanthanoid series i.e. we could have chosen any/" configuration equally well to illustrate the main characteristics of the spectra of lanthanoid complexes. We shall focus on three main features splittings, band widths and absolute excitation frequencies. 

Like other lanthanoids, praseodymium is also used to give color to glass, ceramics, enamels, and other materials. The characteristic color provided by compounds of praseodymium is a bright yellow. 

XANES is one of powerful tools for the study of chemical states of lanthanoid compounds. Most of the conclusions drawn from the XANES for the lanthanoid compounds have been concerned with the chemical states or characteristics of the electronic structures. In particular, valences of lanthanoids have been studied using such spectra (2). They were obtained by assigning some of the peaks in the spectra to different valences. By using the Anderson impurity model, the valences were derived from intensities of shake-up peaks (3). 

N2, <10 ppm H20). All the complexes are microcrystalline powders with the characteristic color of the lanthanoid ions. Complexometric analyses are performed with edta (Titriplex III, Merck), using xylenol orange and urotropine. 

Despite the term traditionally applied to this group of elements, rare earths, their crustal abundance is not particularly low. Cerium ranks around 25 in the listing of all the naturally occurring elements, its abundance being similar to that of Ni or Cu [1]. Even the least abimdant lanthanoid elements, Tb, Tm, and Lu, are more abundant than Ag [2]. Because of their geo-chemical characteristics, however, the rare earth-containing minerals consist of mixtures of the elements with relatively low concentration of them [3]. Accordingly, the number of their exploitable deposits, mainly consisting of phosphates and fluoro-carbonates, is rather small [1,3]. 

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... 

The Ln ions exhibit large ionic radii ranging from 117 pm for La to 100 pm for Lu, Table 2-1. Also well known, the Ln radii steadily decrease throughout the series as a result of the so-called lanthanoid contraction effect. These are very characteristic chemical features of the lanthanoid elements. 

Because of the inner nature of the 4f orbitals, the dififerenees of eleetron configuration between the lanthanoid elements are associated to eleetrons relatively well screened from the chemical surroundings by the outer (5s p ) shell. This implies weak crystal fields splitting effects [28], and a relatively small eovalent contribution to the bonding, particularly in the sesquioxides. Accordingly, the ionic model plays an important role in determining their chemistry [21]. Also related to these chemical characteristics, the lanthanoid compounds exhibit a rich variety of stmctures, often reflected in the occurrence of polymorphism phenomena. 

Fluorescent lanthanoid chelates absorb radiation at the wavelength characteristic of the chelator and emit with the wavelength characteristic of the metal, due to energy transfer from the ligand to the metal ion. Lanthanoid chelates are mainly based on bipyridine, polyaminocarboxylate, and cryptate chelators. A typical anteima attached to some polyaminocarboxylate chelators is carbostyril 124 (7-amino-4-methyl-2(lH)-quinolinone) (see Figure 13), which absorbs at a wavelength of 337 nm and the main emission will be at 615 nm for europium and 543 nm for terbium. 

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. 

The valence shell of a lanthanoid element contains 4/ orbitals and that of an actinoid, 5/ atomic orbitals. The ground state electronic configurations of the /-block elements are listed in Table 27.1. A 4/ atomic orbital has no radial node, whereas a 5/ atomic orbital has one radial node (see Section 1.6). A cmcial difference between the 4/ and 5/orbitals is the fact that the 4/ atomic orbitals are deeply buried and 4/electrons are not available for covalent bonding. Usually for a lanthanoid metal, M, ionization beyond the M ion is not energetically possible. This leads to a characteristic +3 oxidation state across the whole row from La to Lu. 

Lanthanoid aryl complexes, like alkyl derivatives, are unstable in air but more stable on heating. The IR spectra of Ph3Sc and Ph3Y exhibit the absorbtion bands characteristic for Ph groups. Some broadening of the bands in the spectrum of Ph3Y and their comparatively low intensity indicate an association of the complex. 

The IR spectra of all cyclohexylisonitrile complexes of lanthanoids are practically identical [159, 165, 178]. The absorption bands characteristic for five-membered cycle with the symmetry of are present in the spectra. The nonplane deformation C-H vibration is observed at frequencies close to those of ionic compounds such as CpK or CpNa. It indicates a significant polarity of ring-metal bond [159]. 

The sharp bands of low intensity are characteristic for such ions (the molar coefficients of extinction, e = 1-10) in this range. The molar coefficients of extinction for CgHg -complexes are 10-1(X) times larger than those for the f-f-transitions of aqua-ions, and the absorption bands for them are essentially broader. This difference suggests that the visible spectra of CgHg lanthanoid complexes are determined in the whole not by the f-f transitions but due to a charge transfer from the ligand to the metal [30, 31, 34, 37]. 

One of the characteristic for REM compounds properties is their ability to saturate the coordinating sphere of metal atom via the formation of bridging bonds. The alkoxides also display this feature to form the associates [Ln(OR)3]x. When an alkoxide Ln(OR)3 is added to an alkoxide of another group metal the bimetallic alkoxide with bridging RO ligands is formed (Table VIII.5). The alumoalkoxides Ln[Al(OC3H7-i)4]3 obtained for Sc, Y, La and most of lanthanoids are examples of such compounds [53, 54, 109, 111] ... 

Thus, one can conclude that most of the known up to the present carbonylmetalates of lanthanoids do not contain a direct Ln-M bond and have the ionic nature. The distinctive feature of the compounds appears to be the existence of isocarbonyl linkages Ln-O-C-M. At the same time the compounds with short distances (characteristic of a covalent bond) between the Ln and M atoms evidently also can be synthesized in some cases. The problem in the whole needs an additional theoretical study. 

An attempt has been made to find the dependence of v(CO) on the coordination number of lanthanoid atom [3]. It appeared that these characteristics are related by a linear dependence, but for every CN of the Ln atom there are several v(CO) bands in the infrared spectrum. A low quality of the spectra and their complexity did not give the possibility to determine the molecular structure of REM carbonyls and the exact CN of Ln atoms. 

The UV-visible absorption spectra of these compounds exhibit two characteristic bands in the region of 460 and 600-640 nm [12]. The complexes with substituted phthalocyanine ligands have the same spectrum [18]. The maximum of a long-wave band shifts to the blue region along the lanthanoid row (Table XII.2). 

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