Lanthanoids elements

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. 

On the basis of the preceding section, it would appear that there should be almost no more spectroscopy for transition metal complexes than for non-transition metal complexes. All the transitions within the d or / shells are forbidden, and it should be only transitions from these to excited states of the same multiplicity involving other orbital sets that should contribute anything new. Such transitions would be few in number and lie at high energies. In fact, of course, quite the converse is known to be the case. The spectra of transition and lanthanoid element compounds are rich and lie at low energies. 

The intensities of the /<-/ transitions of the tripositive lanthanoid element spectral bands are a good deal lower than those of the transition metals, presumably because the coupling between the electronic and vibrational wave functions is smaller on account of the shielding from the valence electrons. Value of/in the order of KT7 are observed.47-49 109 However, certain of the elements in the divalent state show d -f transitions which, as expected, are much more intense and blanket the /<-/bands.47-49... 

For the lanthanoid elements the spectra often consist of many bands corresponding to the transitions between the states of the free-ion terms, each state perturbed by small ligand field effects to give several components. The bands are quite sharp, perhaps only a few cm-1 wide. The sharpness arises because of the small interaction of the /-electrons with the valence electrons. 

Equations (68) and (69) apply quite well to the majority of the lanthanoid element compounds, where the approximation to an isolated ground state is good. Table 7 exemplifies this. 

Table 7 The Magnetic Properties of Some Lanthanoid Element Compounds...

For the lanthanoid elements, ligand field splittings are so small that quenching of orbital angular momentum is not important. This probability also applies in the actinoid elements. 

Nakamura, S., Ohashi, S., and Akiba, K., Effects of complexing agents on transport of lanthanoid elements across versatic acid liquid membrane. Sep. Sci. Tech., 1992, 27 863-873. 

Lutecium is the heaviest, rarest, and most expensive lanthanoid element. The lanthanoids elements make up Row 6 of the periodic table between barium and hafnium. The periodic table is a chart that shows how chemical elements are related to one another. The lanthanoids are usually shown as a separate row at the bottom of the table. They are also called the rare earth elements. That name does not fit very well for most lanthanoids. They are not really so rare, but were once difficult to separate from each other. However, lutetium is both rare and difficult to separate from the other lanthanoids. 

Molybdenum never occurs free in nature. Instead, it is always part of a compound. In addition to molybdenite, it occurs commonly as the mineral wulfenite (PbMo04). Its abundance in Earth s crust is estimated to be about 1 to 1.5 parts per million. That makes it about as common as tungsten and many of the rare earth (lanthanoid) elements. In 2008, the largest producers of molybdenum in the world included the United States, China, Chile, Peru, and Canada. In the United States, molybdenum ores were found primarily in Colorado, Idaho, Nevada, and New Mexico. According to the U.S. Geological Survey (USGS), the value of the molybdenum from U.S. mines was 4.5 billion that year. 

Thulium is probably the rarest of the lanthanoid elements. Its abundance is estimated at about 0.2 to 1 part per million in Earth s crust. This still makes it more abundant than silver, platinum, mercury, and gold. 

Ytterbium tends to be more reactive than other lanthanoid elements. It is usually stored in sealed containers to keep it from reacting with oxygen in the air. It also reacts slowly with water and more rapidly with acids and liquid ammonia. 

The valence shell of a lanthanoid element contains 4/ orbitals and that of an actinoid, 5/ atomic orbitals. The ground state electronic conhgurations of the /-block... 

In order that the Periodic Table should fit on a single page, the lanthanoids (elements 57-70) and actinoids (elements 89-102) series are usually written below the table. We will not concern ourselves with the chemistry of these elements in this book. 

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]. 

From the chemical point of view, the lanthanoid elements are characterized by a regular variation of their 4f electron configuration throughout the series. Table 2-1. Due to the nature of the orbital group, (n-2) involved in the variation of their electron configuration, these elements are often referred to as the first inner transition series. Inherent to this peculiar electron configuration, the lanthanoid elements show a number of atomic properties that are considered to determine the chemical and structural properties of their compounds, and, particularly, those of their oxides. 

TABLE 2-1. Some relevant properties of the lanthanoid elements... 

As deduced from Table 2-1, the lanthanoid elements show relatively low standard atomization enthalpies and ionization potentials. These properties make them highly active reducing metals, with Allred-Rochow electronegativities ranging fi om 1.01 (Eu) to 1.14 (Lu), similar to that reported for Ca (1.04) [20],... 

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. 

Regarding the hydroxycarbonate phases, there are two well characterized stmctural varieties which are known for most of the lanthanoid elements [80] the ancylite-like, orthorombic, A-Ln(OH)COs, Fig. 2-5, the stmcture of which is described in detail in [81,82] and the hexagonal one, B-Ln(OH)C03, Fig. 2-6, isostmctural of the bastnaesite mineral (LnFCOs), the characterization of which is reported in [83,84]. 

Ln602(0H)g(C03)3, and Ln4(0H)6(C03)3, have been identified for the heavier lanthanoid elements Tm, Yb, and Lu. X-ray powder diffraction data for the five phases mentioned above are reported in [85]. 

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 25.1. A 4/ atomic orbital has no radial node, whereas a 5/ atomic orbital has one radial node (see Section 1.6). A crucial difference between the 4/ and 5/ orbitals is the fact that the 4/ atomic orbitals are... 

Separation of selected lanthanoid elements was performed by HSCCC equipped with a small coiled column of 10 ml capacity. A hexane solution of DEHPA was retained in the mbing column as the stationary phase. The stationary phase retention increased with increasing rotational speed and with decreasing flow rate of the aqueous mobile phase. Mixtures of lanthanoids (Gd, Tb, and Dy) were eluted in increasing order of extractability by introducing the buffered mobile phase. Lanthanoids (Sm through Er) and yttrium were simultaneously enriched in the stationary phase from a large volume of a sample solution, and then separated from each other with reasonable resolution. 

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