Spectra of divalent lanthanides

Spectra of divalent lanthanides Free-ion work since 1965... 

Divalent lanthanide halide complexes studied by absorption spectroscopy involve the Sm(II)-Al-Cl and Eu(II)-Al-Cl vapor complexes. The data are interpreted by the presence of EuAljCln and EuAUCIh vapor molecules (Sorlie and 0ye 1978) while the stoichiometries of the SmCla-AlC vapor complexes are not known (Papatheodorou and Kucera 1979). Spectra of divalent lanthanides are expected to exhibit weak Laporte-forbidden 4f <— 4f transitions and relatively strong bands arising from 5d 4f, 6s <— 4f,. .. transitions. Such transitions are observed in the near-UV region in spectra of both Sm(II)-Al-Cl and Eu(II)-Al-Cl vapor complexes (figs. 12 and 13). 

After this discussion of c.t. transitions on Ln i+ ions we now turn to the divalent lanthanides ions. Here the first allowed transitions in the spectra are 4f- -5d transitions as expected. They have been studied in detail. We will here mention some relevant results. 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

The surfaee-to-bulk emission intensity ratios in the photoemission spectra of compounds with divalent or so ealled mixed-valent lanthanide elements (see section 7) were found to be drastically higher than those with trivalent ones. For Au alloys with divalent Yb or Eu constituents this effect was interpreted in terms of possible surface segregation of the lanthanide elements (Johansson et al. 1982). For the lanthanide dialuminides an alternative explanation, also applicable to the Au alloys, has been proposed (Laubschat et al. 1986). The scattering probability of photoexcited bulk 4f electrons increases with decreasing binding energy leading to a reduction of their mean free path. 

At the present time the electron-spectroscopic manifestations of 4f-hybridization in heavy lanthanide materials are scarce. (The so-called valence fluctuating compounds containing the elements Sm, Eu, Tm or Yb are discarded here (see section 7.3).) Nevertheless, indications for a coupling of the f states with band states have been found in materials where the Eu atoms, according to Mossbauer spectroscopy and the magnetic susceptibility, are in the divalent 4f configuration. The XPS 3d core level spectra of a few systems, like Eu metal, EuAlj, EuCuj, EuAgj (Schneider... 

Here / and are the relative fractions of the normalized absorption jumps in the 4f" and 4f" reference spectra and m is the number of outer valence electrons. This procedure was applied e.g. by Launois et al. (1980), Martin et al. (1980), Ravot et al. (1981) to extract the valence from the L, spectra of mixed valent lanthanide chalcogenides. Figure 14 reproduces the spectra of mixed valent TmSe together with its (isostructural) references TmS (nominally 3" ) and TmTe (nominally 2" ). Divalent Tm in TmTe exhibits a smaller and narrower line than trivalent Tm in... 

Since the available spectroscopic results for divalent actinides are fragmentary, we adopt a consistent interpretation which accounts for all observations and predicts the energies of other bands that might be accessible to observation. The basic aspects of the required tentative model can be deduced in part from available data for divalent lanthanide spectra. 

This review covers all organometallic complexes of Sc, Y and the lanthanides reported in the year 2000 and their reactions. Endohedral fullerene complexes of the lanthanides have, as usual, been excluded. Highlights this year include striking reports of lanthanides in non-classical oxidation states (Sections 3.2 and 5), a remarkable reversible dinitrogen activation described in Section 3.9.2 and evidence for the existence of the divalent hydrides LnH2(THF)2 (Ln = Sm, Yb) (Section 3.10). In addition Evans has assessed the utility of electrospray mass spectrometry for the characterization of organolathanides. The results are promising and the spectra and dissociation patterns show sensitivity to the metal and its oxidation state. ... 

Since the lanthanides that exhibit the divalent state in aqueous solution, Sm, Eu ", and Yb, are all readily oxidized to the trivalent state, attempts to record their absorption spectra have usually proceeded from the rapid dissolution of a soluble anhydrous compound. The absorption spectra shown in fig. 24.16 for Sm ", Eu, and Yb were adapted from results published by Butement and Terrey (1937), Butement (1948), and Christensen et al. (1973). Production of other divalent lanthanide ions by pulse radiolysis, and the observation of their spectra is discussed in section 5.2. 

Charge transfer bands result whenever an easily oxidized ligand is bound to a trivalent lanthanide ion which can be reduced to the divalent state or when the ligand is bound to one of the tetravalent ions (J0rgensen, 1970). Such transitions are commonly observed in the spectra of complexes of samarium(III), euro-pium(III), thulium(III), ytterbium(III), and cerium(IV). The position of these bands in the spectrum is markedly dependent on the ligand and the metal ion. For example, in the ions RCU the charge transfer bands for europium(III),... 

The full-rotational group compatibility tables show how a free-ion J level is broken up into crystal-field levels when the ion is placed in a crystalline environment with a distinct point symmetry. The irreducible representations (irreps) are labelled according to the notations of Koster et al. (1963). The tables are given up to J = 8 for even-electron systems and up to J = 17/2 for odd-electron systems. The double groups are marked by an asterisk. Although higher J values may occur for divalent lanthanide ions, they are of less importance for the study of the energy levels in the ultraviolet, visible and near-infrared parts of the spectra. 

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