Gaseous lanthanide compounds

As part of a continuing study of the electronic spectra of lanthanide compounds in the vapor phase (11), we report here the spectra of the gaseous tribromides and triodides of Pr, Nd, Er, and Tm and of the gaseous 2,2,6,6-tetramethyl-3,5-heptanedionates of Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm (9). Spectra of gaseous lanthanide compounds are virtually unexplored, in contrast to crystal and solution spectra, and can be expected to contribute new information concerning energy levels and intensities of / / transitions. 

Table IX. Oscillator Strengths, P, of Hypersensitive Transitions in Gaseous Lanthanide Compounds...

The thermodynamic functions (and other properties needed for their calculations) of gaseous lanthanide trichlorides have been extensively studied for several decades. In our view, the primary data on molecular constants are adequate for the required revision and refinement of the reduced Gibbs energy for the compounds imder consideration. Moreover, this class of compounds perfectly demonstrates how the contribution from electronic excitation to reduced Gibbs energy can be considered. Therefore, we start by describing the properties of RQ3 (R = La-Lu) molecules. 

Nevertheless, there are a quite specific set of properties of lanthanide compounds closely related to the behaviour of gaseous atoms and positive ions. For instance, a system containing q electrons can show the values of... 

Gaseous 2,2,6,6-Tetramethyl-3,5-Heptanedionates. With Eisentraut and Sievers s (9) discovery of a group of volatile lanthanide chelates, the lanthanide 2,2,6,6-tetramethyl-3,5-heptanedionates abbreviated M(thd)3, an interesting group of compounds for vapor phase spectral investigation became available. 

Hydrides are broadly of three types, saline, covalent and metallic. Saline hydrides are formed by the alkali metals (Gp. lA), the alkaline earths (Gp. IIA) and the lanthanides they have ionic lattices, high melting points and, when fused, are electrolytes. Elements of the B Groups from IIIB to VIIB have covalent hydrides, most of them gaseous at room temperature. The metallic hydrides characteristic of some of the transition elements are in effect alloys and usually lack the stoichiometric composition of normal chemical compounds. 

It is important to realize that the electronic structures listed in table 6 are those of the neutral (un-ionized) gaseous atoms, whereas it is the electronic structure of the ions and compounds that we are chiefly concerned with in chemistry. The relationship of the electronic structure of the gaseous atom of an element to that of its compounds can be rather complicated. For example, in the case of the actinide and lanthanide elements, one would not necessarily predict the predominance of the III oxidation state from the electronic structures of the gaseous atoms there are usually only two so-called valence electrons , the 7s or 6s electrons, which might indicate a preference for the II oxidation state. 

As electron diffraction provides direct measurement of interatomie distanees, it is an ideal method for the determination of the molecular stmcmres of gases. Moreover, moleeules in the gas phase are free from the intermolecular interactions and the influence of fields that ean distort a structure (particularly the conformation) in the crystalline state, or even change it completely. We will see some examples of this in the case histories presented in Chapter 12. But there are, of course, limits to the usefiilness of electron diffraction, of which the most obvious is that gaseous samples are needed. The essential requirement is that the compound to be smdied should have a vapor pressure of about 1 mbar at a temperature at which it is stable. Lower vapor pressures can be used, but the experiments are more difficult to perform. As long as no decomposition occurs, the temperature does not really matter, and such involatile species as alkali metal halides or lanthanide halides and some metal oxides have been studied at temperatures of up to 2000 K or more. However, we should remember that raising the sample temperamre increases amplitudes of vibration, and can change the relative populations of isomers or conformers. 

Lewis acid catalysis is one of the most useful methods in modern organic synthesis. However, many of the common Lewis acids are highly water-labile and have been used in organic synthesis under strictly anhydrous conditions. Contrary to this, it was found that lanthanide triflates catalyzed aldol reactions of formaldehyde (Scheme 3.6). Formaldehyde is one of the most highly reactive Cl electrophiles. In this reaction, not gaseous formaldehyde but a commercially available aqueous solution was used as the formaldehyde source. This invaluable find introduced the concept of Lewis acid catalysis in aqueous media to many chemists. Later, it was also reported that other aldol reactions, with a variety of aldehydes and silyl enol ethers, as well as allylation reactions, proceeded smoothly in aqueous media to afford the desired compounds in high yields. ... 

One of the features that sets actinide and lanthanide spectra apart from those of other elements in the periodic table is that the f orbitals can be considered both as containing the optically active electrons and as belonging to the core of filled shells. As a result of this dominant characteristic, the spectra of these elements, particularly of the lower valence states, are moderately insensitive to changes in the ionic environment. This relative insensitivity of core electrons to external circumstances also means that for these elements there is a close connection between energy levels in compounds and those in gaseous free atoms and ions. 

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