Diatomics lanthanide

Electronic states of diatomic lanthanide and actinide halides 101... 

In this section, we summarize the results of recent experimental and theoretical studies on diatomic lanthanide and actinide halides. It is pointed out that there exists a large set of spectroscopic data on these species. These are already summarized in Huber and Herzberg (1979). The present section is focused primarily on those diatomic molecules for which extensive studies have been made recently. The reader is referred to the excellent book of Huber and Herzberg for previous studies. A recent paper by Gotkis (1991) provides theoretical insight and systematic comparison of bonding in lanthanide halides. 

There are several recent experimental studies on the CeO diatomic molecule. Schall et al. (1986) have studied CeO using the sub-doppler Zeeman spectroscopy. Again, the ligand-field model is so successful in explaining the observed spectra due to the ionic nature of the diatomic lanthanide oxide. Linton et al. (1979, 1981, I983a,b) as well as Linton and Dulick (1981) have studied the electronic spectrum of CeO using absorption, emission as well as laser spectroscopic method. There are many 0-0 bands for... 

Okada et al. (1984) assignment of the bands in the 37000-51000cmregion to 4f 5d transition. The spin-orbit splitting was computed as 2200 cm for the 4f state. The Mulliken-population analysis revealed a 4f occupation of 1.01 electron. It was noted by Kotzian et al. (1991a, b) that due to the compactness of the 4f shell, the 4f valence electron of Ce does not contribute to metal-ligand bonding. This is reminiscent of the bonding discussed before for the diatomic lanthanide... 

In summary there has been considerable progress in our understanding of the spectroscopy of diatomic lanthanide oxides, fluorides and hydrides. Due to the ionic nature of these compounds, ligand-field models and more recently ab initio effective core potentials have made significant impact in the interpretation of the observed... 

Shovm here are only those metals for which polyatomic molecules have been measured under equilibrium conditions. For most of the transition metals, including the lanthanides and actinides, the concentration of the diatom relative to hat of the atomic species is between 10 and 10"7 (iQ). 

Acetylacetonate and substituted acac derivatives are attractive because of their versatility and stability under normal conditions, as well as their ability to deposit metals cleanly under relatively mild conditions . The dipivaloylmethanato (dpm) derivative from stable and volatile lanthanide compounds, e.g. Lu(dpm)3, have in the gas phase D3 symmetry of the coordination polyhedron. According to Kepert s model, bidentate ligands can be approximated by diatomic molecules and it is completely predictable for the structures of these complexes in the gas phase, but the solid-state structures might be different. 

Recently, it has also been shown that the addition of a CPP to 4f-in-core PPs of lanthanide atoms leads to a significant improvement of the atomic first and second ionization potentials [95]. A comparison of calculations without and with a CPP is given in Fig. 15. The somewhat larger deviations for, e.g., La, Ce, Gd and Lu, arise partly from the neglect of spin-orbit corrections. The 4f-in-core PP-bCPP approach also leads to slight overall improvements of the molecular constants for lanthanide diatomics. The explicit correlation of the 4f shell in small-core PP calculations is quite tedious and does not lead to significantly better results [231,232]. 

Besides the two examples discussed above, the DK approach was applied to various other diatomics of heavy-main group elements, like AuH, AuCl, PbO, Pb2, TIH [14,15,18,19]. The close similarity of ZORA and DK results was also confirmed for hydrides, oxides, and fluorides of the f-elements La, Lu, Ac, and Lr at the scalar relativistic level [143]. Differences in bond lengths, vibrational frequencies, and binding energies amounted to at most 0.7 pm, 6 cm" and 6 kJ/mol, respectively. Except for cases where the quality of the basis set may be questioned, calculated results for the lanthanide species agree well with available experimental data. 

During the completion of the present review we became aware of all-electron frozen-core (R ls -4d °) QR-DFT (quasirelativistic density-functional theory) calculations for selected lanthanide diatomics by Wang and Schwarz (1995). Results obtained with the Amsterdam program (Snijders and Baerends 1978, Snijders et al. 1979), gradient-corrected exchange (Becke 1988) and self-interaction corrected (Stoll et al. 1978, 1980) correlation (Vosko et al. 1980) energy funetionals are summarized in table 17. The agreement with experimental data is quite satisfactory. 

There is a strong affinity of lanthanides and actinides for oxygen.This property can be helpful for the activation of oxygenated organic functions. An indirect measure of the oxophilicity of f-elements is the bond strengths D for the gas phase dissociation of the diatomic species MO (8). For lanthanides Kcal/mole (799Kj/mole) for... 

The electronic structures of molecules containing lanthanide and actinide atoms are extremely interesting due to the complex array of electronic states resulting from open f-shell electronic configurations. There are seven possible real projections for the 4f orbitals which could accommodate up to 14 electrons. As a result, numerous possible electronic states of varied spin multiplicities and spatial symmetries result even for a simple diatomic molecule consisting of a lanthanide or actinide element. A wealth of spectroscopic data has been accumulated up to now on several diatomics containing f-block elements. The spectra are considerably complex and definitive assignments are not always feasible. 

The electronic spectra of rare-earth diatomics such as LaO, CeO, PrO, etc. are notoriously complex as noted by Field and co-workers (see, e.g.. Field 1982). Since an array of molecular electronic states result from incomplete 4f shells, several electronic states have nearly the same properties. Furthermore, spectroscopic bands arising from one set of states can be highly perturbed by another set of states in the near proximity. Moreover, there are several isotopes for lanthanides with large nuclear spins which lead to complex hyperfine structures. 

The lanthanide and relativistic contractions have other important consequences on molecular and chemical properties. Figure 4 compares the equilibrium bond distances of NiH, PdH and PtH. Note that the PtH diatomic has a significantly shorter bond distance compared to PdH which is attributed to the contractions of the outer 6s orbital of the Pt atom arising from both lanthanide contraction and relativity. 

Pyykko and Desclaux (1978) (see also Desclaux and Pyykko 1974,1976) have used the one-center Dirac-Fock method to study diatomic hydrides containing very heavy atoms. In particular, the work of Pyykko (1987) on several lanthanide hydrides is worth citing. We will describe the results of these calculations subsequently. It should be stated that the one-center DF method is applicable only to diatomic hydrides. A critical comparison of the properties computed by this method with the state-of-the-art ab initio method suggests that the properties obtained by the one-center method are not always in good agreement with more accurate procedures. Also, the properties of the excited states have not been computed using this method. 

All the methods described above take into account electron-correlation effects but completely ignore spin-orbit coupling which is known to be very significant for molecules containing actinides and lanthanides. A method to introduce electron correlation and spin-orbit coupling for diatomics based on a SCF procedure was developed in 1982 by Christiansen et al. and was called the spin-orbit Cl (SOCI) method. This should be contrasted with the second-order Cl method which is also abbreviated as SOCI. In the spin-orbit Cl method which we prefer to call the relativistic Cl method, all electronic configurations which can mix in the presence of the spin-orbit operators are included variationally. However, this method was restricted to diatomics and only up to 5000 configurations could be included. 

Field (1982) has combined the well-known molecular-orbital pictures of s and p orbitals with the ligand-field theoretical treatment of the d and f orbitals. This approach has provided very simple and yet promising interpretation for lanthanide oxides. Lanthanide oxides such as CeO, PrO, etc. are highly ionic. The conventional MO theory for the AB diatomic molecule by itself ignores the A B or A B ionic character of such compounds. Although indirectly from MO theory, this can be rationalized as well but the amount of effort is usually more significant using the LCAO-MO approach. 

In contrast to the now relatively extensive metal-metal bonded chemistry of sub valent Mg, Al, and Zn, homomet2Jlic bonds involving the 6s and 7s orbitals of the lanthanide and actinides elements are rare (diatomic overlap between the 5f orbitals in Uj and other cases is discussed later). Their absence is largely a consequence of the rdatively low second and third ionization energies (compared to Al), which reduce the stability of the -i-l and +2 oxidation states. Examples... 

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