Electronic spectra of lanthanides

The electronic spectra of lanthanide compounds resemble those of the free ions, in contrast to the norm in transition metal chemistry the crystal-field splittings can be treated as a perturbation on the unsplit levels. Complexes thus have much the same colour as the... 

In contrast to the situation with the 3d transition metals in particular, the 4f-4f transitions in the electronic spectra of lanthanide complexes rarely serve any diagnostic purpose. It may be noted, however, that the spectra of the octahedral [LnXe] " ions (X = Cl, Br) have particularly small extinction coefficients, an order of magnitude lower than the corresponding aqua ions, due to the high symmetry of the environment. 

Question 5.3 Explain why f-f transitions in the electronic spectra of lanthanide complexes are weaker than d-d transitions in the corresponding spectra of transition metal complexes. 

Electronic Spectra of Lanthanide Compounds in the Vapor Phase... 

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. 

Z10 Electronic Spectra of Lanthanide Complexes 39.Z11 Lanthanides in the Dipositive Oxidation State 39.Z11.1 Hydrated species 39. Z 11.2 Other solvated species 39. Z 11.3 Complexes with nitrogen donors... 

The calculation of electronic spectra of lanthanide or actinide compounds poses a considerable challenge to relativistic electronic structure methods. Apart from the mandatory relativistic Hamiltonians, the small energetic separation of valence / and d orbitals in lanthanide and actinide atoms gives rise to a prominent multi-reference nature of the N-electron wave function. Even the most accurate methods of calculating multiple spin-orbit-coupled states such as MRCl+SO and CASPT2+SO suffer from the limited number of configura-... 

Experimental data for the electronic spectra of lanthanides and actinides are available and may serve to parametrize semiempirical approaches or to calibrate ab initio calculations. Total energies, orbital energies, radial orbital expectation values, and maxima from relativistic Dirac-Hartree-Fock as well as nonrelativistic Hartree-Fock calculations have been summarized by Desclaux and form a useful starting point for (qualitative) discussions of the electronic structure of lanthanide and actinide compounds. 

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

There are difficulties associated with the use of ordinary electronic absorption spectra of lanthanide complexes in solution to provide detailed information regarding coordination number and geometry. However, difference spectra versus NdCl3 are reported for Nd3+-ligand (L) solutions for the 4/9/2— -4G5/2, 4G7,2 transitions (L = dipicolinate, oxydiacetate, iminodiacet-ate, malate, methyliminodiacetate and Ar,Ar -ethylenebis Af-(o-hydroxyphenyl)glycinate ). Hypersensitive behaviour was examined and transition dipole strengths were discussed in terms of the nature of the complex species present.431... 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

The influence of the ligand field on the electronic states of lanthanides is small and generally of the order of 200 cm-1. Because the ligand field perturbation of J states are minimal, the f-f electronic transitions are sharp. In addition to f-f transitions, both 4f —> 5d and charge transfer transitions are also observed in the spectra of lanthanides [92]. Lanthanide ions exhibit emission in the solid state, and in some cases in aqueous solutions. Energy transfer from the ligand or intermolecularly from an excited state can give rise to the emission from lanthanide ions. 

At about the same time, Dieke developed and published optical spectra of lanthanides in 1956. After 1960 the raw data were converted into crystal field parameters by the methods developed by Stevens, Elliot and Judd. This was followed by a large body of data on crystal field parameters derived from optical spectra of lanthanides from all parts of the world. The crystal field Hamiltonian as given in equation (8.9) for an f electron is... 

At present there is consensus on the fact that the observed nephelauxetic effect in the spectra of lanthanide compounds is analogous to the phenomenon observed in the spectra of d-transition metal complexes. The nephelauxetic effect cannot be quantitatively interpreted by excluding the covalent interaction of lanthanide ions with surrounding ligands [34]. Jorgensen has proposed [38] two possible mechanisms of interaction for the observed nephelauxetic effect, namely (i) direct participation of lanthanide 4f orbitals in the formation of molecular orbitals also known as symmetry restricted covalency , (ii) transfer of some part of the ligand electron density to the unfilled 6s and 6p orbitals of the lanthanide also known as central field covalency . 

Certain transitions in the electronic spectra of some tripositive lanthanide ions (notably Nd, Ho, Er) exhibit hypersensitivity to the enviromnent, which can sometimes be used to indicate geometry, as in the case of the neodymium(lll) aqua ion shown in Figure 6. 

In lanthanide elements, the 5s and 5p shells are on the outside of the 4f shell. The 5s and 5p electrons are shielded, any force field (the crystal field or coordinating field in crystals or complexes) of the surrounding elements in complexes have little effect on the electrons in the 4f shell of the lanthanide elements. Therefore, the absorption spectra of lanthanide compounds are line-like spectra similar to those of free ions. This is different from the absorption spectra of d-block compounds. In d-block compounds, spectra originate from 3d 3d transitions. The nd shell is on the outside of the atoms so no shielding effect exists. Therefore, the 3d electrons are easily affected by crystal or coordinating fields. As a result, d-block elements show different absorption spectra in different compounds. Because of a shift in the spectrum line in the d-block, absorption spectra change from line spectra in free ions to band spectra in compounds. 

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