Spectral properties of transition-metal

One of the most important applications of correlation diagrams concerns the interpretation of the spectral properties of transition-metal complexes. The visible and near ultra-violet spectra of transition-metal completes can generally be assigned to transitions from the ground state to the excited states of the metal ion (mainly d-d transitions). There are two selection rules for these transitions the spin selection rule and the Laporte rule. 

Although the physical basis of the crystal field model is seen to be unsound, the fact remains that, in summarizing the importance of the symmetry of the ligand environment, it qualitatively reproduces many of the features of the magnetic and spectral properties of transition metal complexes. This early qualitative success established its nomenclature in the fields of these properties. While we shall have little more to say about crystal field theory as such, much of the rest of this article will be couched in the language of the crystal field model, and for that reason some little trouble has been taken to outline its development. 

In recent years, knowledge of electronic-spectral properties of transition metal ion complexes has increased rapidly. The chiroptical properties of amino acids and peptide complexes have been used to determine the absolute configuration of these compounds. This chapter is not an exhaustive review, and the interested reader is referred to the literature references for further detail. 

The known spectral properties of transition metal complexes in which the metal ion has the 3d configuration are quite limited. Several recent books (1,2) emphasize this fact by their rather meager presentation of data. 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

Although little use is made now of the theory presented in this chapter, it contains the basis of all of those that are used. It provides the foundation, particularly for the understanding of spectral and magnetic properties all else is elaboration and refinement. A knowledge of simple crystal field theory is therefore essential to an understanding of the key properties of transition metal complexes and particularly those covered in Chapters 8 and 9. This chapter deals exclusively with transition metal complexes. In one or more of their valence states, the ions of transition metals have their d orbitals incompletely filled with electrons. As a result, their complexes have characteristics not shared by complexes of the main group elements. It is the details of the description of these incompletely filled shells which is our present concern this is in contrast to the discussion of the previous chapter where the topic was scarcely addressed. Ions of the lanthanides and actinides elements have incompletely filled f orbitals and so necessitate a separate discussion which will be given in Chapter 11. 

The spectral properties of this excited state species are of great interest since they represent the effects of the (d 2) (d 2 y2), d 2 metal centered transition upon... 

Fan, J.,Whitehold, J.A., Olenyuk, B., Levin, M.D., Stang, P.J. and Fleischer, E.B. (1999) Self assembly of porphyrin arrays via coordination to transition metal bisphosphine complexes and the unique spectral properties of the product metallacyclic ensembles. J. Am. Chem. Soc., 121 (12), 2741-2752. 

Quinoxaline-2,3-dithione, as reported earlier,1 is useful for its coordinating properties with transition metals. The metal complexes of the dithione with Cu, Ni, Zn, Pd, and Pt have been prepared,171 and the spectral properties of the Ni and Pd complexes examined.171,172 UV data indicate that quinoxaline-2,3-dithione (153) is present as such, rather than as 2,3-dimercaptoquinoxaline the highly colored nature of its complexes is attributed to charge transfer.171... 

Perhaps a more fundamental application of crystal field spectral measurements, and the one that heralded the re-discovery of crystal field theory by Orgel in 1952, is the evaluation of thermodynamic data for transition metal ions in minerals. Energy separations between the 3d orbital energy levels may be deduced from the positions of crystal field bands in an optical spectrum, malting it potentially possible to estimate relative crystal field stabilization energies (CFSE s) of the cations in each coordination site of a mineral structure. These data, once obtained, form the basis for discussions of thermodynamic properties of minerals and interpretations of transition metal geochemistry described in later chapters. 

Considerable interest centres on the Mantle constituting, as it does, more than half of the Earth by volume and by weight. Attention has been focussed on several problems, including the chemical composition, mineralogy, phase transitions and element partitioning in the Mantle, and the geophysical properties of seismicity, heat transfer by radiation, electrical conductivity and magnetism in the Earth. Many of these properties of the Earth s interior are influenced by the electronic structures of transition metal ions in Mantle minerals at elevated temperatures and pressures. Such effects are amenable to interpretation by crystal field theory based on optical spectral data for minerals measured at elevated temperatures and pressures. 

In preparing Table I two criteria were used. First, the compropor-tionation constant for symmetric mixed-valence complexes should be large. It is important to recognize that for complexes in which the metal-metal distance is small significant contributions to Kc arise from factors other than those associated with metal-metal coupling. Second, the breakdown of Hush theory (4) for the spectral properties of MMCT transitions should be apparent. In practice this means that the pre-... 

Limited information is available on the uv-visible spectral properties of BjO, and related materials, both as pure materials and with transition-metal impurities. The lowest-energy fundamental absorption in B2O3 is at about 8.6 eV (Izumitani and Hirota, 1985), close to the value of 8.4 eV calculated by Tossell for B(OH)j (Tossell, 1986). To assess the environment encountered by metal ion impurities, crystal-field (d-d) transitions, uv absorption spectral transitions (e.g., 6s-)-6p in Pb ), or ESR spectra of odd electron species can be studied. For example, Lin and Angell... 

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