Crystal field theory, stabilization

Thus, the ligand-stabilized d-orbital splitting pattern is qualitatively consistent with the expectation of crystal-field theory, but the physical origin of this splitting should be attributed to attractive donor-acceptor interactions such as (4.86b) rather than to any inherent electrostatic repulsions toward the incoming ligands. More accurate treatment of the spectroscopic 10Dq value should, of course, be based on separate consideration of the two spectroscopic states. 

The above discussion has considered the stabilization of complexes in terms of the crystal field theory. It is desirable to consider the same topic in terms of modern molecular orbital theory. Although the development and sophisticated consideration of the MO treatment is far beyond the scope of this chapter, an abbreviated, qualitative picture will be presented, focusing again on the energy levels of the highest occupied and lowest empty orbitals and again using the square planar d case. 

Crystal Field Theory (CFT) has also been used considerably to rationalize visible absorption spectra, hydration energies, stabilities of complexes, rates and mechanism of reaction, and redox potentials of transition element ions. These applications of CFT are summarized in a book by Basolo and Pearson 1B6). 

There are several approaches for obtaining spectral data for low-abundance transition metal ions, rare minerals and crystals of small dimensions. Data for a transition element in its chemical compounds, such as hydrates, aqueous solutions, molten salts or simple oxides, may be extrapolated to minerals containing the cation. Such data for synthetic transition metal-doped corundum and periclase phases used to describe principles of crystal field theory in chapter 2, appear in table 2.5, for example. There is a growing body of visible to near-infrared spectral data for transition metal-bearing minerals, however, and much of this information is reviewed in this chapter and the following one. These results form the data-base from which crystal field stabilization energies (CFSE s) of most of the transition metal ions in common oxide and silicate minerals may be estimated. 

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. 

The crystal chemistry of many transition metal compounds, including several minerals, display unusual periodic features which can be elegantly explained by crystal field theory. These features relate to the sizes of cations, distortions of coordination sites and distributions of transition elements within the crystal structures. This chapter discusses interatomic distances in transition metal-bearing minerals, origins and consequences of distortions of cation coordination sites, and factors influencing site occupancies and cation ordering of transition metals in oxide and silicate structures, which include crystal field stabilization energies... 

One of the most successful applications of crystal field theory to transition metal chemistry, and the one that heralded the re-discovery of the theory by Orgel in 1952, has been the rationalization of observed thermodynamic properties of transition metal ions. Examples include explanations of trends in heats of hydration and lattice energies of transition metal compounds. These and other thermodynamic properties which are influenced by crystal field stabilization energies, including ideal solid-solution behaviour and distribution coefficients of transition metals between coexisting phases, are described in this chapter. 

Thus crystal field theory can in principle explain all the trends of variation of stability constants shown in Fig. 3.15. 

A mass-spectral study performed on Sc(acac)3, Sc(dpm)3 and La(dpm)3 vapors shows that, due to volatility and thermal stability, this group of S-drketonates is suitable for low-temperature gas-phase transport of metals. The similarity of the mass spectra of Sc, Y and La -diketonates may point to a similarity of the processes involved in the thermal destruction of the molecular species present in vapor. Within the framework of crystal field theory, an unpaired electron of a central metal ion (Sc +, Y + or La + with the nrf configuration) occupies a orbital, which has the lowest energy in D2h symmetry. This results in stabilization of the ML2 radical and enhances the thermal stability of the complex. 

Covalent radius 34 Crystal field stabilization energy 100, 109 Crystal field theory 11, 98,100,112... 

Among the early successes of crystal field theory was its ability to account for magnetic and spectral properties of complexes. In addition, it provided a basis for understanding and predicting a number of their structural and thermodynamic properties. Several such properties are described in this section from the crystal field point of view. Certainly other bonding models, such as molecular orbital theory, can also be used to interpret these observations. Even when they are, however, concepts from crystal field theory, such as crystal (or ligand) field stabilization energy, are often invoked within the discussion. 

We have assumed implicitly that Cr3+ ions will not occur in tetrahedral holes in an oxide lattice. Cr + is a species. In the crystal field theory, such ions lead to particularly good stabilization of octahedral coordination. For example, the coordinate bond energy of Cr(H20) + is 120 kcal, of which 9 represent crystal field stabilization energy (CFSE) (35). In fact, there are no known chromias with crystal struc-... 

Formation constants for many metal complexes have been compiled by Ramunas Motekaitis and Art Martell, and these as well as techniques for measuring them in the laboratory will be covered in Chapters 3 and 8. One can, however, predict the relative stability of a desired complex based on simple bonding theories. Crystal field theory, as well as the Irving-Williams series and Pearson s hard-soft-acid-base theory (see the next section) enable us to predict what might happen in solution. 

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