Ligand field theory splitting

Libby, W., 713 ligand, 671, 672 ligand field splitting, 682 ligand-field theory, 688 ligand-to-metal transition, 686 light, 4, 6... 

At an early stage in the development of ligand field theory, it was found that A the splitting parameter for tetrahedral MX4, should be equal to (4/9) of A0, the splitting parameter for octahedral MX6, and experimental data are in good agreement with this prediction. However, this takes no account of the fact that the M—X distance in tetrahedral MX4 is usually some 8—10% shorter than in octahedral MX6. In the pointcharge crystal field model, A is proportional to R-5, so that if the difference in R between MX4 and MX6 is taken into account, we predict (At/A0) to be 0.6—0.7, compared with the experimental value of about 0.5. An AOM treatment (131) leads to better results, since here we find ... 

The role of electronic structure in Mn and Co site preference and mobility can to some extent be understood through ligand-field theory (LFT). LET qualitatively explains how the degeneracy of the 3d orbitals is broken when a free TM ion is surrounded by coordinating anions. The ligand-field splitting of d orbitals in octahedral and tetrahedral coordination is pictured in Figure 6. ... 

The analogy between the two theories is only formal. Crystal field theory is a purely electrostatic approach that does not take into consideration the formation of MOs and the nature of the bond. According to crystal field theory, optical and magnetic properties are ascribed to crystal field splitting between two AOs, whereas in ligand field theory energy splitting occurs between AOs, and... 

These spectral differences can be understood through ligand field theory.(6) For two coordinate Cu(I) with an approximately linear geometry, the electric dipole allowed Is —> Ap transition is predicted to split Into Is —> Ap and Is —> Ap components. 

The loss of degeneracy of atomic or molecular levels in a molecular entity with a given symmetry by the attachment or removal of ligands to produce reduced symmetries. Ligand field theory treats metal ligand complexation as a consequence of molecular orbital formation, whereas crystal field splitting considers ligands as point... 

The extension to non-octahedral complexes is possible, but must be carried out with great care The orbitals chosen can be expressed as a linear combination of the usual orbitals for an octahedron, and electron-electron repulsions can then be calculated from those for the octahedral case. It is not necessarily adequate for tetrahedral complexes of the first row transition elements, to use ligand field theory in the strong field limit, even for powerful ligands in V(mesityl)4 the ligand field splitting is only 9250 cm ... 

It should be noted that the splitting A0 is generally of the order of 1-3 eV, whereas the elevation of the set of d levels as a whole is of the order of 20-40 eV. Thus it should always be borne in mind that the crystal and ligand field theories focus attention on only one relatively small aspect of the overall energy of formation of a complex. 

The crystal and ligand field theories were developed to deal with only those properties of the complexes that are derived directly from the set of electrons originally occupying the d orbitals of the metal ion. Since these orbitals are the principal parents of the e and t2 MOs of the complex it is not unreasonable to treat the latter as though they were nothing more than split (crystal field theory) or split and somewhat diluted (ligand field theory) metal d orbitals. It is clear, however, that such a view can be only an approximation—indeed, a fairly ruthless one. Yet, with judicious empirical choice of one or more... 

Two main aspects of the HDW approach are important for the present discussion. One aspect is that the model is valid only in the absence of orbital angular momentum contributions to the system, so that its use has been broadly restricted to (say) binuclear complexes of iron(LLl), octahedral nickel(II), or distorted octahedral copper(II). The other aspect concerns the fact that the parameter is a global parameter of the system sequestering many contributions from various ligands and orbitals and, in this respect, may be compared with how Dq in ligand-field theory represents the net effect of electrostatic and covalent (o and rr) effects in the tjg — eg orbital splitting. Attempts to localize our conception of the exchange phenomenon have been made by various... 

The splittings of the d function manifold are conventionally dealt with by the formalism and techniques we call ligand field theory. The representation of the ligand effects by a one-electron potential is the central feature of the theory, conferring upon it great simplicity and hence utility. Recent studies (28) have shown that the approach is based on a much sounder footing than a brief acquaintance with quantum chemistry would indicate. 

According to ligand field theory, the ligand field splitting is the energy separation between nonbonding and antibonding molecular orbitals built principally from d-orbitals. 

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