Static Ligand Field

In this section, we will be concerned with the low-symmetry components of the ligand field (LF) in hexacoordinated complexes. Cases of tetragonal, trigonal and orthorhombic symmetry breaking will be developed, but first attention will be devoted to the general selection rules governing these interactions. 

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Though we find AEq(T) curves corresponding to E(xAi) —E(hBz) rs 100 cm-1, which fit the experimental A Eq(T) data, we present arguments why an energy separation of E(F4X) E(5B2) >300 cm-1 seems to be more realistic. In addition to the static molecular structure of Fig. 23 one has to consider lattice vibrations which have the effect that the potential V of the iron contains higher terms than the static ligand field potential F<°> (702) ... 

Two states /a and /b that are eigenfunctions of a Hamiltonian Hq in the absence of some external perturbation (e.g., electromagnetic field or static electric field or potential due to surrounding ligands) can be "coupled" by the perturbation V only if the symmetries of V and of the two wavefunctions obey a so-called selection rule. In particular, only if the coupling integral (see Appendix D which deals with time independent perturbation theory)... 

The intensities of polarized ligand field spectra ns-n ) of V(ethyl-dtp)3 doped into the corresponding indium(III) compoimd exhibit a relatively small temperature dependence. The source of the large intensities of the d-d transition is the static distortion of the ligand field n ) and not vibronic effects to any appreciable extent. 

The static Jahn—Teller distortions appear as structural changes which may be associated with the origin of low symmetry terms in the ligand field potential. Here, the separation of the orbital... 

Finally, it has to be recalled that internal strains or crystal packing forces have to be present, in order to stabilize one of several equivalent minima so that the system is seen as statically distorted. It has been stated that even small low-symmetry perturbations can be amplified by vibronic coupling A mechanism of this kind may be more effective in lifting electronic degeneracies than low-symmetry ligand field components. The important point is whether JT contributions are greater or comparable with the low-symmetry ones if it is so, JT terms must be included in to the Hamiltonian and have to be considered before or together with the static low-symmetry crystal field. 

Fig. 3 a, b. Static or dynamic JT effect as a function of temperature or spectroscopic technique. ESR (a) and ligand field (b) spectra of elpasolite-type mixed crystals Ba2Zni, Cu, WO at different temperatures (adapted from Ref. 42)... 

Inspecting Table 2 there is no doubt that electron-electron interactions and high-symmetry contribution of the ligand field have to be taken into account first. The problem arises when one wants to introduce vibronic interactions, which are often greater than or comparable with spin-orbit coupling or low-symmetry static fields. In such cases JT effect cannot be neglected, but has to be treated simultaneously with the other interactions. 

The relatively simple strucmre of Cr(acac)3 was used as a model for electronic structure evaluation. This molecule has a high degree of photostability and opportunity for excitation to both ligand-field and charge-transfer states. The lowest-energy state corresponds to E. The E A2 transition is responsible for the static emission line at 794 nm. The... 

Superhyperfine interactions are rarely observed in the ESR spectrum of the VO + ion because the unpaired electron interacts only weakly with the ligand nuclei, so that often the size of the coupling is less than the ESR bandwidth. This problem has been surmounted through the use of ENDOR spectroscopy. In ENDOR spectroscopy, molecules with their V=0 axes either parallel or perpendicular to the direction of the static magnetic field are selectively irradiated. In this way, the anisotropic superhyperfine coupling constants of and N and the N quadrupolar coupling constants can be obtained. 

Cobalt(ni).—The optical activity associated with d-d transitions in dissymmetric cobalt(iii) complexes has been examined in terms of a model which incorporates both ligand field theory and the one-electron static-coupling theory of optical activity. Expressions have been obtained which relate the sign of the net or total d-d rotatory strength to specific stereochemical features in the ligand environment. Sector rules based on these expressions have been proposed and tested. A model for predicting the photo-reactions and quantum yields of cobalt(iii) complexes has been devised. ... 

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