Symmetry-based ligand field parameters

Furthermore, based on earlier calculations (39) for the type 1 copper protein plastocyanin, ligand-field parameters for the blue copper in laccase have been derived. These reports (37,38) also include a structural representation of the type 1 center composed of a flattened tetrahedron (D2d symmetry) with two imidazole side-chains, a cysteine sulfur, and a fourth ligand (which probably is methionine sulfur), bound to the metal ion. Although no such low-temperature experiments have been performed with ascorbate oxidase, one might anticipate similar structural features for the blue type 1 centers. 

Many spectroscope and magnetic studies have been concerned with empirical correlations between these parameters and features of structural and chemical interest in the molecules. It should be noticed, however, that these symmetry-based parameters are global (like /HDvv which is discussed earlier), referring to the field of all ligands as a whole. [The same is true of recent more comprehensive symmetry-defined parameters proposed by Donini et al. (17).] Being based on the minimum assumptions of ligand field theory, and hence, for some, preferred as more basic, these parameters lack possibilities for immediate chemical relevance and appeal. 

Fig. II Symmetry-based parameterization (nonadditive ligand field model, LFModel 1) for tetragonally distorted CuCl42 (d9) and NiC142 (dH), the parameters should be taken with a sign minus when going to orbitals (d1) term notations pertain to CuCl42-(d9)...

The optical properties of transition-metal (TM) ions in crystals have generally analyzed by the ligand-field theory developed by Sugano et al. In the ligand-field theory, the multiplet energies of the TM ions in the octahedral (or tetrahedral) symmetry are expressed in terms of the Racah parameters B and C) and the crystal-field parameter (A). However, in the analysis based on this theory, the parameters axe determined from the experimental optical spectra under a certain trial assignment of the observed peaks. Therefore, correct parameters cannot be obtained unless the optical spectrum of the material is available and well understood. Even if the correct parameters are... 

---