Ligands, anisotropic

The Ligand Combination Approach to Creating Anisotropic Frameworks... 

In the structures of strongly anisotropic groups, as in the compounds ( 113)381—0— P(S)F2 (55) and (CH3)3SnOPSF2 42), the 0-atom is the favoured coordination ligand, whereas in the tin compound the sulfur atom coordinates too. Obviously sulfur coordination if favoured to F-coordination. 

In contrast to the EFG analyzed before, aU of these expectations from ligand field theory are largely confirmed by the DFT calculations. Despite the fact that the S = 2 state has a smaller prefactor for the isotropic Fermi contact term, the core polarization in the presence of four unpaired electrons is much larger and, consequently, the isotropic Fe-HFC is predicted to be roughly a factor of two larger in magnitude for as compared to 2g. Similarly, the dipolar MFCs are comparable for both spin states, which must be due to a considerable contribution from anisotropic covalency in the 5 = 2 species which partially compensates for the smaller prefactor. 

Gresh N, Cisneros GA, Darden TA, Piquemal J-P (2007) Anisotropic, polarizable molecular mechanics studies of inter-, intra-molecular interactions, and ligand-macromolecule complexes. A bottom-up strategy. J Chem Theory Comput 3 1960... 

In Ag-SAPO-ll/C2H4 zeolite the EPR at 77 K shows the spectra of Ag° atoms and C2H5 radicals. After annealing at 230 K those species disappeared and then an anisotropic EPR sextet was recorded. Based on DFT calculation the structure of complex was proposed in which two C2H4 ligands adopted eclipsed confirmation on either side of the Ag atom. As a result the overwhelming spin density is localised on ethylene orbitals. 

Such modification of the anisotropy would be hard to achieve, though, with Gd(III) ions, as their anisotropy would be minimal even with engineered ligand changes so this approach would be better suited to 3d chemistry, where the metal ion orbitals can interact more strongly with those of the ligands. Figure 9.9 tells us that isotropic 3d-metals are better for lower temperature work than anisotropic ones. 

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