Anisotropic covalency

Another typical example for anisotropic covalency is found in five-coordinate ferric compounds with intermediate spin S = 3/2 (also discussed in Sect. 8.2). Crystal field theory predicts a vanishing valence contribution to the EFG, whereas large quadrupole splittings up to more than 4 mm s are experimentally found. 

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. 

The surface chemistry of carbon is rather complex. At a single adsorption site several chemically inequivalent types of heteroatom bonds may form. Strong interactions between surface functional groups further complicate the list of surface chemical structures as derived for the most relevant carbon-oxygen system An additional dimension of complexity is presented by the large variety of substrate structures of carbon which arise from anisotropic covalent bonding rather than by a isotropic metallic interaction. 

Table 5 AILFT ligand field parameter values for octahedral CrX63 complexes allowing for anisotropic covalence (LFModel 3)a...

The effects of anisotropic covalency on the quadrupole splitting of high-spin ferric compounds has been considered theoretically for compounds of... 

If D is less than zero, must be greater than k, where the k terms allow for anisotropic covalent reduction of the spin-orbit coupling. The ki values directly relate to differences in covalent delocalization of the different d orbitals due to formation of metal-ligand bonds. ... 

Rather unexpectedly. Equation (la) fails to predict the correct magnitude and sign of D for [FeCy (f)caic= +0.24 cm vs. /)exp= 0.04 cm ). It was determined that the failure results from anisotropic covalency, i.e., the covalency of the axially split 3d y and Mxz,xy orbitals differ. The greater covalency of the 34, orbital in the flattened 7 structure decreases the spin-orbit coupling to the " T i(z) component rektive to the coupling to the Tfx,y) in Equation (lb), [Pg.694]

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