Ligand-centered contributions

Finally, if the electron is considered to be delocalized on a large part of the molecule as the result of n delocalization, the problem is further complicated. Pseudocontact shift is also provided by spin density in a p orbital which has an orbital contribution to the magnetic moment different from zero. This is very relevant for heteronuclei like 15N or 13C which may bear unpaired spin density on pz orbitals. The picture is of the type of Fig. 2.1 where, besides spin polarization, dipolar interactions occur. For example, fractions of unpaired electrons delocalized onto nitrogen p orbitals in Fe(CN) - are predicted to affect 

---

R m calculations in the presence of ligand-centered contributions are possible for metal complexes with ligands having dominant ti spin density delocalization mechanisms. With certain approximations, the relaxation rates of protons and carbon atoms in sp2 CH moieties can be expressed [33-35] as the sum of a... 

Schmidtke et al. [30] presented a simplified model of the K integrals, which retains the Stevens delocalization coefficients featuring in the LCAO expansion of the metal d-orbitals over the ligands but does not allow for differences in the radial parts of the metal function. More-center and ligand-centered contributions are neglected. In this approximation the K integrals can be expressed as follows ... 

However, these reactions jure associated with terminal rather than bridging carbonyl groups. In metal "ketonic compounds, the center for attack is normally the metal. This may be rationalized in terms of additional delocalization of the positive charge from the carbon to the metal center and implies a "carbyne contribution to the carbonyl bridge, of the form discussed above for the methylated products, again emphasizing the possibility that in systems of this type reaction may occur at either the metal or ligand center... 

One contribution to this problem is the compariajn of the PE spectrum of bis-tr-allylnickel with that of 1,3,5-hexatriene. The upper part of Scheme 2.5-3 diows the interpretation of the PE spectrum of bis- r-allylnickel by C. D. Batich Important for our comparison is that the marked occupied orbitals of the unified ligand system (El) are ligand-centered and practically do not interact with metal orbitals. 

Separation of pseudocontact and contact (including any ligand centered pseudocontact) contributions to the hyperline shifts of heme and axial ligand protons for oxidized rat microsomal cytochrome at 313 K (heme numbering as in Fig. 5.7B) [127]... 

The first criterion really related to the content of this book is the analysis of T and 72. As the dominant contribution to nuclear relaxation is dipolar in nature, Tfl and linewidths will decrease as we move farther from the paramagnetic center. Even the contact contribution to relaxation often decreases with the number of chemical bonds from the paramagnetic center. A caveat, however, should be given. Spin density transfer causes ligand-centered relaxation. Significant spin density on ax orbital of an sp2 carbon may relax an attached proton more than the paramagnetic center itself, owing to the different distances and to the sixth power dependence on distance. 

---