Ligand electrons, spin polarization

It is a somewhat surprising fact that information about vibrational frequencies and bond lengths in very common and important ions is very sparse. Clearly, many more such determinations are necessary. Only this year, for example, have data become available which enable metal-ligand displacements on electron removal for the aquo Fe(II)/Fe(III) and Co(II)/Co(III) systems to be obtained. In view of the importance of these partially unpublished data, I have reproduced them in Table VI. The values of A for the Fe and Co systems are 0.128-0.137, and 0.208 8 respectively. The variability of the Fe results points to the fact that caution must be exercised in using data obtained in crystal lattices for a solution environment. Spin-polarized neutron diffraction studies on the structure of solu-... 

The case of the hexa(pyridine) nickel(II) complex has been extensively debated in the early literature of NMR of paramagnetic complexes [17-21]. The shift pattern with a-H > y-H > (i-H (Fig. 2.11 and Table 2.3) was soon recognized to be predominantly of a-type. The ligand has a a MO system which has the correct symmetry to overlap with the dx2 y2 and dz2 orbitals. However, spin polarization can induce V2 spin density in the it system. Once some unpaired spin density is in a p orbital, it spin-polarizes the electrons of the C—H a bond, thus producing a further mechanism for transferring spin density on the proton. The proton A/h value from this mechanism is proportional to the spin density on the carbon pz orbital, p , through a proportionality constant >ch ... 

An excellent introduction to this topic is provided by La Mar. (40) This review clearly distinguishes between the three possible spin-transfer mechanisms, namely ligand (L) -> metal (M) a-spin transfer, L -> M 7t-spin transfer, and M L 7r-spin transfer. In addition to these direct spin delocalization mechanisms, where a fraction of unpaired electron density originally associated with the metal is actually placed into an MO that includes the ligand to some extent, there are indirect spin delocalization or spin polarization mechanisms. These involve essentially a redistribution of electron spin in an MO otherwise... 

To investigate the bond between thiophene and M in the S (or 771) mode, DV calculations were performed for the complex ion [Cp (C0)2Fe (17 -T)]+ (Cp=cyclopentadienyl, T=thiophene) [77], Due to the strong covalency between the Fe and the ligands, the electron configuration is a low-spin closed-shell, so that no spin-polarization is present. 

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