Ligand field, axial

For Cr(m) complexes, D is relatively small (comparable to the X-band microwave quantum, 0.317 cm-1) and all three fine structure lines are observable. This is not always the case. Consider high-spin Fe(m) in an axial ligand field with D > > hv0, E = 0. With the same Hamiltonian as above and the magnetic field along the z-axis, the energies are ... 

Thus, for example, high-spin d5 Fe(m) in an axial ligand field should show a resonance around g = 2 and another resonance near g - 6 when B is perpendicular to the symmetry axis. In solution, where the complex tumbles rapidly and averages the g-values, the resonance is expected to be so broad as to be undetectable, but, in frozen solution, both resonances should be observable. 

For a pseudo-axial ligand field the appropriate perturbation Hamiltonian, H, is, in the absence of a magnetic field, (c.f. Section 2)... 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

The intermediate-spin state has been suggested both for ferrous (21) and ferric hemeproteins (22, 23). Direct participation of the intermediate-spin iron porphyrins in biological processes might be less probable however, any reasonable theoretical account of the varied electronic structure of iron in hemeproteins should explain the fact that the so-called intermediate-spin iron porphyrin can exist only when the axial ligand field is extremely weak. 

Figure 4. The lowest states of an Fe(II) d6 ion in a porphyrin as a function of the axial ligand field...

A Tanabe-Sugano diagram is drawn also for the d5 system in Fe(III) porphyrins. The ground state of the d5 system is constantly 6A i, regardless of the values for even reasonable values of A in Fe(II) porphyrin. For A = 30B, the ground state is the low-spin state 2E or 2B2 for a stronger axial ligand field (A - 25B). In fact, the low-spin... 

For the [Anm(Cot)2] anions magnetic moment measurements have been carried out for the Np, Pu, and Am derivatives (63, 64), all in the form of the 2 THF solvates. Thus X-ray diffraction data show that KNp(Cot)2 2 THF and its Pu and Am analogues are all iso-structural, and similar results for the related diglyme solvates indicate that it is extremely probably that these are iso-structural with the salt KCe(Cot)2 diglyme, which is known (23) to have a sandwich structure with D8d symmetry about the Ce atom. Consequently it is reasonable to assume that the pseudo-axial ligand field model is applicable to these [Anin(Cot)2] systems, and to interpret the magnetic data in these terms. 

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