Low-spin case

Figure 7.8 A plot of LFSE against d configuration for high-spin and low-spin cases...

The octahedral complexes of these ions correspond to the low-spin case, lA g, with fie(r x 0. 

High spin cobalt(II), a S = 3h ion, is more interesting from the NMR point of view as the electron relaxation times are shorter that in the low spin case. The electron configurations are reported in Fig. 5.31. High spin cobalt(II) is commonly encountered in four- (tetrahedral), five- (either square pyramidal or... 

In Figure 7, the MCD spectra of high- as well as low-spin ferric iron chlorin derivatives are displayed (38, 39). The high-spin ferric-fluoride adducts of methylchlorin-reconstituted myoglobin and horseradish peroxidase are compared in Figure 7A. For the ferric low-spin case, the... 

On the other hand, in Co(NH3)53+ the ammonia lone pair orbitals are closer in energy to the metal ion orbitals, a situation that produces a larger degree of mixing between the two sets of orbitals. This in turn gives a relatively large amount of d-orbital splitting, and the low-spin case results [see Fig. 20.32(b)]. 

Remember to take into account the number of udditionul pairing energies required in low-spin cases before converting units from cm to k.F mol to izive CFSE = - 24.1 k.F mol... 

For example, a ion could have five unpaired electrons, three in t2g and two in eg orbitals, as a high-spin case or it could have only one unpaired electron, with all five electrons in the t2g levels, as a low-spin case. The possibilities for all cases, d through d , are given in Table 10-5. 

We emphasize that the present discussion focuses only on high-spin open-shell systems to which a single-determinant reference wavefunction is applicable. Coupled cluster techniques for low-spin cases, such as open-shell singlets, have been pursued in the literature for many years, however, and provide a fertile area of research (Refs. 158, 167-170). 

When the coupled spin-containing units are structurally different and also have different total spins, Si and S2, the allowed spin states for the dimer vary from (Si + S2) to Si — S2I in — 1 steps. In the high spin ferromagnetic case (Si + S2), one still talks about ferromagnetic interactions, but in the low spin case, necessarily different from zero, one talks about ferrimagnetic interactions (in a certain way, all the allowed states from (Si + S2) — 1 to Si — S2I can be considered as ferrimagnetic). 

In this section the parameter L is used to quantify the distortions from sphericality, and define the relationship between cluster shape and the number of valence electrons. Figure 3 illustrated the splitting of the If shell under oblate and prolate distortions. The resultant orbital stabilisation energies in terms of p are shown in Table 8 for clusters with 4 to 8 valence electrons. Both high and low spin cases are considered. The most stable geometries predicted by this simple model, are summarised in Table 9. 

For the low spin case we refer to the set of Kramers doublets whose spacing is shown in Fig. 23. The second doublet is situated at an energy of more than 1000 cm-i above the first (lowest) doublet. This is far in excess of thermal energy at room temperature — therefore the population in the second doublet will be small, and even smaller in the third doublet. The contribution of the latter to the susceptibility will be ignored. 

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