Low-spin compounds

Low-spin compounds exhibit lower isomer shifts than high-spin compounds, whereas the isomer shifts of intermediate-spin compounds often resemble those of the corresponding low-spin compounds. 

Table 4.2 summarizes the Lewis-like oxidation states, d count, and coordination numbers for low-spin compounds of all the normal-valent transition metals. While these represent only a subset of the empirically known species, the Lewis-like... 

The sensitivity to structure of hemochrome spin states is represented by the two examples reported by Scheidt et a/.,88 i.e. [Fe(0EP)(3-Clpy)2]C104 exists in the low-spin state at 98 K but equilibrates to a 1 1 mixture of the low- and high-spin species at 293 K in the solid state, and Fe(TPP)(NCS)(py) crystallizes as a low-spin compound with a bent Fe—N—CS geometry, while a high-spin species with a linear FeNCS structure is obtained for Fe(OEP)(NCS)(py). Slightly longer Fe—N dis-... 

The Co(II) compounds are usually low-spin, five-coordinate monomeric species, LCo(DH)2(II) (II). However, L2Co(DH)2 low-spin compounds can be isolated (II). These latter materials are known to be five coordinate in solution and have EPR and visible spectra and magnetic moments ( 2 BM) identical to those of LCo(DH)2 (12). The X-ray structure of neither type of solid is known when L = P-donor ligand but the bis(pyridine) compound has been characterized structurally (13) and exhibits long Co-N(pyridine) bonds of 2.25 A. 

Brill and Williams (16) noted that the weak bands around 17—19 kK in essentially high-spin systems were little affected in position by axial coordination, and appear in much the same place as the normal —(l bands of the low-spin compounds. They therefore suggested that the... 

The absence of charge transfer bands in low-spin systems was explained by asserting that the metal eg orbitals (dxz, yz) were filled in the low-spin compounds, so that no Eu charge transfer states were possible. However, ESR evidence (74) indicates that the unpaired electron is in the dyZ orbital, so that two Eu charge transfer bands are still possible. 

Most of the essentially high-spin ferrimyoglobin derivatives studied as single crystals (96,97) exhibited a shoulder around 9 kK. This could be a consequence of the splitting of the Eu excited state. However, other possibilities are worthy of consideration. Most, possibly all, of these compounds contain some of the low-spin form which has a weak band at about this energy, as shown in the near infra-red spectra of the essentially low-spin compounds MbCN, cytochrome-c (97) and cytochrome-65 (100). However, even MbF, which, from magnetic evidence (20,22,64) is... 

We have assigned the near infra-red band of the low-spin compounds to an ay-polarized charge transfer band, which might be regarded approximately as the excitation... 

---