Low-spin configuration

Figure 7.40 (a) Weak-field, high-spin and (b) strong-field, low-spin configurations in d, d, d ... 

Figure 19.16 The possible high-spin and low-spin configurations arising as a result of the imposition of an octahedral crystal field on a transition metal ion.

Organometallic compounds apart, oxidation states below - -2 are best represented by complexes with tris-bidentate nitrogen-donor ligands such as 2,2 -bipyridyl. Reduction by LiAlH4 in thf yields tris(bipyridyl) complexes in which the formal oxidation state of vanadium is -1-2 to —1. Magnetic moments are compatible with low-spin configurations of the metal but. 

It is possible to observe spin-allowed, d d bands in the visible region of the. spectra of low-spin cobalt(lll) complexes because of the small value of 0Dq, (A), which is required to induce spin-pairing in the cobalt(lll) ion. This means that the low-spin configuration occurs in complexes with ligands which do not cause the low -energy charge transfer bands whieh so often dominate the spectra of low-spin complexes. 

All complexes in this oxidation state which have been characterized are octahedral and diamagnetic with the low-spin configuration. 

The change from high- to low-spin configurations is necessarily discontinuous. A given complex is either on one side of the divide or the other. We conclude this section with a look at how the steric role of the d shell can affect angular geometries within a series of just high-spin, nominally tetrahedral nickel(ii) complexes. 

Similar effects are observed in the iron complexes of Eqs. (9.13) and (9.14). The charge on the negatively charged ligands dominates the redox potential, and we observe stabilization of the iron(iii) state. The complexes are high-spin in both the oxidation states. The importance of the low-spin configuration (as in our discussion of the cobalt complexes) is seen with the complex ions [Fe(CN)6] and [Fe(CN)6] (Fq. 9.15), both of which are low-spin. 

A typical example of a correlation diagram for Fe is given in Fig. 4.3. It summarizes the isomer shifts for a great variety of iron complexes with oxidation states (1) to (VI) in the order of the respective high-spin, intermediate-spin, and low-spin configurations. The plot of the corresponding values marked by grey, hatched and open bars demonstrates three major trends ... 

A high spin complex is the Crystal Field designation when all t g and eg orbitals are singly occupied before pairing begins. A low spin complex, on the other hand, is the Crystal Field designation, where electrons paired in t2g orbitals before eg orbitals are occupied. The low spin configuration can exist only for octahedral... 

The 2,2/-bi-2/-thiazolines 16 17 18 (Dq(Ni2+) values 1160, 1120 and 1160 cm-1, respectively) are related to the bithiazoles, and the perchlorate salts of the [Fe N6]2+ derivatives of all three systems are low spin at room temperature but their behaviour above room temperature has not been reported. The low spin configuration for [Fe 173](C104)2 in particular appears inconsistent with the behaviour of related systems containing substituents adjacent to the donor atoms, and with its relatively low Dq(Ni2+) value. Nelson and co-workers have pointed out that the steric effect here is not so pronounced because the methyl substituents are not coplanar with the diimine group [21]. 

The variation of the associated cation has also been investigated for 5-halo-salicylaldehyde thiosemicarbazone compounds [105]. The favouring of the low spin configuration for both (cation) [Fe(5-Br-thsa)2] and (cation) [Fe(5-Cl-thsa)2] follows the order of the associated cations Na+>-Li+>K+>NH4+ even though the effect of variation of the monovalent cation is not very pronounced. Overall the low spin state is favoured to the greater extent in the salts of the 5-chloro derivative. On the other hand, the Zn2+ salt of this derivative, which crystallises as a sesqui hydrate, shows a more extended transition in the range 80-300 K [105]. 

At Xu = 0 Fe was found to be of intermediate stability in octahedral coordination. Unfortunately the valences of the Fe ions could not be clearly determined using the spin integration method of section 5 because the net spin on the Fe did not match a high-spin or low-spin configuration consistent with the average formal oxidation state (+4). This suggests that the d-orbital filling takes on an... 

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