Octahedral configuration changes

Fig. 7.4 Equivalent representations of the complex M(AA)j. The usual octahedral representation is shown in A. In B, viewing is down the Cj axis three metal-ligand bonds point towards the viewer, and three point away. In C, the opposite faces of the octahedron are shown when viewing is down the Cj axis. Representation B will be often used to illustrate configurational changes.

As cobalt(III) complexes (d6), cobalamins strictly adhere to an octahedral geometry, with an axial benzimidazole nitrogen ligand. It appears that in protein-bound B12 this axial ligand is replaced by a histidine provided by the peptide chain, with consequent configurational changes that influence the reactivity of the... 

The absorption spectra and Stokes radii (calculated from the results of conductivity measurements) of solvated complex ions of first transition metals were reported by Abe and Wada [Ab 80] in HMPA. They found that the Stokes radii of these complex ions are sensitive to the configurational change, since the diameter of an HMPA molecule is very large as compared with the crystallographic diameters of central metal ions. Mn(II), Fe(II), Co(II) and Cu(II) ions were found to be tetrahedrally solvated while in the case of Ni(II), equilibria are established among octahedral, square planar and tetrahedral solvated species. Zn(II) forms probably a complex ion solvated octahedrally by HMPA. 

An especially interesting recent example is Benedetti et al 5 use of circular dicliroism (CD) spectroscopy to detect a pressure-induced change of the configuration at the metal centre of the octahedral chiral A- and A-tris... 

Low-spin, octahedral complexes are formed by ligands such as bipy, phen and CN , and their stability is presumably enhanced by the symmetrical configuration. [Fe(bipy)3] + and [Fe(phen)3] + are stable, intensely red complexes, the latter being employed as the redox indicator, ferroin , due to the sharp colour change which occurs when strong oxidizing agents are added to it ... 

In the next chapter we look at the intensities of d-d electronic transitions. We shall see that transitions between terms of the same spin-multiplicity are much more intense than those involving a change of spin. It is for this reason that our focus in the present chapter has been on the former. We have seen that for d d , d and configurations in octahedral or tetrahedral environments, there is only one so-called spin-allowed transition. For

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