Six-coordinate system

The comparison of Fe(III) spin transition systems with those of other metal ions reveals the greater variety of chromophores for which spin crossover is observed in iron(III). This is reflected in a generally more diverse coordination environment as well as a far broader range of donor atom sets. For six-coordinate systems the spin crossover generally involves an S=l/2<->S=5/2 change, whereas for five-coordinate materials an intermediate (quartet) spin state is involved in S=l/2<->S=3/2 transitions. There is just one report of such a transition in a six-coordinate system and that is considerably distorted [126]. 

Six-Coordinate Systems with Ground States of Nondegenerate Octahedral... 

A study of indium(III) iodine bond lengths as a function of coordination number of the indium shows a systematic change. The symmetry or the charge of the adduct has only a minor effect. For well-authenticated four-, five-, and six-coordinate systems, the average In—distances are about 2.68, 2.73, and 2.83 A, respectively. " " ... 

Such a procedure was used successfully by Wemer to demonstrate that six-coordinate complexes have an octahedral structure. The method starts with the assumption that a six-coordinate system has a structure in which the six ligands are situated at positions symmetrically equidistant from the central atom. If it is further assumed that three of the more probable structures are (1) planar, (2) a trigonal prism, and (3) octahedral (Table 1.5), then it is possible to compare the number of known isomers with the number theoretically predicted for each of these structures. Such a comparison shows that for the second and third compounds in Table 1.5 planar and trigonal prism structures predict there should be three isomers. Instead, complexes of these types were... 

In six-coordinated systems, the hybrid orbitals involve the s, p py p, and atomic orbitals. The resulting six sp (f or cf sp hybrid orbitals point toward corners of an octahedron. For [CoFe] the d orbitals used have the same principal energy level as the s and p orbitals. A complex of the nsnp ruf type is called an outer-orbital complex because it uses outer d... 

Therefore, the rules that predict rate behavior for six-coordinated systems will often not apply to complexes having smaller coordination numbers. Because rate behavior is dependent on mechanism and since reactions of metal complexes proceed by a variety of paths, it is impossible to make generalizations that apply to all complexes regardless of the type of mechanism by which they react. In spite of this, the rules that are outlined in this section are surprisingly consistent with the data on the rate behavior of octahedral complexes. 

A reaction path analogous to that used for Ni(II) affords the [Co(18S6)] cation (Eq. 22a) (Fig. 10 Table 1), where 18S6 wraps around the metal ion in an all facial fashion to yield the meso isomer. Several lines of evidences indicate adoption of the low-spin ( Eg) state. These include observation of a pronounced Jahn-Teller axial elongation (0.21 A) as well as Co-S distances consistent with the ionic radius of the low-spin ion [104]. Magnetic susceptibility measurements confirm the low-spin formulation (Table 5) [98, 99]. EPR g values (g approx. 2, g > 2 Table 5) establish the existence of a ground state (consistent with axial elongation of a low-spin six-coordinate system). 

For a substitution reaction involving a six-coordinated species we can envision two possible transition states. One involves an Sn2 mechanism (substitution, nucleophilic, and bimolecular) where a six-coordinate complex accepts the ligand to be added. This produces a seven-coordinate system as the intermediate that then releases one of the substituted ligands, thus returning to a new six-coordinate system. The second possible transition state involves an Sn2 mechanism... 

Restricting ourselves to a six-coordinated system and to the valence bond theory and crystal field theory, it is possible to illustrate the bonding in complexes and to designate the nomenclature using [CoFe] and [Co(NH3)e] + as examples. It is first necessary to know that [CoFs] is paramagnetic with four unpaired electrons, whereas [Co(NH3) ] + is diamagnetic. On the basis of the valence bond theory the electronic structures are designated as ( 5) or cPsp (6) hybridizations. 

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