Oxidation state, formal effect

The difficulty of assigning a formal oxidation state is more acutely seen in the case of 5-coordinate NO adducts of the type [Co(NO)(salen)]. These are effectively diamagnetic and so have no unpaired electrons. They may therefore be formulated either as Co -NO or Co -NO+. The infrared absorptions ascribed to the N-O stretch lie in the range 1624-1724 cm which is at the lower end of the range said to be characteristic of NO+. But, as in all such cases which are really concerned with the differing polarities of covalent bonds, such formalism should not be taken literally. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

The dependence on formal oxidation state can be attributed to electrostatic lowering of the metal d orbitals thereby narrowing the gap with ligand orbitals. The effect of principal quantum number may be due to better overlap of the larger Ad and 5d orbitals with ligand orbitals, compared to the more compact 3d orbitals. Thus 0 increases in the... 

The last requirement to be met is that the metal d orbitals are close enough in energy to the C—H and H—H orbitals for effective interaction to take place. This requirement is readily met across most of the first and second transition row series. The energies of the d orbitals sweep a broad range as the atomic number and formal oxidation state are changed, and the size, shape, and energy may be finely tuned by the appropriate choice of ligands. 

The formation of compounds in the formal oxidation state of VI is well established for all four elements, for example, the sexivalent fluorides and the TeF ion. However, oxidation to this highest valency state becomes progressively more difficult as group VIB is descended since the inert pair effect causes the elements to behave as though two of their valence electrons are absent. Some examples of the compounds formed by the group VIB elements and their stereochemistries are described in Table 1. 

Compared to other 1,1-dithiolates, the R2DED ligands are quite unique in their properties. These ligands appear to be more effective than the dithio-carbamate ligands in stabilizing metal ions in high formal oxidation states and are characterized by a conjugated n system (Fig. 54). 

Finally it should be noted that formation of the perfluorobicyclo[3.3.0]-octa-2,7-diene-4,6-diyl ligand allows pyramidalization of four fluorinated carbons and may reflect a thermodynamic preference for sp2-hybridized carbon atoms in coordinated OFCOT to undergo rehybridization to sp3, provided that the ancillary ligands present on the metal can support an increase in the formal oxidation state and that the constraints of the 18-electron rule are obeyed. The origins of this thermodynamic effect for uncoordinated fluorinated alkenes have been discussed in detail (2). Extensions to nickel, palladium, and platinum systems are described in Section IX. 

Raman and Mossbauer studies.97 Thus the compounds are partially oxidized and should be more correctly expressed as [M(DPG)2KIs)o.2 with the nickel in a formal oxidation state of 2.20.97 The electrical conductivity in the Ni atom chain direction is 10-2 fi 1 cm-1, 105 times that of the unoxidized parent compound.97 98 The temperature dependence of the conductivity indicates that the compound is a semiconductor with AE = 0.19+0.01 eV. Table 3 indicates that changing the halide has little effect on the conductivity but that the Ni complex is more conducting than the Pd analogue. 

There is quite a number of theoretical approaches to the understanding of experimentally observed chemical shifts. It was early realized that chemical shifts could be related to the formal oxidation state of the element under study. Further investigations revealed that the effective charge q (A) of an atom A in a molecule is the important parameter and numerous correlations based on the equation... 

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