Ambiguity in oxidation states

Ligand deprotonation may result in some interesting ambiguities in oxidation state, which ultimately allow us to consider new patterns of reactivity. We begin by considering the iron(m) complex of a macrocyclic amine 5.18. Deprotonation of the amino group in the complex gives an iron(m)-amido complex 5.19 (Fig. 5-34). 

NITROSYL COMPLEXES AND OTHERS WHERE AN AMBIGUITY IN OXIDATION STATE... 

Older texts often employ an alternative nomenclature in which the suffixes -ous and -ic are encountered. In general, these labels only apply to the most common oxidation states of the metals, -ic referring to the higher oxidation state and -ous to the lower. Using this nomenclature, copper(ii) is referred to as cupric and copper(i) as cuprous. The system works well if there are only two common oxidation states for a metal ion, but if there are more, the scheme becomes either ambiguous or unwieldy as a variety of prefixes are added. 

The well-known ambiguities in limiting descriptions of metal and ligand oxidation states are summarized below. 

In characterizing the aims, methods, and values of chemistry, some chemists recently have stressed the irreducibility of chemistry to physics. Hoffmann, for instance, in reflecting on the question, has written that "most of the useful concepts of chemistry (for the chemist aromaticity, the concept of a functional group, steric effects, and strain) are imprecise. When reduced to physics they tend to disappear."40 Further, it is the very ambiguity of some chemical concepts, like "oxidation state," that appeals to chemists. For chemists, the oxidation state of an atom varies according to its role in a molecule, appearing to vary from, perhaps, + 3 to -2 for an atom. Physicists are uneasy with this "elastic" heuristic device.41... 

Once the basic structure has been determined, bond valences can be used to resolve a number of problems of interpretation. Diffraction experiments can identify the location of each atom, but cannot identify its oxidation state. In most structures the oxidation state is determined by the requirements of electroneutrality (Rule 11.1), but in some structures more than one assignment is possible. Bond valence sums can usually resolve this ambiguity. 

When Or is bound to a transition metal its formal oxidation state and that of the metal are ambiguous. In the compounds discussed above the MoVI-peroxide formulation is used. However, in the absence of structural data an MoIV-dioxygen or Mov-superoxide formulation is possible. The hypothetical reaction of MoIV and 02 can be considered an oxidative addition, wherein the extent of charge transfer determines the proper formulation. In the complexes discussed here the O—O distance lies in the range 1.44 to 1.55 A. Comparison of these... 

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