Quinone diimine ligands

Let us now illustrate some significant examples of the redox propensity of related (but less widely studied61) quinone diimines. As happens for 1,2-dioxolenes, such ligands also exhibit the reversible electron transfer sequence o-benzoquinone diiminejo-benzosemiquinone diimine monoanion/ o-phenylendiamine dianion illustrated in Scheme 4. 

The group of noninnocent ligands is continuously growing and comprises dithiolates, diimines, quinones, phenolates, and their derivatives with all combinations of coordinating N, O, and S atoms, and many others. Some examples are found in Sect. 8.2. 

The formal oxidation number of the metal differs by two in these two cases. In molecular orbital terms the problem is one of the extent to which electrons are in metal d orbitals or delocalized over the ligand. Undoubtedly, in general, considerable delocalization occurs, which accounts for the ability of these complexes to exist with such a range of electron populations. The exact specification of orbital populations in any given case is a difficult and subtle question. The same problem arises for quinone and diimine complexes. 

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