Noninnocent ligands

Scheme 13 The FeN2S2 center bearing a coordinatively unsaturated iron(III) with a noninnocent ligand...

The terms innocent and noninnocent ligands are widely used to emphasize the fact that some ligands do not necessarily possess a closed-shell configuration. These terms can be used meaningfully only in conjunction with the spectroscopic oxidation state of the metal ion. In this book, we generally refer to spectroscopic oxidation numbers, unless stated differently. 

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. 

Comman, C.R., G.J. Colpas, J.D. Hoeschele, J. Kampf, and V.L. Pecoraro. 1992. Implications for the spectroscopic assignment of vanadium biomolecules Structure and spectroscopic characterization of monooxovanadium(V) complexes containing catecholate and hydroxamate-based noninnocent ligands. J. Am. Chem. Soc. 114 9925-9933. 

The numerous properties of metallo-dithiolene complexes are a direct consequence of the fantastic redox interplay between the redox-active metal and the dithiolene ligand. A considerable amount of experimental evidence over the last 40 years has pointed to the fact that dithiolenes are highly noninnocent ligands (20, 86, 110, 113, 280-282). That is, they may be viewed in a valence bond description as existing somewhere between the extremes of neutral (dithione/dithiete) and dianionic (dithiolate) forms (Fig. 1). This variable formal... 

Rauchfuss s team is working on noninnocent ligands, a family of ligands in which the oxidation state is unclear. A quinone is a typical noninnocent ligand that has not been used very much in hydrogen activation. Using such non-... 

Transition metal ions with organic radicals exist in the active sites of metalloproteins. The best understood example is galactose oxidase, which features a single Cu(II) ion coordinated to a modified tyrosyl radical. Many combined experimental and theoretical studies have focused on electronic properties of metal complexes with redox active ligands, yet reactivity beyond characterization has been limited. We will demonstrate the influence of the metal complex redox state on H2 activation by anilino-phenolate noninnocent ligands. 

The majority of reported Co complexes in oxidation states higher than 3-1- contain noninnocent ligands, and this hence renders the assignment of metal oxidation state difficult. This problem has existed since Werner s time indeed, he initially formulated the superoxo-bridged dimers [(NH3)4Co(02)(NH2)Co(NH3)4] + and [(en)2Co(02)(NH2)Co(en)2]" + as mixed-valence peroxo-bridged Co(III)/(IV) complexes. One-electron oxidation of the unusual square-planar Co(III) complex (31) using Ce(IV) gives a deep blue low-spin species. This is soluble and stable in benzene, and exhibits a reversible one-electron reduction... 

Attempts to achieve macrocycles that are capable of stabilizing highly oxidized transition metal complexes has led to the design of noninnocent ligands. The structures of high-valent chrominm(V) oxo species with the two tetraamido N ligands (36) and (37) were determined. Both stmctnres were found to contain distinctly nonplanar amide gronps, and in (36) all fonr amides are nonplanar. 

There is a certain reluctance to assign the oxidation number +3 to palladium, whereas there is not much trouble in assigning it +4 in other complexes. Thus, Pd(III) candidates are looked at with suspicion, not because it is difficult to accept that Pd can have charge densities midway between Pd(II) and Pd(IV), but probably because in many instances where a formal oxidation state +3 can be assigned, either the complex has some noninnocent ligands which do not allow a clear decision, or the complex is a dimer or a higher polymer and can be assigned mixed oxidation states. 

Enedithiolate anions are redox-active ligands (see equation 1), capable of coordinating with a variety of metals as neutral dithioketones (1), thioketone radical thiolate monoanions (2) and ene-l,2-dithiolate dianions (3). The complexes are named dithiolenes irrespective of their oxidation state to remind one that they are noninnocent ligands . 

Chemical shifts 6( V) for oxovanadium complexes carrying the innocent ligand salicyli-deneaminatoethylimidazole and (centre and right) the noninnocent methylhydroxamate and bis(tert-butyl)catecholate. The noninnocent ligands induce downfield shifts (deshielding). ... 

In this chapter and in an upcoming review (Part II), the chemistry of mononuclear paramagnetic organometallic complexes of the second- and third-row groups 9(VIIIB) and lO(VlII) platinum metals (Rh, Ir, Pd, Pt) are summarized. In this chapter, the higher valent species (Rh(II), Ir(II), Rh(IV), Ir(IV), Pd(ni), Pt(III) and complexes with oxidized redox noninnocent ligands) are described. In the next chapter, the lower valent species (Rh(0), lr(0), Pd(I), Pt(I), and complexes with reduced redox noninnocent ligands will be discussed. 

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