Non-innocent ligand

Lever ABP, Gorelesky SI (2004) Ruthenium Complexes of Non-Innocent Ligands Aspects of Charge Transfer Spectroscopy 107 77-114 LiB, see He J (2005) 119 89-119... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

Such a mechanism would have to involve the nitrosyl ligand acting in a non-innocent manner, changing from a three-electron donor to a one-electron donor in the intermediate complex. Such participation of the nitrosyl ligand has precedent in related systems (108). 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Given the redox non-innocent character of the quinoidal ligand, the present diamagnetic complex can in principle be formulated either as Cr(0)-(quinone)3, or as Cr(VI)-(catechol)3, or as Cr(III)-(semiquinone)3. As a matter of fact, based also on the C-O distance, it has been preferentially assigned as [Crni(3,5-di-Bu sq)3].54... 

We finally point out that heteroleptic-dioxolene complexes with different redox non-innocent ligands can exhibit even further difficulty assignable redox changes.60... 

As briefly alluded to, there are different classes of redox-active ligands in addition to the above mentioned ones. For example, we have seen in Chapter 5, Section 8, that azo-groups (in particular, 2-(phenylazo)pyr-imidine) are able to undergo two separate one-electron reduction processes. Conjugated polynitriles (mnt, tcne, tcnq) also constitute an important class of redox-active molecules and the electrochemical behaviour of their metal complexes has been reviewed.107 The same holds as far as alkyldithiocarbamates (Rdtc) and their metal complexes are concerned,108 or nitrosyl complexes in their possible NO+[NO fNO redox sequence.109 Thus, we would like to conclude the present Chapter by discussing a few less known redox non-innocent ligands. 

Of particular interest are those complexes which serve as model compounds for biological systems such as the active center of the MoFe protein in nitrogenase.107 These complexes will be discussed in detail in the relevant chapters. It should be noted here, however, that WS2- is a non-innocent ligand, and thus complexes such as [Co(WS4)2]" (n = 2,3)108>109 and [Fe(WS4)2]" (n = 2, 3) can easily be prepared.110,111,112 In addition, some unusual complexes have been synthesized with WS2- as ligands. One such example is [Fe3W3S12]4 which contains the Fe3(/r3-S)2 center as depicted in Figure 11. 

Ligands such as aniline (an), 1,2-diaminobenzene (dab, o-phenylenediamine) and 2,2 -dia-minobiphenyl886 887 are classed separately, not because their ability to bind to a central metal is any less than the ligands discussed previously, but because of their potential non-innocent behavior888 with respect to internal redox reactions. Indeed, the dark blue complex isolated from the air oxidation of Con/dab in aqueous ethanol (a conventional route to yellow Co(diamine)3+ systems) has been shown to have structure (117) with five-coordinate Co11.888 Related diimine complexes have been reported for Ni11889 as well as the conventional Ni(dab)(+,890 Co(dab)3+ 891 and Pt(dab) + 892 systems. 

The non-innocent ligand catechol forms very stable osmium(VI) complexes, as do a number of substituted catechols. 

Only a few complexes containing iridium(II) are known, most of which are of a special type as they are either dimeric and diamagnetic or contain non-innocent ligands. More likely, many reported iridium(II) complexes are hydrides of iridium(III) or iridium(III) complexes containing a o Ir—C bond rather than a supposed n Ir—C bond. Furthermore, iridium(II) complexes, with a d1 electronic configuration, are expected to be paramagnetic. 

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