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• Metal coordination affects the redox behavior of dye chromophores. • Electron transfer sites may involve the metal and/or the dye ligand. • Variable coordination patterns are observed for metal complexes of indigo. • Radical formation and mixed valency may occur in oligonuclear complexes. • Long wavelength absorptions are common in metal complexes of dyes. The chromophores of several classes of well-established dyes, e.g., indigo, 9,10-anthraquinones, formazanes, diketopyrrolopyrroles (DPPs), or certain azo compounds, are not only responsible for intense low-energy absorptions but are also suited for dye ligand-based electron transfer and for (multiple) metal chelate coordination at N and O donor centers, after due deprotonation. Coupling with the redox activity of bonded transition metals such as ruthenium can lead to intricate electronic structures with variable oxidation state combinations, especially when the non-innocent dye-derived ligands are bridging two or more metal centers. Employing an array of experimental methods such as crystal structure determination, voltammetry, EPR, IR and UV–vis-NIR spectroelectrochemistry, and using supporting DFT calculations, it has been possible to analyze the geometrical and electronic situations in compounds such as [L x Run(µ-BL)RumL x ]k, involving radical formation of the bridging chromophore and/or metal-metal mixed valency (BL: bridging ligand derived from dye). Among the prominent suitable examples for BL components are deprotonated indigo with variable ligand oxidation states and coordination patterns (N,O; N,N′; O,O′; N,O:N′,O′; N,N′:O,O′), the non-innocent nindigo ligands with indigo-O replaced by NR, the widely employed 9,10-anthraquinones with additional O or NR donor substituents at various chelate-forming positions, bidirectionally non-innocent formazanato ligands, the DPP(diketopyrrolopyrrole) π system used for technical pigments, heterocyclic quinones, and selected bis-chelating azo compounds with strong and potentially useful NIR absorbance. The deprotonated bridging ligands can adopt several oxidation states such as. BL0, BL −, BL2−, BL 3− and BL4− for the indigo and anthraquinone examples. With the increasing application of classical dye chromophores in information technology systems based on optical and electronic properties, their hitherto neglected potential for metal coordination, including metal-metal bridging and coordination-enhanced electron transfer reactivity, awaits to be exploited – the scope of such capability is illustrated in this article. [ABSTRACT FROM AUTHOR]

Metal dithiolene complexes have increasingly developed over the last fifteen years due to their peculiar electronic structures that makes them very important units for magnetic, conducting and optical molecular materials. This review deals with the different families of chiral metal dithiolenes reported to date in the literature, with the objective of underlining the role of chirality on the resulting materials, in order to disclose possible directions for future developments. Chirality in such materials has been introduced through two strategies, via chiral cations in combination with anionic complexes or using pre-synthesized chiral dithiolene ligands. The corresponding materials resulting from both strategies are thoroughly discussed, with a special focus on the solid state architectures largely influenced by the chiral information. Chiral dithiolene ligands have generally provided homoleptic Ni(II) and Au(III) anionic and neutral bis(dithiolene) complexes, while heteroleptic complex have been described with Ni(II) and Mo(II). The neutral Au(III) complexes are especially interesting in the context of chiral single component conductors. [ABSTRACT FROM AUTHOR]

Highlights: [•] Valence tautomeric (VT) transitions in metal complexes involve a stimulated intramolecular electron transfer between ligand and metal. [•] Cobalt-dioxolene complexes and complexes with other redox-active metal–ligand combinations display VT transitions. [•] Valence tautomerism has been reported for mononuclear, polynuclear and polymeric complexes. [•] The combination of VT behavior with other physical and chemical properties can generate bifunctional molecule-based materials. [Copyright &y& Elsevier]

Abstract: The nickel ion has a remarkably rich coordination chemistry among the first-row transition metals. Complexes with sulfur containing ligands are particularly notable, since they can manifest classical/metal-based (innocent) or inverted/ligand-based (non-innocent) behavior depending on the chemical composition of the S-ligands and the coordination geometry. Using sulfur K-edge X-ray absorption spectroscopy (XAS), we established a spectrochemical series for [Ni(II)S4] complexes containing thiolate, aliphatic dithiolate, olefinic and aromatic enedithiolate, conjugated dithiocarbamate, and aliphatic thioether ligands. The pre-edge intensities at the sulfur K-edge follow an increasing trend from tetrathiolate through dithiolate and enedithiolate to tetrathioether. In order to obtain quantitative sulfur orbital compositions from XAS data, we generalized the earlier methods of estimating the sulfur 1s→3p transition dipole integral for a broad range of S-ligands by considering chemical shifts of spectroscopic features due to changes in the sulfur effective nuclear charge and S-ligand coordination to Ni. The XAS-based experimental orbital compositions are compared with a comprehensive set of density functional theory-based, electronic structure calculations. The 1,2-dithiolate ligands gave indication of inverted bonding, independently whether the coordinated sulfur centers are connected by constrained single or double C,C bonds. Despite intense pre-edge features, both dithiocarbamate and thioether complexes can be described with classical inorganic bonding description. [Copyright &y& Elsevier]

This review touches the most common instances where non-innocent (“suspect”) behaviour of redox-active ligands, either substrates or supporting components, is observed in a biochemical context. These ligands include the O2/O2·−/O22−, NO+/NO·/NO−, o-quinone/o-semiquinone/catecholate and tyrosyl/tyrosinate redox systems, the tetrapyrrole (porphyrinic) ligands, the pterins and flavins, and the dithiolene/ene-dithiolate ligands in molybdo- and tungstopterin. These non-innocent ligands are discussed in their interaction with biological iron, copper, manganese, molybdenum or tungsten centers. [Copyright &y& Elsevier]

Abstract: This review provides an overview of density functional theory (DFT) calculations in a consequence with spectroelectrochemical measurements on mononuclear and symmetrically or unsymmetrically bridged di- and tetranuclear ruthenium complexes of vinyl and TCNX ligands. The DFT approach is used for the calculations of molecular structures, vibrational frequencies, electronic and electron paramagnetic resonance (EPR) spectral data. DFT calculations enable us to identity the primary redox site and the electron and spin-density distribution between the individual components for the individual redox congeners. The DFT technique reproduces the spectral properties of the presented complexes and their radical ions. The generally close correspondence between experimental and quantum chemical results demonstrate that modern DFT is a powerful tool to address issues like ligand non-innocence and electron and spin delocalization in systems containing both redox-active metal ions and redox-active ligands. [Copyright &y& Elsevier]

Abstract: Ruthenium complexes with the non-innocent ligands (NILs) benzoquinone, iminobenzoquinone and benzoquinonediimine and their redox derivatives exhibit intriguing electronic properties. With the proper ligand set the NIL π* orbitals mix extensively with the ruthenium dπ orbitals resulting in delocalized electron distributions and non-integer oxidation states, and in most of these systems a particular ruthenium oxidation state dominates. This review critically examines the electronic structure of Ru–NIL systems from both an experimental and computational (DFT) perspective. The electron distribution within these complexes can be modulated by altering both the ancillary ligands and the NIL, and in a few cases the resultant electron distributions are exploited for catalysis. The Ru–NIL systems that perform alcohol oxidation and water oxidation catalysis are discussed in detail. The Tanaka catalyst, an anthracene-bridged dinuclear Ru complex, is an intriguing example of a Ru–NIL framework in catalysis. Unlike other known ruthenium water oxidation catalysts, the two Ru atoms remain low valent during the catalytic cycle according to DFT calculations, some experimental evidence, and predictions based on the behavior of the related mononuclear species. [Copyright &y& Elsevier]

[Display omitted] • Redox-active ligands expand the application of lanthanides to redox monitoring. • The redox-active ligand provides the complexes with a redox activity. • Lanthanide complexes are sensitive probes for ROS and RNS. • Detection is possible by Luminescence and Magnetic Resonance Imaging. Lanthanides complexes are indispensable in fields related to medicine and biology. Their success lies in the great number of unpaired electrons (4f7 configuration of the Gd3+ ion) that makes them ideal contrasts agents for magnetic resonance imaging. On the other hand their narrow emission bands and long lifetime of their excited state provide them unique luminescence properties. The aqueous solution chemistry of the lanthanides is dominated by the (+III) oxidation state under oxygenated atmosphere. This severely limits their use as redox probes, and not surprisingly very few redox-switches employing lanthanide ions were reported until 2010. A very promising approach based on redox-reactive and redox non-innocent ligands has recently emerged in the literature, which overcomes this limitation and expands the application of lanthanides to redox monitoring. Herein the ligand acts as a redox-sensor by either changing its oxidation state or reacting with reactive oxygen (or nitrogen) species (ROS/RNS). Its response induces changes in the environment of the lanthanide ion, which acts as a reporter of the redox status. The detection is based on a modification of the magnetic or optical properties of the complexes, with detection by conventional spectroscopic techniques. We summarize in this review article the recent advances in this burgeoning field, with special emphasis on the detection of biological relevant ROS, RNS and redox status. [ABSTRACT FROM AUTHOR]

[Display omitted] • Transition metal catalysts derived from non-innocent ligands in water oxidation. • Role of redox active non-innocent ligands are discussed. • Various types of mechanisms are illustrated for such an oxidation process. • Cyclic voltametric, kinetic, electrochemical and theoretical studies are considered. Transition metal complexes acting as catalysts towards water oxidation concern a challenging research topic with high relevance in solar and fuel conversion technologies. Non-innocent ligands with redox behavior can contribute with an important role on this field. We review herein the use as catalysts of complexes (containing single or double metal sites) with such a type of ligands on water oxidation and the corresponding reaction mechanisms as established by kinetic, electrochemical and theoretical studies. [ABSTRACT FROM AUTHOR]