In biological reduction processes the dihydronicotinamides NAD(P)H often transfer hydride to an unsaturated substrate bound within an enzyme active site. In many cases, metal ions in the active site bind, polarize and thereby activate the substrate to direct attack by hydride from NAD(P)H cofactor. This review looks more widely at the metal coordination chemistry of organic donors of hydride ion--organo-hydrides--such as dihydronicotinamides, other dihydropyridines including Hantzsch's ester and dihydroacridine derivatives, those derived from five-membered heterocycles including the benzimidazolines and benzoxazolines, and all-aliphatic hydride donors such as hexadiene and hexadienyl anion derivatives. The hydride donor properties--hydricities--of organo-hydrides and how these are affected by metal ions are discussed. The coordination chemistry of organo-hydrides is critically surveyed and the use of metal-organo-hydride systems in electrochemically-, photochemically- and chemically-driven reductions of unsaturated organic and inorganic (e.g. carbon dioxide) substrates is highlighted. The sustainable electrocatalytic, photochemical or chemical regeneration of organo-hydrides such as NAD(P)H, including for driving enzyme-catalysed reactions, is summarised and opportunities for development are indicated. Finally, new prospects are identified for metal-organo-hydride systems as catalysts for organic transformations involving 'hydride-borrowing' and for sustainable multi-electron reductions of unsaturated organic and inorganic substrates directly driven by electricity or light or by renewable reductants such as formate/formic acid.

A proposal for a redox-linked conformational gate to proton translocation--a proton pump gate--based upon a transition-metal redox-switchable hemilabile ligand (RHL) system is made. Consideration of the requirements for such a system reveals copper(II) to be the ideal metal centre. To test the proposal and, thereby, to provide an artificial proton pump gate, the copper coordination chemistry of three tris(pyridylmethyl)amine (tpa) ligands with one "leg" (PY*) substituted at the 6-position of the pyridine ring by a dimethoxyphenyl (L(1)), a hydroquinone (H(2)L(2)) or a quinone (L(3)) substituent has been investigated. Crystal structures of sp-[Cu(kappa(4)N-L(1))Cl]Cl.3 H(2)O (sp=square pyramidal), sp-[Cu(kappa(3)N-H(2)L(2))Cl(2)] and tbp-[Cu(kappa(4)N,kappaO-HL(2))][PF(6)] (tbp=trigonal bipyramidal) have been determined. The Cu(I) complexes [Cu(L)(MeCN)(n)](+) (L=L(1), H(2)L(2)) display physicochemical properties consistent with a "dangling" PY* leg; from the NMR spectra, the barriers to inversion of the ligand amine donor for the Cu(I) complexes are estimated to be within the range of about 30-45 kJ mol(-1). In the Cu(II) complexes, coordination of the PY* leg is finely balanced and critically depends on the nature of the PY* substituent and the availability of potential co-ligand(s). For example, tbp-[Cu(kappa(4)N-L(1))Cl](+) reacts cleanly with Cl(-) ions to afford sp-[Cu(kappa(3)N-L(1))Cl(2)]; Vis/NIR spectrophotometric titrations suggest the affinity of tbp-[Cu(kappa(4)N-L(1))Cl](+) for Cl(-) ion in dichloromethane is 9.7 x 10(2) and is at least 10(4)-fold greater than that of tbp-[Cu(kappa(4)N-L(3))Cl](+). The complex sp-[Cu(kappa(3)N-H(2)L(2))Cl(2)] has a "dangling" PY* leg, in which an intramolecular OH(hydroquinone).N(pyridine) hydrogen bond "ties-up" the pyridyl nitrogen atom, and reacts with Brønsted bases to give tbp-[Cu(kappa(4)N,kappaO-HL(2))](+). Two-electron oxidation of sp-[Cu(kappa(3)N-H(2)L(2))Cl(2)] is linked to loss of two protons and a conformational change, and affords tbp-[Cu(kappa(4)N-L(3))Cl](+). The [Cu(kappa(3)N-H(2)L(2))Cl(2)]-[Cu(kappa(4)N-L(3))Cl](+) system provides a first demonstration of the critical step in the proposed proton pumping cycle, namely a redox-driven and proton-linked conformational change. The possible biological relevance of this work to proton pumping in cytochrome c oxidase is mentioned.

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The application of ruthenium phthalocyanine complexes as sensitizing dyes in dye-sensitized solar cells (DSCs) is explored. Four monomeric complexes are reported which vary in peripheral substitution and axial ligand anchoring groups. Sensitizing dyes containing two ruthenium centers are also presented. These dyads, which contain ruthenium phthalocyanine and bipyridyl chromophores, were prepared using a protection/deprotection strategy that allows for convenient purification. DSCs fabricated using the phthalocyanine complexes and dyads were less efficient than those incorporating a standard DSC dye. However, on the basis of the number of molecules bound to the TiO(2) electrode surfaces, several of the new complexes were more efficient at photocurrent generation. The results highlight the importance of molecular size, and thus the dye coverage of the electrode surface in the design of new sensitizing dyes.