Redox transitions of chromium, manganese, iron, cobalt and nickel protoporphyrins in aqueous solution

The electrochemical redox behavior of immobilized chromium, manganese, iron, cobalt, and nickel protoporphyrins IX has been investigated over the pH 0-14 range. In the investigated potential domain the metalloporphyrins were observed in four different oxidation states (M(I), M(II), M(III) and M(IV)). The metalloporphyrins differ in the potentials at which the redox transitions occur, but the observed pH dependence of the redox transitions was similar for the different metalloporphyrins and revealed that the M(II)/M(III) and M(III)/M(IV) transitions were accompanied by a hydroxide transfer at high pH. The fact that the metalloporphyrins are immobilized on graphite does not seem to have a large influence on their redox behavior, as can be deduced from the comparable behavior of immobilized metalloporphyrins on gold and of watersoluble metalloporphyrins in solution. We also performed density functional theory (DFT) calculations on the metalloporphyrins in different oxidation states. The geometries and spin states predicted by these calculations agree well with experimentally determined values; the calculations were also able to predict the electrochemical potentials of the [M(II)]/[M(III)-OH] redox transition to within about 300 mV.     

Divergent Stabilities of Tetravalent Cerium,Uranium, and Neptunium Imidophosphorane Complexes**

The study of the redox chemistry of mid-actinides (U−Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=P^tBu(pyrr)₂]⁻; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium ( 1-M , 2-M , M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M^4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U^5+/4+, U^6+/5+, and to an unprecedented, well-behaved Np^5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.     

Probing the intrinsic electronic structure of the bis(dithiolene) anions [M(mnt)2]2- and [M(mnt)2]1- (M = Ni, Pd, Pt; mnt = 1,2-S2C2(CN)2) in the gas phase by photoelectron spectroscopy

A detailed understanding of the electronic structure of transition metal bis(dithiolene) complexes is important because of their interesting redox, magnetic, optical, and conducting properties and their relevance to enzymes containing molybdenum and tungsten bis(dithiolene) centers. The electronic structures of the bis(dithiolene) anions [M(mnt)(2)](n-) (M = Ni, Pd, Pt; mnt = 1,2-S(2)C(2)(CN)(2); n = 0-2) were examined by a combination of photodetachment photoelectron spectroscopy (PES) and density functional theory calculations. The combined experimental and theoretical data provide insight into the molecular orbital energy levels of [M(mnt)(2)](2-) and the ground and excited states of [M(mnt)(2)](1-) and [M(mnt)(2)]. Detachment features from ligand-based orbitals of [M(mnt)(2)](2-) occur at similar energies for each species, independent of the metal center, while those arising from metal-based orbitals occur at higher energies for the heavier congeners. Electronic excitation energies inferred for [M(mnt)(2)](1-) from the PES experiments agree well with those obtained in optical absorption experiments in solution, with the PES experiments providing additional insight into the changes in energy of these transitions as a function of metal. The singly charged anions [M(mnt)(2)](1-) were also prepared and studied independently. Electron detachment from the ground states of these doublet anions accessed the lowest singlet and triplet states of neutral [M(mnt)(2)], thereby providing a direct experimental measure of their singlet-triplet splitting.     

Chloride supporting electrolytes for all-vanadium redox flow batteries

This paper examines vanadium chloride solutions as electrolytes for an all-vanadium redox flow battery. The chloride solutions were capable of dissolving more than 2.3 M vanadium at varied valence states and remained stable at 0-50 °C. The improved stability appeared due to the formation of a vanadium dinuclear [V(2)O(3)·4H(2)O](4+) or a dinuclear-chloro complex [V(2)O(3)Cl·3H(2)O](3+) in the solutions over a wide temperature range. The all-vanadium redox flow batteries with the chloride electrolytes demonstrated excellent reversibility and fairly high efficiencies. Only negligible, if any, gas evolution was observed. The improved energy capacity and good performance, along with the ease in heat management, would lead to substantial reduction in capital cost and life-cycle cost, making the vanadium chloride redox flow battery a promising candidate for stationary applications.     

Redox-rich spin-spin-coupled semiquinoneruthenium dimers with intense near-IR absorption

Using the [RuCl(μ-tppz)ClRu](2+) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine] platform for bridging two o-quinone/catecholate two-step redox systems (unsubstituted, Q(n), or 3,5- di-tert-butyl-substituted, DTBQ(n)), we have obtained the stable complexes [(Q(?-))Ru(II)Cl(μ-tppz)ClRu(II)(Q(?-))] (1) and the structurally characterized [(DTBQ(?-))Ru(II)Cl(μ-tppz)ClRu(II)(DTBQ(?-))] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (λ(max) ≈ 920 nm; ε > 50,000 M(-1) cm(-1)), and variable intra- and intermolecular spin-spin interactions. Density functional theory calculations, electron paramagnetic resonance (EPR), and spectroelectrochemical results (UV-vis-near-IR region) for three one-electron-reduction and two one-electron-oxidation processes were used to probe the electronic structures of the systems in the various accessible valence states. EPR spectroscopy of the singly charged doublet species showed semiquinone-type response for 1(+), 2(+), and 2(-), while 1 exhibits more metal based spin, a consequence of the easier reduction of Q as compared to DTBQ. Comparison with the analogous redox series involving a more basic N-phenyliminoquinone ligand reveals significant differences related to the shifted redox potentials, different space requirements, and different interactions between the metals and the quinone-type ligands. As a result, the tppz bridge is reduced here only after full reduction of the terminal quinone ligands to their catecholate states.     

Transition metal ion-substituted polyoxometalates entrapped in polypyrrole as an electrochemical sensor for hydrogen peroxide

A conducting polymer was used for the immobilization of various transition metal ion-substituted Dawson-type polyoxometalates (POMs) onto glassy carbon electrodes. Voltammetric responses of films of different thicknesses were stable within the pH domain 2-7 and reveal redox processes associated with the conducting polymer, the entrapped POMs and incorporated metal ions. The resulting POM doped polypyrrole films were found to be extremely stable towards redox switching between the various redox states associated with the incorporated POM. An amperometric sensor for hydrogen peroxide detection based upon the POM doped polymer films was investigated. The detection limits were 0.3 and 0.6 μM, for the Cu(2+)- and Fe(3+)-substituted POM-doped polypyrrole films respectively, with a linear region from 0.1 up to 2 mM H(2)O(2). Surface characterization of the polymer films was carried out using atomic force microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy.     

Synthesis, characterization, and photophysical properties of a series of supramolecular mixed-valence compounds

The synthesis and characterization of 10 cyano-bridged trinuclear mixed-valence compounds of the form [(NH3)5M-NC-FeII(CN)4-CN-M'(NH3)5]n+ (M = RuIII, OsIII, CrIII, or PtIV; n = 2, 3, or 4) is reported. The electronic spectra of these supramolecular compounds exhibit a single intervalent (IT) absorption band for each nondegenerate Fe-->M/M' transition. The redox potential of the Fe(II) center is shifted more positive with the addition of each coordinated metal complex, while the redox potentials of the pendant metals vary only slightly from their dinuclear counterparts. As a result, the Fe-->M IT bands are blue-shifted from those in the corresponding dinuclear mixed-valence compounds. The energies of these IT bands show a linear correlation with the ground-state thermodynamic driving force, as predicted by classical electron transfer theory. Estimates of the degree of electronic coupling (Hab) between the metal centers using a theoretical analysis of the IT band shapes indicate that most of these values are similar to those for the corresponding dinuclear species. Notable exceptions occur for the Fe-->M IT transitions in Os-Fe-M (M = Cr or Pt). The enhanced electronic coupling in these two species can be explained as a result of excited state mixing between electron transfer and/or ligand-based charge transfer states and an intensity-borrowing mechanism. Additionally, the possibility of electronic coupling between the remote metal centers in the Ru-Fe-Ru species is discussed in order to explain the observation of two closely spaced redox waves for the degenerate Ru(III) acceptors.     

How a Redox‐Innocent Metal Promotes the Formal Reductive Elimination of Biphenyl Using Redox‐Active Ligands

One of the most compelling strategies for utilizing redox‐active ligands is to perform redox events at the ligands to avoid accessing prohibitively high energy oxidation states at the metal center. This has been demonstrated experimentally in many systems, yet there is little understanding of the fundamental electronic structures involved with these transformations or how to control them. Here, the reductive elimination of biphenyl from [M(isq)₂Ph₂] (M=Ti, Zr, and Hf and isq=2,4‐di‐tert‐butyl‐6‐tert‐butyliminosemiquinone) was studied computationally. It was found that the metal remains in the +IV oxidation state and all redox chemistry was mediated by the redox‐active ligands. Two types of electron‐transfer mechanisms were identified, an asymmetric unpaired electron transfer (UET) and a symmetric pairwise electron transfer (PET), the former always being lower in energy. The energetic differences between these two mechanisms were explained through simple molecular orbital theory arguments. Despite the metal’s redox‐inactivity, it still has a marked influence on the calculated energetics of the reaction, with the Ti systems being much more reactive than the Zr/Hf systems. This primarily originates from the shorter Ti?Ph bond, which leads to a stronger filled‐filled interaction between these ligands at the reactant state. This greater reactant destabilization leads to the lower activation energies.     

Determination of U(IV) and U(VI) by ion chromatography-inductively coupled plasma mass spectrometry and its application to kinetic studies

The ions normally formed by actinides in aqueous solutions by the oxidation states III-VI are M3-, M4+, MO2+ and MO2(+2), respectively. Oxidation state representatives such as Am3+, Th4+, NpO+ and UO+, which resist oxidation state changes, were used to investigate a method to separate the oxidised species (MO2 and MO2(2+)) from the reduced species (M3+ and M4+). With this method the hexavalent state of uranium could be separated from the tetravalent state of uranium in aqueous media in less than 8 min. Uranium concentrations down to 10(-9) M could be analysed without changing the redox composition during the separation. The oxidation kinetics of the tetravalent uranium for different hydrochloric acid concentrations was investigated. The measurements showed good agreement with values found in the literature, although the uranium concentrations were one million times lower.     

Reversible Switching of Redox‐Active Molecular Orbitals and Electron Transfer Pathways in CuA Sites of Cytochrome c Oxidase

The Cu_A site of cytochrome c oxidase is a redox hub that participates in rapid electron transfer at low driving forces with two redox cofactors in nearly perpendicular orientations. Spectroscopic and electrochemical characterizations performed on first and second‐sphere mutants have allowed us to experimentally detect the reversible switching between two alternative electronic states that confer different directionalities to the redox reaction. Specifically, the M160H variant of a native Cu_A shows a reversible pH transition that allows to functionally probe both states in the same protein species. Alternation between states exerts a dramatic impact on the kinetic redox parameters, thereby suggesting this effect as the mechanism underlying the efficiency and directionality of Cu_A electron transfer in vivo. These findings may also prove useful for the development of molecular electronics.     

玻碳电极上六氰亚铁酸钯膜的现场FTIR光谱电化学研究

本文报道六氰亚铁酸钯膜修饰电极在ＨＣｌ,ＫＣｌ和ＮａＣｌ溶液中的现场反射ＦＴＩＲ光谱电化学研究,结果表明该修饰膜具有内外两层结构,分别为Ｐｄ２Ｆｅ（ＣＮ）６和Ｍ２ＰｄＦｅ（ＣＮ）６,其中Ｍ为支持电解质一价阳离子．在１ｍｏｌ／ＬＮａＣｌ中,内层的氧化电位Ｅｍ＝０．８７Ｖ（ｖｓ．Ａｇ／ＡｇＣｌ）,外层为０．７７Ｖ．在１ｍｏｌ／ＬＨＣｌ或ＫＣｌ中两层的氧化还原波重叠为一个大ＣＶ波峰而难以分辨,然而现场ＦＴＩＲ光谱电化学清晰地分辨出这两种结构在所有３种溶液中ＣＮ的不同振动频率,发现Ｈ＋离子是最佳的支持电解质,能使这两种结构同时发生氧化反应     

Darstellung und Eigenschaften der Diphthalocyaninate des Yttriums und Indiums

Synthesis and Properties of the Diphthalocyaninates of Yttrium and Indium Blue di(phthalocyaninato(2–))metalates of tervalent yttrium and indium are obtained by the reaction of yttrium acetate or anhydrous indium chloride with molten phthalodinitrile in the presence of potassium methylate and isolated as complex salts with organic cations. Anodic oxidation of (ⁿBu₄N)[M(Pc^2?)₂] (M = Y, In) yields crystals of green paramagnetic di(phthalocyaninato)metal(III)-dichloromethane solvate, [M(Pc)₂] · CH₂Cl₂ (μ_eff = 1.8/1.9 B.M. (Y/In)). Red brown di(phthalocyaninato)metal(III)-polybromide, [M(Pc^?)₂]Br_x is prepared by oxidation with bromine in excess. The redox properties of the di(phthalocyaninato)metalates(III) are investigated by cyclic voltammetry and difference pulse polarography. A quasi reversible (ΔE ? 60 mV) one electron process at 0.09 V (Y) and ?0.07 V (In) is assigned to the redox couple [M(Pc^2?)₂]^?/[M(Pc)₂]. Electronic absorption spectra as well as MIR/FIR and resonance Raman spectra are reported. The characteristic features of the three oxidation states and the influence of the ionic radius and the electron configuration of the metal ion are discussed.     

Novel hypervalent complexes of main-group metals by intramolecular ligand-->metal electron transfer

New fascinating electronic features of the simple diketoamine chelate ligand HN[CH2C(tBu)=O]2 (1) are described. Unexpectedly, the corresponding trianionic amido-dienolate form of 1 is capable of reducing main-group metal atoms M after initial coordination and intramolecular L-->M two-electron transfer and of stabilizing main-group elements in unusual low oxidation states. This is impressively shown by the synthesis and structural characterization of the novel Ge and Sn complexes 4-6 by redox reactions of lithiated 1 with the corresponding metal halides GeCl4 and MCl2 (M=Ge, Sn). Surprisingly, conversion of tris-lithiated 1 with GeCl4 readily consumes two molar equivalents of GeCl4 and results in the formation of the neutral GeCl3 complex 4 and GeCl2. The former represents the second example of a structurally characterized neutral octahedrally coordinated germanium compound. Reaction of dilithiated 1 with GeCl2 does not lead to the expected ClGe(+2) complex but affords the novel dimeric germylene 5, whereas similar reaction using SnCl2 furnishes the monomeric stannylene (ClSn(+2) complex) 2 and elemental tin due to the higher oxidation potential of Sn(+2). Unexpectedly, a similar redox reaction of dilithiated 1 with PbCl2 furnishes the first air- and water-stable lithium 1,2-diketoimine-enolate 7 and elemental lead. Compound 7 is tetrameric in the solid state and consists of a strongly distorted Li4O4 cubic core with trigonal-bipyramidal coordinated Li+ ions.     

Electronic structure and solvation of copper and silver ions: a theoretical picture of a model aqueous redox reaction

Electronic states and solvation of Cu and Ag aqua ions are investigated by comparing the Cu(2+) + e(-)--> Cu(+) and Ag(2+) + e(-)--> Ag(+) redox reactions using density functional-based computational methods. The coordination number of aqueous Cu(2+) is found to fluctuate between 5 and 6 and reduces to 2 for Cu(+), which forms a tightly bound linear dihydrate. Reduction of Ag(2+) changes the coordination number from 5 to 4. The energetics of the oxidation reactions is analyzed by comparing vertical ionization potentials, relaxation energies, and vertical electron affinities. The model is validated by a computation of the free energy of the full redox reaction Ag(2+) + Cu(+) --> Ag(+) + Cu(2+). Investigation of the one-electron states shows that the redox active frontier orbitals are confined to the energy gap between occupied and empty states of the pure solvent and localized on the metal ion hydration complex. The effect of solvent fluctuations on the electronic states is highlighted in a computation of the UV absorption spectrum of Cu(+) and Ag(+).     

Redox and spectroscopic orbitals in Ru(II) and Os(II) phenolate complexes

A detailed spectroscopic and electrochemical study of a series of novel phenolate bound complexes, of general formulas [M(L-L)(2)(box)](PF(6)), where M is Os and Ru, L-L is 2,2-bipyridine or 2,2-biquinoline, and box is 2-(2-hydroxyphenyl)benzoxazole, is presented. The objectives of this study were to probe the origin of the LUMOs and HOMOs in these complexes, to elucidate the impact of metal and counter ligand on the electronic properties of the complex, and to identify the extent of orbital mixing in comparison with considerably more frequently studied quinoid complexes. [M(L-L)(2)(box)](PF(6)) complexes exhibit a rich electronic spectroscopy extending into the near infrared region and good photostability, making them potentially useful as solar sensitizers. Electrochemistry and spectroscopy indicate that the first oxidation is metal based and is associated with the M(II)/(III) redox states. A second oxidative wave, which is irreversible at slow scan rates, is associated with the phenolate ligand. The stabilities of the oxidized complexes are assessed using dynamic electrochemistry and discussed from the perspective of metal and counter ligand (LL) identity and follow the order of increasing stability [Ru(biq)(2)(box)](+) < [Ru(bpy)(2)(box)](+) < [Os(bpy)(2)(box)](+). Electronic and resonance Raman spectroscopy indicate that the lowest energy optical transition for the ruthenium complexes is a phenolate (pi) to L-L (pi) interligand charge-transfer transition (ILCT) suggesting the HOMO is phenolate based whereas electrochemical data suggest that the HOMO is metal based. This unusual lack of correlation between redox and spectroscopically assigned orbitals is discussed in terms of metal-ligand orbital mixing which appears to be most significant in the biquinoline based complex.     

Reactivity of MnII with superoxide. Evidence for a [MnIIIOO]+ unit by low-temperature spectroscopies

Manganese superoxide dismutase cycles between the MnIII and MnII states to produce oxygen and hydrogen peroxide from superoxide. The formation of an adduct has been suggested, but its nature remains questionable because both [MnIIOO-] and [MnIIIOO2-] redox states have been proposed. Study of the reactivity of superoxide with manganese complexes is of current interest. The reaction of [(L)MnII]2+ [L = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine] with potassium superoxide has been investigated at low temperature in an anhydrous solvent using various techniques. Upon the addition of ca. 2 equiv of potassium superoxide, the [(L)MnII]2+ colorless solution turned blue and the UV-vis spectrum displayed a band at 590 nm (165 M(-1) cm(-1)) and a shoulder at 430 nm (100 M(-1) cm(-1)). Electrospray ionization mass spectrometry showed a peak (m/z = 434.1) assigned to [(L)MnO2]+. The X-band electron paramagnetic resonance spectrum parallel mode displayed a six-line signal separated by 6.6 mT and centered at 86 mT (g = 8.1). These results support the formation of an [MnIIIOO]+ adduct.     

Effect of metal ionic radius and chelate ring alternation motif on stabilization of trivalent nickel and copper in binuclear complexes with double cis-oximato bridges

Oxime ligands are able to form stable binuclear species with copper(II) ions in aqueous solution. They also have a strong tendency to decrease the Mn+/(n-1)+ redox potentials of the central ions. Ligands possessing the hydroxyimino groups together with other powerful sigma-donor groups can be very efficient chelating agents able to facilitate the stabilisation of high oxidation states of 3d-metals. Here we report the synthesis, structural characterization and redox behaviour of mononuclear and binuclear complexes based on hydroxyiminoamide tetradentate open-chain ligands. In all mononuclear anionic complexes the central atom is situated in a square-planar surrounding of four nitrogen atoms. This pseudo-macrocyclic conformation is due to the presence of short intramolecular hydrogen bonds uniting the cis-oximate oxygen atoms. The square-planar surrounding of the strong sigma-donors facilitates efficient stabilization of the trivalent state of copper and nickel ions. In cyclic voltammetry studies the quasi-reversible processes M2+-->M3+ can be observed. In the binuclear complexes the coordinatively saturated octahedral ion M[prime or minute] is bound to the two oxygen atoms of the bridging oximate groups and the four nitrogen atoms of the tetradentate ligand tren. Two metal ions (M and M') are linked by the double cis-oximate bridge and are incorporated in a six-membered bimetallic chelate ring. Metallamacrocycle formation leads to certain changes in the structural parameters of the binuclear complexes as compared to those observed in the mononuclear species. Also the study of the electrochemical activity of binuclear complexes has shown important differences in their redox behaviour as compared to their mononuclear precursors.     

Interaction of NO with nanosized Ru-, Pd-, and Pt-Doped SnO2: electron paramagnetic resonance, Mössbauer, and electrical investigation

The mechanism of NO interaction with nanosized Ru(Pd,Pt)-doped SnO(2) was studied by electron paramagnetic resonance, M?ssbauer, and electric resistance measurements. Three steps were proposed for the reaction between the semiconductor oxide and the gaseous component: (i) the formation of bielectronic oxygen vacancies (V(o)) in SnO(2); (ii) their single-ionization (V(o)(*)) with injection of electrons into the SnO(2) conduction band; (iii) the subsequent transfer of electrons from V(o)(*) to [Ru(Pd,Pt)](4+). The last process induces the formation of further oxygen vacancies which reduce the transition metal centers to lower oxidation states; the redox processes is enhanced and the electrical resistance in transition metal-doped SnO(2) is stronger modified with respect to the undoped material.     

Heterogeneous Kinetics of Metal- and Ligand-Based Redox Reactions within Adsorbed Monolayers

Dense monolayers of [Os(bpy)(2)py(p3p)](2+), where bpy is 2,2'-bipyridyl, py is pyridine, and p3p is 4,4'-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Three well-defined waves, corresponding to osmium- and bipyridyl-based redox reactions, are observed in cyclic voltammetry of these monolayers, where the supporting electrolyte is tetrabutylammonium tetrafluoroborate (TBABF(4)) dissolved in acetonitrile. These reactions correspond to the charge states 3+/2+, 2+/1+, and 1+/0, respectively. Chronoamperometry, conducted on a microsecond time scale, has been used to measure the heterogeneous electron transfer rate constant, k/s(-1), for all three redox processes. For concentrations of TBABF(4) above 0.1 M, heterogeneous electron transfer is characterized by a single unimolecular rate constant. Standard heterogeneous electron transfer rate constants, k degrees, have been evaluated by extrapolating Tafel plots of ln k vs overpotential, eta, to zero driving force to yield values of (4.8 +/- 0.3) x 10(4) s(-1), (2.5 +/- 0.2) x 10(5) s(-1), and (3.3 +/- 0.3) x 10(4) s(-1) for k degrees (3+/2+), k degrees (2+/1+), and k degrees (1+/0), respectively. For large values of eta, these Tafel plots are curved for all three redox reactions, and while those corresponding to metal-based electron transfer are asymmetric with respect to eta, those corresponding to ligand-based reactions are symmetric. Temperature-resolved measurements of k reveal that the electrochemical activation enthalpy, DeltaH(), decreases from 43.1 +/- 2.8 kJ mol(-1) for the 3+/2+ reaction to 25.8 +/- 1.9 kJ mol(-1) for the 1+/0 process. Probing the temperature dependence of the formal potential gives the reaction entropy, DeltaS(rc) degrees. The reaction entropy depends on the state of charge of the monolayer with values of 212 +/- 18, 119 +/- 9, and 41 +/- 5 J mol(-1) K(-1) being observed for the 3+/2+, 2+/1+, and 1+/0, redox transformations, respectively. The electronic transmission coefficient, kappa(el), describing the probability of electron transfer once the nuclear transition state has been reached, is considerably less than unity for all three redox processes. However, kappa(el) is larger for the bipyridyl-based reductions, (2.4 +/- 0.9) x 10(-5), than for the metal-based reaction, (1.5 +/- 0.7) x 10(-6). This large difference in electronic transmission coefficients may be a consequence of the redox potentials of the bridging ligand and the remote redox sites being comparable, so that the electronic states of the bridging ligand contribute to the electron tunneling pathway.