Diruthenium Complexes [{(acac)2RuIII}2(μ-OC2H5)2], [{(acac)2RuIII}2(μ-L)](ClO4)2, and [{(bpy)2RuII}2(μ-L)](ClO4)4 [L = (NC5H4)2−N−C6H4−N−(NC5H4)2, acac = Acetylacetonate, and bpy = 2,2‘-Bipyridine]. Synthesis, Structure, Magnetic, Spectral, and Photophysical Aspects

Independent Tuning of the pKa or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes

Two series of RuII(acac)2(py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|ΔE1/2| ≫ |ΔpKa|·0.059 V) while the changes to the imidazole primarily affect its acidity (|ΔpKa|·0.059 V ≫ |ΔE1/2|). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.

Unprecedented metal-metal bonded {Ru4(μ3-O)2} butterfly core in oxido-carboxylato bridged mixed valence cluster-structural elucidation and electronic forms in accessible redox states

Unprecedented metal-metal bonded oxido-carboxylato bridged mixed valence tetraruthenium cluster [(acac)₆Ru(μ₃-O)₂(μ-CH₃COO)₃] 1 (S = 1/2) (acac = acetylacetonate) with {Ru₄(μ₃-O)₂} "butterfly" core has been achieved via the reaction of Ru(acac)₂(CH₃CN)₂ with excess CH₃COONa (Ru : CH₃COONa = 1 : 15) in refluxing EtOH-H₂O (5 : 1). Structural analysis of 1 ascertained a unique Ru-Ru bonded (Ru2-Ru3: 2.5187(6) Å (DFT: 2.560 Å)) {Ru₂(Ru-Ru)(μ₃-O)₂} butterfly core, unlike the reported other Fe₄ or Mn₄ derived "butterfly" core. The connectivity of Ru₄(μ₃-O)₂ core in 1 with three Ru₂(μ-CH₃COO)₃ and two each Ru-(acac)₂/Ru-acac units resulted in interconnected four RuO₆ octahedral entities. The doubly bridged μ₃-O^2- ions of the nearly planar central metal-metal bonded Ru₂O₂ core (Ru2-Ru3, "body" or "hinge") linked to the remaining two "wing-tip" Ru atoms (Ru1 and Ru4). Complex 1 with a S = 1/2 spin state displayed paramagnetically shifted ¹H NMR over a wide chemical shift range in CDCl₃ (δ, 13 to -30 ppm) and a metal based anisotropic EPR (g₁ = 2.17, g₂ = 2.01, g₃ = 1.86; Δg = g₁-g₃ = 0.31 and 〈g〉 = [1/3(g₁² + g₂² + g₃²]^1/2 = 2.01) at 100 K in CH₃CN-toluene. The metal based one-electron reversible oxidation at 0.49 V and reduction at 0.485 V versus SCE of 1 led to the EPR inactive (even at 4 K) spin-coupled Ru^IIIRu^IIIRu^IVRu^IV (S = 0) and Ru^IIIRu^IIIRu^IIIRu^III (S = 0) electronic configurations for 1⁺ and 1^-, respectively. Mixed valence 1 and 1⁺ exhibited low-energy near-infrared (NIR) absorption bands at 1350 nm and 1156 nm, respectively, in CH₃CN. A combined experimental (UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) analysis indicated a delocalised mixed valence form of 1 ({Ru₂(Ru-Ru)(μ₃-O)₂}).

A Diruthenium Complex of an N-Pyridyl-o-aminophenol Derivative: A Route to Mixed Valency

An N-pyridyl-o-aminophenol derivative that stabilises mixed-valence states of ruthenium ions is disclosed. A diruthenium complex, [(L_IQ ⁰ )Ru₂ Cl₅ ]⋅MeOH (1⋅MeOH) is successfully isolated, in which L_IQ ⁰ is the o-iminobenzoquinone form of 2-[(3-nitropyridin-2-yl)amino]phenol (L_AP H₂ ). In 1, L_IQ ⁰ oriented towards one ruthenium centre is a non-innocent NO-donor redox ligand, whereas another oriented towards another ruthenium centre is an innocent pyridine-donor redox ligand. Complex 1 is a diruthenium(II,III) mixed-valence complex, [Ru^II (L_IQ ⁰ )(μ-Cl)₂ Ru^III ], with a minor contribution from the diruthenium(III,III) state. [Ru^III (L_ISQ ^.- )(μ-Cl)₂ Ru^III ] contains L_ISQ ^.- , which is the o-iminobenzosemiquinonate anion radical form of the ligand. Complexes 1^- and 1⁺ are diruthenium(II,II), [Ru^II (L_IQ ⁰ )(μ-Cl)₂ Ru^II ], and diruthenium(III,III), [Ru^III (L_IQ ⁰ )(μ-Cl)₂ Ru^III ], complexes, respectively, of L_IQ ⁰ . Complex 1^2- is a diruthenium(II,II) complex of the o-iminobenzosemiquinonate anion radical (L_ISQ ^.- ), [Ru^II (L_ISQ ^.- )(μ-Cl)₂ Ru^II ], with a minor contribution from the diruthenium(III,II) form, [Ru^III (L_AP ^2- )(μ-Cl)₂ Ru^II ]. Complex 1²⁺ is a diruthenium(III,IV) mixed-valence complex of L_IQ ⁰ , [Ru^III (L_IQ ⁰ )(μ-Cl)₂ Ru^IV ]. Complexes 1 and 1²⁺ exhibit inter-valence charge-transfer transitions at λ=1300 and 1370 nm, respectively.

2,2'-Dipyridylamines: more than just sister members of the bipyridine family. Applications and achievements in homogeneous catalysis and photoluminescent materials

2,2'-Dipyridylamines (dpa) and related compounds belong to the family of polydentate nitrogen ligands. More than a century has passed since their first report but new complexes and applications have been emerging in recent years owing to the versatility of dpa-based architectures. This review aims to present and highlight the main achievements attained with dpa-containing metal complexes in the domains of homogeneous catalysis and luminescent materials.

Triply bridged dinuclear ruthenium complexes bearing alkylbis(2-pyridylmethyl)amine in the mixed-valence state of Ru(ii)–Ru(iii)

Five diruthenium complexes in the mixed-valence state of Ru(ii)–Ru(iii), triply bridged by halogeno and methoxido ligands, were newly synthesized and compared.

Sensitivity of the valence structure in diruthenium complexes as a function of terminal and bridging ligands

The compounds [(acac)2Ru(III)(μ-H2L(2-))Ru(III)(acac)2] (rac, 1, and meso, 1') and [(bpy)2Ru(II)(μ-H2L(•-))Ru(II)(bpy)2](ClO4)3 (meso, [2](ClO4)3) have been structurally, magnetically, spectroelectrochemically, and computationally characterized (acac(-) = acetylacetonate, bpy = 2,2'-bipyridine, and H4L = 1,4-diamino-9,10-anthraquinone). The N,O;N',O'-coordinated μ-H2L(n-) forms two β-ketiminato-type chelate rings, and 1 or 1' are connected via NH···O hydrogen bridges in the crystals. 1 exhibits a complex magnetic behavior, while [2](ClO4)3 is a radical species with mixed ligand/metal-based spin. The combination of redox noninnocent bridge (H2L(0) → → → →H2L(4-)) and {(acac)2Ru(II)} → →{(acac)2Ru(IV)} or {(bpy)2Ru(II)} → {(bpy)2Ru(III)} in 1/1' or 2 generates alternatives regarding the oxidation state formulations for the accessible redox states (1(n) and 2(n)), which have been assessed by UV-vis-NIR, EPR, and DFT/TD-DFT calculations. The experimental and theoretical studies suggest variable mixing of the frontier orbitals of the metals and the bridge, leading to the following most appropriate oxidation state combinations: [(acac)2Ru(III)(μ-H2L(•-))Ru(III)(acac)2](+) (1(+)) → [(acac)2Ru(III)(μ-H2L(2-))Ru(III)(acac)2] (1) → [(acac)2Ru(III)(μ-H2L(•3-))Ru(III)(acac)2](-)/[(acac)2Ru(III)(μ-H2L(2-))Ru(II)(acac)2](-) (1(-)) → [(acac)2Ru(III)(μ-H2L(4-))Ru(III)(acac)2](2-)/[(acac)2Ru(II)(μ-H2L(2-))Ru(II)(acac)2](2-) (1(2-)) and [(bpy)2Ru(III)(μ-H2L(•-))Ru(II)(bpy)2](4+) (2(4+)) → [(bpy)2Ru(II)(μ-H2L(•-))Ru(II)(bpy)2](3+)/[(bpy)2Ru(II)(μ-H2L(2-))Ru(III)(bpy)2](3+) (2(3+)) → [(bpy)2Ru(II)(μ-H2L(2-))Ru(II)(bpy)2](2+) (2(2+)). The favoring of Ru(III) by σ-donating acac(-) and of Ru(II) by the π-accepting bpy coligands shifts the conceivable valence alternatives accordingly. Similarly, the introduction of the NH donor function in H2L(n) as compared to O causes a cathodic shift of redox potentials with corresponding consequences for the valence structure.

Electronic structures and selective fluoride sensing features of Os(bpy)2(HL2−) and [{Os(bpy)2}2(μ-HL2−)]2+ (H3L: 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid)

The non-innocence of coordinated HL²⁻ in [(bpy)₂Os(HL²⁻)]ⁿ (1ⁿ) and (2ⁿ) and their selectivity towards F⁻ recognition were evaluated.

Strong metal-metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1-5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1(+)-5(+) in the NIR region at 1660 nm-1750 nm (ε ≈ 2700 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 1800 cm(-1)). DFT calculations of 1(+) and 3(+) yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) for 1(+)-5(+) are about 3000 cm(-1), comparable to that for the Creutz-Taube ion at 3185 cm(-1). The DFT calculations predict that the Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

Theoretical Determination of the Excited States and of g-Factors of the Creutz−Taube Ion, [(NH3)5−Ru−pyrazine−Ru−(NH3)5]5+

The Creutz-Taube complex [(NH3)5-Ru-pyrazine-Ru-(NH3)5]5+ is studied using wave function-based methods, namely the CASSCF/MS-CASPT2 method. Spin-orbit effects have been calculated with the SO-RASSI program. The nature of the ground state is analyzed, and all the excited states up to 50,000 cm(-1) are calculated. They form a quasi continuum from 25,000 cm(-1), theoretical bands are assigned to UV-visible spectra, and MCD bands are assigned by calculating transition moments from first principles for both spectroscopies. g-Factors are calculated from first principles and modeled by a model Hamiltonian: they compare well to experimental values and are shown to be the same as in the monomeric species.

Complex series [Ru(tpy)(dpk)(X)]n+ (tpy = 2,2':6',2''-terpyridine; dpk = 2,2'-dipyridyl ketone; X = Cl-, CH3CN, NO2(-), NO+, NO*, NO-): substitution and electron transfer, structure, and spectroscopy

The complex framework [Ru(tpy)(dpk)]2+ has been used to study the generation and reactivity of the nitrosyl complex [Ru(tpy)(dpk)(NO)]3+ ([4]3+). Stepwise conversion of the chloro complex [Ru(tpy)(dpk)(Cl)]+ ([1]+) via [Ru(tpy)(dpk)(CH3CN)]2+ ([2]2+) and the nitro compound [Ru(tpy)(dpk)(NO2)]+ ([3]+) yielded [4]3+; all four complexes were structurally characterized as perchlorates. Electrochemical oxidation and reduction was investigated as a function of the monodentate ligand as was the IR and UV-vis spectroscopic response (absorption/emission). The kinetics of the conversion [4]3+/[3]+ in aqueous environment were also studied. Two-step reduction of [4]3+ was monitored via EPR, UV-vis, and IR (nu(NO), nu(CO)) spectroelectrochemistry to confirm the {RuNO}7 configuration of [4]2+ and to exhibit a relatively intense band at 505 nm for [4]+, attributed to a ligand-to-ligand transition originating from bound NO-.

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (tptz)-Derived [RuII(tptz)(acac)(CH3CN)]+ and Mixed-Valent [(acac)2RuIII{(μ-tptz-H+)-}RuII(acac)(CH3CN)]+

Mononuclear [Ru(II)(tptz)(acac)(CH3CN)]ClO4 ([1]ClO4) and mixed-valent dinuclear [(acac)2Ru(III){(mu-tptz-Eta+)-}Ru(II)(acac)(CH3CN)]ClO4 ([5]ClO4; acac = acetylacetonate) complexes have been synthesized via the reactions of Ru(II)(acac)2(CH3CN)2 and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz), in 1:1 and 2:1 molar ratios, respectively. In [1]ClO4, tptz binds with the Ru(II) ion in a tridentate N,N,N mode (motif A), whereas in [5]ClO4, tptz bridges the metal ions unsymmetrically via the tridentate neutral N,N,N mode with the Ru(II) center and cyclometalated N,C- state with the Ru(III) site (motif F). The activation of the coordinated nitrile function in [1]ClO4 and [5]ClO4 in the presence of ethanol and alkylamine leads to the formation of iminoester ([2]ClO4 and [7]ClO4) and amidine ([4]ClO4) derivatives, respectively. Crystal structure analysis of [2]ClO4 reveals the formation of a beautiful eight-membered water cluster having a chair conformation. The cluster is H-bonded to the pendant pyridyl ring N of tptz and also with the O atom of the perchlorate ion, which, in turn, makes short (C-H- - - - -O) contacts with the neighboring molecule, leading to a H-bonding network. The redox potentials corresponding to the Ru(II) state in both the mononuclear {[(acac)(tptz)Ru(II)-NC-CH3]ClO4 ([1]ClO4) >> [(acac)(tptz)Ru(II)-NH=C(CH3)-OC2H5]ClO4 ([2]ClO4) > [(acac)(tptz)Ru(II)-NH2-C6H4(CH3)]ClO4 ([3]ClO4) > [(acac)(tptz)Ru(II)-NH=C(CH3)-NHC2H5]ClO4 ([4]ClO4)} and dinuclear {[(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(NC-CH3)]ClO4 ([5]ClO4), [(acac)2Ru(III){(mu-tptz-H+(N+-O-)2)-}Ru(II)(acac)(NC-CH3)]ClO4 ([6]ClO4), [(acac)2Ru(III){(mu-tptz-H+)-}Ru(II)(acac)(NH=C(CH3)-OC2H5)]ClO4 ([7]ClO4), and [(acac)2Ru(III){(mu-tptz-Eta+)-}Ru(II)(acac)(NC4H4N)]ClO4 ([8]ClO(4))} complexes vary systematically depending on the electronic nature of the coordinated sixth ligands. However, potentials involving the Ru(III) center in the dinuclear complexes remain more or less invariant. The mixed-valent Ru(II)Ru(III) species ([5]ClO4-[8]ClO4) exhibits high comproportionation constant (Kc) values of 1.1 x 10(12)-2 x 10(9), with substantial contribution from the donor center asymmetry at the two metal sites. Complexes display Ru(II)- and Ru(III)-based metal-to-ligand and ligand-to-metal charge-transfer transitions, respectively, in the visible region and ligand-based transitions in the UV region. In spite of reasonably high K(c) values for [5]ClO4-[8]ClO4, the expected intervalence charge-transfer transitions did not resolve in the typical near-IR region up to 2000 nm. The paramagnetic Ru(II)Ru(III) species ([5]ClO4-[8]ClO4) displays rhombic electron paramagnetic resonance (EPR) spectra at 77 K (g approximately 2.15 and Deltag approximately 0.5), typical of a low-spin Ru(III) ion in a distorted octahedral environment. The one-electron-reduced tptz complexes [Ru(II)(tptz.-)(acac)(CEta3CN)] (1) and [(acac)2Ru(III){(mu-tptz-Eta+).2-}Ru(II)(acac)(CH3CN)] (5), however, show a free-radical-type EPR signal near g = 2.0 with partial metal contribution.

Controlling Metal–Ligand–Metal Oxidation State Combinations by Ancillary Ligand (L) Variation in the Redox Systems [L2Ru(μ-boptz)RuL2]n, boptz=3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine, and L=acetylacetonate, 2,2′-bipyridine, or 2-phenylazopyridine

The new compounds [(acac)2Ru(mu-boptz)Ru(acac)2] (1), [(bpy)2Ru(mu-boptz)Ru(bpy)2](ClO4)2 (2-(ClO4)2), and [(pap)2Ru(mu-boptz)Ru(pap)2](ClO4)2 (3-(ClO4)2) were obtained from 3,6-bis(2-hydroxyphenyl)-1,2,4,5-tetrazine (H2boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J = -36.7 cm(-1)) Ru(III) centers. We have investigated the role of both the donor and acceptor functions containing the boptz2- bridging ligand in combination with the electronically different ancillary ligands (donating acac-, moderately pi-accepting bpy, and strongly pi-accepting pap; acac = acetylacetonate, bpy = 2,2'-bipyridine pap = 2-phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal-ligand-metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru(mu-boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co-ligand for both Ru(III) and Ru(II) is demonstrated by the adoption of the mixed-valent form in [L2Ru(mu-boptz)RuL2]3+, L = bpy, whereas the corresponding system with pap stabilizes the Ru(II) states to yield a phenoxyl radical ligand and the compound with L = acac- contains two Ru(III) centers connected by a tetrazine radical-anion bridge.

A New Coordination Mode of the Photometric Reagent Glyoxalbis(2-hydroxyanil) (H2gbha): Bis-Bidentate Bridging by gbha2- in the Redox Series {(μ-gbha)[Ru(acac)2]2}n (n = −2, −1, 0, +1, +2), Including a Radical-Bridged Diruthenium(III) and a RuIII/RuIV Intermediate

The bis-bidentate bridging function of gbha2- with N,O-/N,O- coordination was observed for the first time in the complex (mu-gbha)[Ru(III)(acac)2]2 (1). Density functional theory calculations of 1 yield a triplet ground state with a large (deltaE > 6000 cm(-1)) singlet-triplet gap. Intermolecular antiferromagnetic coupling was observed (J approximately -5.3 cm(-1)) for the solid. Complex 1 undergoes two one-electron reduction and two one-electron oxidation steps; the five redox forms [(mu-gbha)[Ru(acac)2]2]n (n = -2, -1, 0, +1, +2) were characterized by UV-vis-NIR spectroelectrochemistry (NIR = near infrared). The paramagnetic intermediates were also investigated by electron paramagnetic resonance (EPR) spectroscopy. The monoanion with a comproportionation constant K(c) of 2.7 x 10(8) does not exhibit an NIR band for a Ru(III)/Ru(II) mixed-valent situation; it is best described as a 1,4-diazabutadiene radical anion containing ligand gbha*3-, which binds two ruthenium(III) centers. A Ru(III)-type EPR spectrum with g1 = 2.27, g2 = 2.21, and g3 = 1.73 is observed as a result of antiferromagnetic coupling between one Ru(III) and the ligand radical. The EPR-active monocation (K(c) = 1.7 x 10(6)) exhibits a broad (deltanu(1/2) = 2600 cm(-1)) intervalence charge-transfer band at 1800 nm, indicating a valence-averaged (Ru3.5)2 formulation (class III) with a tendency toward class II (borderline situation).

3,6-Bis(2′-pyridyl)pyridazine (L) and its deprotonated form (L − H+)−as ligands for {(acac)2Run+} or {(bpy)2Rum+}: investigation of mixed valency in [{(acac)2Ru}2(μ-L − H+)]0and [{(bpy)2Ru}2(μ-L − H+)]4+by spectroelectrochemistry and EPR

Crystallographically characterised 3,6-bis(2'-pyridyl)pyridazine (L) forms complexes with {(acac)2Ru} or {(bpy)2Ru2+}via one pyridyl-N/pyridazyl-N chelate site in mononuclear Ru(II) complexes (acac)2Ru(L), 1, and [(bpy)2Ru(L)](ClO4)2, [3](ClO4)2. Coordination of a second metal complex fragment is accompanied by deprotonation at the pyridazyl-C5 carbon {L --> (L - H+)-} to yield cyclometallated, asymmetrically bridged dinuclear complexes [(acac)2Ru(III)(mu-L - H+)Ru(III)(acac)2](ClO4), [2](ClO4), and [(bpy)2Ru(II)(mu-L - H+)Ru(II)(bpy)2](ClO4)3, [4](ClO4)3. The different electronic characteristics of the co-ligands, sigma donating acac- and pi accepting bpy, cause a wide variation in metal redox potentials which facilitates the isolation of the diruthenium(III) form in [2](ClO4) with antiferromagnetically coupled Ru(III) centres (J = -11.5 cm(-1)) and of a luminescent diruthenium(II) species in [4](ClO4)3. The electrogenerated mixed-valent Ru(II)Ru(III) states 2 and [4]4+ with comproportionation constants Kc > 10(8) are assumed to be localised with the Ru(III) ion bonded via the negatively charged pyridyl-N/pyridazyl-C5 chelate site of the bridging (L - H+)- ligand. In spectroelectrochemical experiments they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g1-g3 approximatly 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the pi acceptor ligated system [4]4+, signifying stabilised metal d orbitals.