Oxidation of [Ir(η-C5Me5)(C3S5)] [C3S5 = 4,5-disulfanyl-1,3-dithiole-2-thionate(2−)] and X-ray crystal structure of [IrBr(η-C5Me5)(μ-C2S4)IrBr(η-C5Me5)]

[Ir(η⁵-C₅Me₅)(C₃S₅)] [C₃S₅²⁻ = 4,5-disulfanyl-1,3-dithiole-2-thionate(2−)] was prepared by a reaction of [NMe₄]₂[C₃S₅] with [Ir(η⁵- C₅Me₅)Cl₂]₂ in ethanol. It was reacted with bromine to afford a paramagnetic species [IrBr(η⁵-C₅Me₅)(C₃S₅)] with the Ir-Br bond and in the one-electron-oxidized state, and a diamagnetic dinuclear species [IrBr(η⁵-C₅Me₅)(μ-C₂S₄)IrBr(η⁵-C₅Me₅)]. ESR spectra for the one-electron-oxidized species in solution are discussed. The X-ray crystal structural analysis for the latter complex revealed the geometry consisting of dinuclear IrBr(η⁵-C₅Me₅) moieties bridged by the C₂S₄²⁻ ligand.     

Crystallographic and theoretical studies of (η-C5Me5)Ru(2,4-dimethyl-η-pentadienyl) and [(η-C5Me5)Rh(2,4-dimethyl-η-pentadienyl)][BF4]

The determination of the solid state structure of Cp*Ru(2,4-dimethyl-η⁵-pentadienyl) (1), where Cp* = pentamethylcyclopentadienyl, fills the gap in the series of previously established structures of closely related compounds. Compound 1 does not exhibit the ideal C_S symmetry and its conformation is intermediate between the C_S-synperiplanar eclipsed and C_S-antiperiplanar arrangements of the ligands. Density functional theory studies indicate that the C_S-synperiplanar eclipsed, C_S-antiperiplanar, and intermediate conformations of 1 and Cp*Rh(2,4-dimethyl-η⁵-pentadienyl)⁺ (2) do not differ by more than a few tenths of 1 kcal/mol. The geometrical features of cation 2 are similar to those of 1, and in both complexes the pentadienyl ligands are not planar. The metal-carbon distances to the Cp* ligands in 1 and 2 are comparable, while the metal-carbon distances to the pentadienyl moiety are somewhat shorter in the Ru complex. A study of the conformational flexibility of the Cp* ligand in 5610 organometallic complexes showed that it usually shields the central metal by 36.2(10)%, provided the metal-centroid(Cp*) distances are normalized to 2.28 Å. The corresponding values in 1 and 2 are 37.2% and 37.4%, respectively.     

Metal-sulfur dπ-pπ buffering of the oxidations of metal-thiolate complexes: Photoelectron spectroscopy of (η-C5H5)Fe(CO)2SR (SR = SCH3, SBu) and (η-C5H5)Re(NO)(PR3)SCH3 (PR3 = PPr3, PPh3)

The metal-sulfur bonding present in the transition metal-thiolate complexes CpFe(CO)₂SCH₃, CpFe(CO)₂S^tBu, CpRe(NO)(PⁱPr₃)SCH₃, and CpRe(NO)(PPh₃)SCH₃ (Cp = η⁵-C₅H₅) is investigated via gas-phase valence photoelectron spectroscopy. For all four complexes a strong dπ-pπ interaction exists between a filled predominantly metal d orbital of the [CpML₂]⁺ fragment and the purely sulfur 3pπ lone pair of the thiolate. This interaction results in the highest occupied molecular orbital having substantial M-S π^∗ antibonding character. In the case of CpFe(CO)₂SCH₃, the first (lowest energy) ionization is from the Fe-S π^∗ orbital, the next two ionizations are from predominantly metal d orbitals, and the fourth ionization is from the Fe-S π orbital. The pure sulfur pπ lone pair of the thiolate fragment is less stable than the filled metal d orbitals of the [CpFe(CO)₂]⁺ fragment, resulting in a Fe-S π^∗ combination that is higher in sulfur character than the Fe-S π combination. Interestingly, substitution of a tert-butyl group for the methyl group on the thiolate causes little shift in the first ionization, in contrast to the shift observed for related thiols. This is a consequence of the delocalization and electronic buffering provided by the Fe-S dπ-pπ interaction. For CpRe(NO)(PⁱPr₃)SCH₃ and CpRe(NO)(PPh₃)SCH₃, the strong acceptor ability of the nitrosyl ligand rotates the metal orbitals for optimum backbonding to the nitrosyl, and the thiolate rotates along with these orbitals to a different preferred orientation from that of the Fe complexes. The initial ionization is again the M-S π^∗ combination with mostly sulfur character, but now has considerable mixing among several of the valence orbitals. Because of the high sulfur character in the HOMO, ligand substitution on the metal also has a small effect on the ionization energy in comparison to the shifts observed for similar substitutions in other molecules. These experiments show that, contrary to the traditional interpretation of oxidation of metal complexes, removal of an electron from these metal-thiolate complexes is not well represented by an increase in the formal oxidation state of the metal, nor by simple oxidation of the sulfur, but instead is a variable mix of metal and sulfur content in the highest occupied orbital.     

Synthesis and electrochemical aspects of group 14 iron complexes of the type (η-C5H5)Fe(L)2ER3, L2 = (CO)2, (Ph2P)2CH2; E = C, Si, Ge, Sn

The dicarbonyl and diphosphine complexes of the type (η⁵-C₅H₅)Fe(L)₂ER₃ (L₂ = (CO)₂ (a), (Ph₂P)₂CH₂ (b); ER₃ = CH₃ (1a/b); SiMe₃ (2a/b), GeMe₃ (3a/b), SnMe₃ (4a/b)) were synthesized and studied electrochemically. Cyclic voltammetric studies on the dicarbonyl complexes 1a-4a revealed one electron irreversible oxidation processes whereas the same processes for the chelating phosphine series 1b-4b were reversible. The E_ox values found for the series 1a-4a were in the narrow range 1.3-1.5 V and in the order Si > Sn ≈ Ge > C; those for 1b-4b (involving replacement of the excellent retrodative π-accepting CO ligands by the superior σ-donor and poorer π-accepting phosphines) have much lower oxidation potentials in the sequence Sn > Si ≈ Ge > C. This latter oxidation potential pattern relates directly to the solution ³¹P NMR chemical shift data illustrating that stronger donation lowers the E_ox for the complexes; however, simple understanding of the trend must await the results of a current DFT analysis of the systems.     

Reaction of cyanamide and their derivatives with (η-C5H5)Mn(CO)3 and (η-C5H4CH3)Mn(CO)3

The reaction of cyanamide and its derivatives with the (η⁵-C₅H₅)Mn(CO)₂(THF) and (η⁵-C₅H₄CH₃)Mn(CO)₂(THF) complexes affords the cyanamide substituted complexes of types (η⁵-C₅H₅)Mn(CO)₂(NCN(R′)(R″)) (2a-d) and (η⁵-C₅H₄CH₃)Mn(CO)₂(NCN(R′)(R″)) (3a-e). All complexes were characterized by spectroscopy (¹H, ¹³C NMR, IR), elemental and mass spectroscopy analysis. Complex 2b (η⁵-C₅H₅)Mn(CO)₂(NCN(CH₃)₂) was additionally examined by single crystal X-ray structure determination.     

Syntheses and structural analyses of chiral rhenium containing amines of the formula (η-C5H5)Re(NO)(PPh3)((CH2)nNRR′) (n = 0, 1)

The reaction of the racemic chiral methyl complex (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₃) (1) with CF₃SO₃H and then NH₂CH₂C₆H₅ gives [(η⁵-C₅H₅)Re(NO)(PPh₃)(NH₂CH₂C₆H₅)]⁺ ([4a-H]⁺; 73%), and deprotonation with t-BuOK affords the amido complex (η⁵-C₅H₅)Re(NO)(PPh₃)(NHCH₂C₆H₅) (76%). Reactions of 1 with Ph₃C⁺ X⁻ and then primary or secondary amines give [(η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂NHRR′)]⁺ X⁻ ([6-H]⁺ X⁻; R/R′/X = a, H/NH₂CH₂C₆H₅/BF₄; a′, H/NH₂CH₂C₆H₅/PF₆; b, H/NH₂CH₂(CH₂)₂CH₃/PF₆; c, H/(S)-NH₂CH(CH₃)C₆H₅/BF₄); d, CH₂CH₃/CH₂CH₃/PF₆; e, CH₂(CH₂)₂CH₃/CH₂(CH₂)₂CH₃/PF₆; f, CH₂C₆H₅/CH₂C₆H₅/PF₆; g, -CH₂(CH₂)₂CH₂-/PF₆; h, -CH₂(CH₂)₃CH₂-/PF₆; i, CH₃/CH₂CH₂OH/PF₆ (62-99%). Deprotonations with t-BuOK afford the amines (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂NRR′) (6a-i; 99-40%), which are more stable and isolated in analytically pure form when R ≠ H. Enantiopure 1 is used to prepare (R_ReS_C)-[6c-H]⁺, (R_ReS_C)-6c, (S)-[6g-H]⁺, and (S)-6g. The crystal structures of [4a-H]⁺, a previously prepared NH₂CH₂Si(CH₃)₃ analog, [6a′,d,f,h-H]⁺, (R_ReS_C)-6c, and 6f are determined and analyzed in detail, particularly with respect to cation/anion hydrogen bonding and conformation. In contrast to analogous rhenium containing phosphines, 6a-i show poor activities in reactions that are catalyzed by organic amines.     

Oxidation of [Ir(η-C5Me5)(C8H4S8)] and crystal structures of [IrI(η-C5Me5)(C8H4S8)](I3) and [IrI(η-C5Me5)(C8H4S8)](I3)1/2(I7)1/2

[Ir(η⁵-C₅Me₅)(C₈H₄S₈)] (1) [ = 2-{(4,5-ethylenedithio)-1,3-dithiole-2-ylidene}-1,3-dithiole-4,5-dithionate(2−)] was reacted with iodine in dichloromethane to afford one-electron- and two-electron-oxidized species [IrI(η⁵-C₅Me₅)(C₈H₄S₈)] (2), [IrI(η⁵-C₅Me₅)(C₈H₄S₈)](I₃) (3) and [IrI(η⁵-C₅Me₅)(C₈H₄S₈)](I₅) (4). The oxidized species exhibit electrical conductivities of (1.1-5.0) × 10⁻⁶ S cm⁻¹ measured for compacted pellets at room temperature. The X-ray crystal structures of the two-electron-oxidized complexes 3 and 4 revealed the Ir-I bonds for both of them and the presence of for 3 and ions for 4 as the counter anions. They have many S-S and S-I non-bonding contacts to form two-dimensional molecular interaction sheets in the solid state.     

Syntheses and crystal structures of the first iridium complexes with m- and p-terphenyl (tp). {[Ir2(p-tp)(cod)2](BF4)2 · 2CH2Cl2}3 and [Ir(m-tp)(η-C5Me5)](BF4)2

Novel two iridium terphenyl complexes were prepared and their structures were characterized crystallographically. The reaction of [Ir(cod)₂]BF₄ with p-terphenyl (p-tp) in CH₂Cl₂ was carried out to afford dinuclear Ir(I) complex {[Ir₂(p-tp)(cod)₂](BF₄)₂ · 2CH₂Cl₂}₃ (cod=1,5-cyclooctadiene) (1 · 2CH₂Cl₂), whereas the reaction of the intermediate [Ir(η⁵-C₅Me₅)(Me₂CO)₃]³⁺ in Me₂CO with m-terphenyl (m-tp) was done to provide mononuclear Ir(III) complex [Ir(m-tp)(η⁵-C₅Me₅)](BF₄)₂ (2). In complex 1 · 2CH₂Cl₂, two Ir atoms are η⁶-coordinated to both sides of terminal benzene rings from the upper and lower sides in the p-tp ligand, while one Ir atom is η⁶-coordinated to one side of the terminal benzene ring in the m-tp ligand in complex 2. Each crystal structure describes the first coordination mode found in metal complexes with the m- and p-tp ligands.     

First crystallographic determination of dichloro-bridged dinuclear rhodium Cp* complex with neutral Me2CO molecules. [Rh2(Cp*)2(μ-Cl)2(Me2CO)2](BF4)2 (Cp* = η-C5Me5)

The single crystals of dichloro-bridged dinuclear Rh-Cp* complex with neutral Me₂CO molecules, [Rh₂(Cp*)₂(μ-Cl)₂(Me₂CO)₂](BF₄)₂ (Cp* = η⁵-C₅Me₅), was isolated and the structure was in first determined crystallographically.     

The reaction between orthodiphenylphosphinobenzaldehyde and [(η-C5Me5)MCl(μ-Cl)]2 (M=Rh and Ir)

The benzaldehyde functionalized phosphine Ph₂PC₆H₄CHO-2 underwent reaction with [(η⁵-C₅Me₅)MCl(μ-Cl)]₂ (M=Rh, Ir) to form (η⁵-C₅Me₅)MCl₂(κP-Ph₂PC₆H₄CHO-2), which underwent activation of the aldehyde C-H bond to form (η⁵-C₅Me₅)MCl(κP,κC-Ph₂PC₆H₄CO-2). Formally the reaction involves oxidative addition of C-H across the metal and reductive elimination of HCl. The structure of (η⁵-C₅Me₅)RhCl(κP,κC-Ph₂PC₆H₄CO-2) has been determined by single-crystal X-ray diffraction.     

Five-coordinate gold(III) complexes of the Kläui ligands [(η-C5H5)Co{P(O)(OR)2}3] (R = Me, Et)

The reactions of cycloaurated gold(III) dichloride complexes [LAuCl₂] (L = 2-C₆H₄CH₂NMe₂ or 2-C₆H₄PPh₂NPh) with monoanionic tripodal oxygen donor Kläui ligands [(η⁵-C₅H₅)Co{P(O)(OR)₂}₃]⁻ (R = Me or Et) results in the formation of cationic gold(III) salts [LAu{OP(OR)₂}₃Co(η⁵-C₅H₅)]⁺. An X-ray structure determination on [(2-C₆H₄PPh₂NPh)Au{OP(OR)₂}₃Co(η⁵-C₅H₅)]BF₄ shows that the Kläui ligand coordinates strongly to the gold through two oxygen atoms, and weakly through the third, giving the gold(III) a distorted square pyramidal geometry. This is the first structurally characterised example of this geometry for gold(III) with ligands other than those containing rigid bipyridine or phenanthroline backbones. In solution at room temperature there is rapid interchange (on the NMR timescale) between the oxygen atoms of the Kläui ligands, which is frozen out on cooling.     

Macropolyhedral boron-containing cluster chemistry: two-electron variations in intercluster bonding intimacy. Contrasting structures of 19-vertex [(η-C5Me5)HIrB18H19(PHPh2)] and [(η-C5Me5)IrB18H18(PH2Ph)]

Fused double-cluster [(η⁵-C₅Me₅)IrB₁₈H₁₈(PH₂Ph)] (8), from syn-[(η⁵-C₅Me₅)IrB₁₈H₂₀] (1) and PH₂Ph, retains the three-atoms-in-common cluster fusion intimacy of 1, in contrast to [(η⁵-C₅Me₅)HIrB₁₈H₁₉(PHPh₂)] (6), from PHPh₂ with 1, which exhibits an opening to a two atoms-in-common cluster fusion intimacy. Compound 8 forms via spontaneous dihydrogen loss from its precursor [(η⁵-C₅Me₅)HIrB₁₈H₁₉(PH₂Ph)] (7), which has two-atoms-in-common cluster-fusion intimacy and is structurally analogous to 6.     

Synthesis, characterization and crystal structure of the bimetallic cyano-bridged [(η-C5H5)(PPh3)2Ru(μ-CN)Ru(PPh3)2(η-C5H5)][PF6]

The bimetallic cyano-bridged [(η⁵-C₅H₅)(PPh₃)₂Ru(μ-CN)Ru(PPh₃)₂(η⁵-C₅H₅)][PF₆] (1) was prepared by reaction of [(η⁵-C₅H₅)(PPh₃)₂RuCl] with N,N′-bis(cyanomethyl)ethylenediamine. The single crystal structure determined by X-ray diffraction showed crystallization on the triclinic P1 space group with a perfect alignment of the cyanide bridges. This accentric crystallization was explored having in view the NLO properties at the macroscopic level, determined by the Kurtz Powder technique. Besides the very low efficiency values for the second harmonic generation, the value obtained for the bimetallic complex 1 showed to be higher than one of the parent complex [(η⁵-C₅H₅)(PPh₃)₂RuCN] (2).     

New thiocyanato and azido adducts of the redox-active Fe(η-C5Me5)(η-dppe) center: Synthesis and study of the Fe(II) and Fe(III) complexes

The new thiocyanato- (5) and azido- (6) complexes were synthesized and studied under their Fe(II) and Fe(III) redox states. For the first time among the various [Fe(η⁵-C₅Me₅)(η²-dppe)]-based cationic radicals studied so far, the magnitude and spatial orientation of the g-tensor diagonal values were experimentally determined for 5[PF₆]. These data are in good agreement with those issued from a DFT modelization. The changes experienced by the electronic structure of the Fe(II) complexes subsequent to oxidation are reminiscent of these previously observed for the known arylalkynyl analogues, albeit some differences can be pointed out. Thus, the differences observed in the ¹H NMR spectra of 5[PF₆] and 6[PF₆] are attributed to a slower electronic spin relaxation and to the differently oriented magnetic anisotropy. The sizeable spin density evidenced by DFT on the terminal atom of the ligands of the Fe(III) complexes renders these new paramagnetic metallo-ligands quite appealing for accessing larger polynuclear molecular assemblies with magnetically interacting centers.     

Anionic ruthenium(II) alkyls with ancillary diene and dienyl ligands: Synthesis and structures of [(η,η-C7H8)RuMe4] and [(η,η-C8H11)RuMe3]

Treatment of the ruthenium(II) diene complexes [(η²,η²-nbd)RuCl₂]_n or [(η²,η²-cod)RuCl₂]_n with 4 equiv. of methyllithium in the presence of N,N,N′,N′-tetramethylethylenediamine (tmed) yields the methyl complexes [Li(tmed)]₂[(η²,η²-nbd)RuMe₄] (1) and [Li(tmed)]₂[(η³,η²-C₈H₁₁)RuMe₃] (2), respectively, where nbd = norbornadiene and cod = 1,5-cyclooctadiene. In the latter compound, the cyclooctadiene ligand has been deprotonated to afford a η³,η²-1,2,3:5,6-cyclooctadienyl group. Both complexes were studied by ¹H and ¹³C{¹H} NMR spectroscopy, and the crystal structure of 2 was determined. One lithium atom in 2 is four-coordinate and bridges between one ruthenium-bound methyl group and one of the wingtip allylic carbon atoms in the η³,η²-C₈H₁₁ ligand. The other lithium atom is five-coordinate, and forms contacts with the other two Ru-Me groups and with the other wingtip carbon atom of the allyl unit.     

Synthesis, characterization and applications in ethylene polymerization of asymmetric ansa-titanocene complexes. Molecular structure of [Ti{Me2Si(η-C5Me4)(η-C5H3iPr)}Cl2]

The ansa-titanocene complexes, [Ti{Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃R)}Cl₂] (R = Me (5), iPr (6), tBu (7), SiMe₃ (8)), were obtained from the reaction of Li₂{Me₂Si(C₅Me₄)(C₅H₃R)} (R = Me (1), iPr (2), tBu (3), SiMe₃ (4)) with [TiCl₄(THF)₂], respectively. Compounds 5-8 have been tested as catalysts in the polymerization of ethylene and compared with the ansa-titanocene complexes [Ti{Me₂Si(η⁵-C₅H₄)₂}Cl₂] and [Ti{Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₄)}Cl₂]. The resulting polyethylene showed molecular weights of about 200 000 g mol⁻¹ and polydispersity values of approximately 3. In addition, the molecular structure of 6 has been determined by single crystal X-ray diffraction studies.     

A facile reaction of bibenzyl trisulfide with [(η-C5H5)Cr(CO)3]2: formation of a thiolato-bridged dichromium complex of [CpCr(CO)2(SBz)]2 (Bz = PhCH2-)

The reaction of [CpCr(CO)₃]₂ (Cp = η⁵-C₅H₅) (1) with an equivalent of Bz₂S₃ at ambient temperature gave [CpCr(CO)₂]₂S (3) [L.Y. Goh, T.W. Hambley, G.B. Robertson, Organometallics 6 (1987) 1051], novel complexes of [CpCr(CO)₂(SBz)]₂ (4) and together with [CpCr(SBz)]₂S (5) as main products. Thermolytic studies showed that 4 underwent complete decarbonylation to give [CpCr(SBz)]₂S (5). Final thermal decomposition of 3 and 5 eventually yielded Cp₄Cr₄S₄ (6) (Goh et al., 1987) after prolonged reaction at 100 °C. However, the reaction of [CpCr(CO)₂]₂ (CrCr) (2) with Bz₂S₃ was much slower at ambient temperature which required 72 h to complete eventually yielding 3 and 5. All the products have been characterized by elemental and spectral analyses. 4 has been structurally determined.     

Syntheses and crystal structures of mononuclear [2.2]paracyclophane complexes of rhodium and iridium supported by the pentamethylcyclopentadienyl ligand [M(η-pcp)(η-C5Me5)](BF4)2 (M=Rh and Ir)

Mononuclear [2.2]paracyclophane complexes of Rh and Ir, [M(η⁶-pcp)(η⁵-C₅Me₅)](BF₄)₂ (M=Rh (1) and Ir (2); pcp=[2.2]paracyclophane) were crystallized and their structures were first characterized crystallographically. On both pcp complexes the metal atom is bonded to the benzene ring on one side of the pcp ligand in the η⁶-coordination mode. The metal atom is also supported by the η⁵-C₅Me₅ ligand to afford a triple-decker sandwich structure. In Rh pcp complex 1 the average RhC(pcp) and RhC(C₅Me₅) distances are 2.284(2) and 2.161(2) Å, respectively. The average C(pcp)C(pcp) distance of 1.407(4) Å with the Rh atom is longer than that (1.388(4) Å) without a Rh atom. Similarly, the average IrC(pcp) and IrC(C₅Me₅) distances in Ir pcp complex 2 are 2.275(3) and 2.174(3) Å, respectively. The average C(pcp)C(pcp) distance of 1.410(4) Å with the Ir atom is longer than that (1.388(4) Å) without an Ir atom. It is interesting that the average interannular distances of 2.97 Å for 1 and 2 between two decks of the pcp ligand are shorter than that (3.09 Å) of the metal-free pcp ligand, indicative of the decrease of the repulsive π-interaction between benzene rings. The Rh pcp complex gave the well-resolved ¹H NMR signals of [Rh(η⁶-pcp)(η⁵-C₅Me₅)]²⁺, whereas the Ir pcp complex exhibited two kinds of ¹H NMR signals which were assigned as [Ir(η⁶-pcp)(η⁵-C₅Me₅)]²⁺ and [Ir₂(η⁶-pcp)(η⁵-C₅Me₅)₂]⁴⁺ in (CD₃)₂CO at 23 °C.     

Adduct formation of [(η-C7H7)Zr(η-C5H5)] with phosphines and N-heterocyclic carbenes: An experimental and theoretical study

The reaction of [(η⁷-C₇H₇)Zr(η⁵-C₅H₅)] with two Lewis bases, tetramethylimidazolin-2-ylidene and PMe₃, is reported and their stability probed via spectroscopic and theoretical methods. The strongly σ-basic N-heterocyclic carbene forms a stable adduct which has been structurally characterised, whilst the PMe₃ ligand coordinates weakly to the metal centre. Variable temperature ³¹P NMR spectroscopy has been used to determine the activation energy for this process (ΔG^‡ = 40.5 ± 1.9 kJ mol⁻¹). DFT calculations have been performed on both complexes and the structures discussed. In addition, the enthalpies for the formation of these compounds have been calculated [ΔH⁰(Zr-IMe) = −56.3 kJ mol⁻¹; ΔH⁰(Zr-PMe₃) = −2.3 kJ mol⁻¹] and show that the N-heterocyclic carbene forms a thermodynamically much more stable adduct than that with PMe₃.     

Mixed-valent and radical states of complexes [(bpy)2M(μ-abpy)M′(bpy)2], M,M′ = Ru or Os, abpy = 2,2′-azobispyridine: Electron transfer vs. hole transfer mechanism in azo ligand-bridged complexes

The title complexes were obtained as M^IIM′^II species [(bpy)₂M(μ-abpy)M′(bpy)₂](PF₆)₄, M,M′ = Ru or Os, using the new mononuclear precursor [(bpy)₂Os(abpy)](PF₆)₂ for the osmium-containing dinuclear complexes. One-electron reduction produces radical complexes [(bpy)₂M(μ-abpy)M′(bpy)₂]³⁺ and [(bpy)₂M(abpy)]⁺ with significant contributions from the metals, as evident from the EPR effects on successive replacement of ruthenium by osmium with its much higher spin-orbit coupling constant. The diruthenium and diosmium radical complexes were also studied by EPR at high-frequency (285 GHz), the latter shows an unusually large g anisotropy g₁ − g₃ = 0.25 in frozen solution. Further reduction was monitored by UV/Vis spectroelectrochemistry. Oxidation produced Os^III EPR signals for [(bpy)₂Os(abpy)]³⁺ and [(bpy)₂Os(μ-abpy)Ru(bpy)₂]⁵⁺, indicating a Ru^IIOs^III species for the latter. The diosmium(III,II) and diruthenium(III,II) mixed-valent species remained EPR silent at 4 K, however, they exhibit weak inter-valence charge transfer (IVCT) bands at about 1460 nm. Whereas the cyclic voltammetric response towards reduction is only marginally different for the three dinuclear complexes, successive replacement of ruthenium by osmium causes the first oxidation potential to decrease. The much higher comproportionation constant K_c for the mixed valent diosmium(III,II) state (K_c > 10¹⁵) in comparison to the diruthenium(III,II) analogue with K_c = 10¹⁰ confirms the electron transfer alternative for the valence exchange mechanism, in contrast to the hole transfer established for analogous dinuclear complexes with the formally related diacylhydrazido(2−) bridging ligands.