The synthesis and structural characterization of the λ⁵-bis(phosphine sulfide) and the bimetalliccomplexes bis[phosphino-M(CO)₅] (M = Mo, W) of the 3,4-dimethyltetrathiafulvalene (ortho-DMTTF)-based rigid dimer (PPh)₂(o-DMTTF)₂, containing a central 1,4-dihydro-1,4-diphosphi-nine ring, are described. Single-crystal X-ray analyses have been performed for the trans isomers(PhPX)₂(o-DMTTF)₂ (X = S, Mo(CO)₅, and W(CO)₅) and for the cis isomer [PhPW(CO)₅]_2-(o-DMTTF)₂. Planar or slightly folded boat-type conformations are observed for the central six-membered ring, together with different packings characterized by short intermolecular S · · · Scontacts. The optical signature of the oxidized species in the case of the free ligand (PPh)_2-(o-DMTTF)₂ has been evidenced by UV-vis spectroelectrochemistry measurements. SolutionEPR measurements on the radical cation species of (PPh)₂(o-DMTTF)₂ definitely assess the fulldelocalization of the unpaired electron over both electroactive TTF units, with an associatedcoupling of 0.48 G with 12 equivalent protons. The EPR signal of the dication proves the radicalnature of this species, in favor of a triplet ground state. The radical cation of the cis-[PhPW(CO)₅]_2-(o-DMTTF)₂ isomer was also investigated by EPR, for which the observed hyperfine structuredemonstrates the extended delocalization of the electron, together with a larger coupling constantwith the phosphorus nuclei. DFT calculations for the radical cation of (PPh)₂(o-DMTTF)₂ afford aboat-type conformation for the central ring and a SOMO consistent with a full delocalization of theelectron over both TTF units. Moreover, the calculations indicate that in the case of the dication of(PPh)₂(o-DMTTF)₂ the triplet state is more stable by 11.7 kcal mol^-1 than the singlet state.

⁷⁷Se-enriched CpNi(bds) (bds = 1,2-benzenediselenolate), has been synthesized and its g tensor and ⁷⁷Se hyperfine tensors have been obtained from its frozen solution electron paramagnetic resonance (EPR) spectrum. These parameters are consistent with those calculated by density functional theory (DFT); it is shown that 10% of the spin is localized on each selenium and that the direction associated to the maximum ⁷⁷Se couplings is aligned along the g_min direction, perpendicular to the Ni(bds) plane. EPR measurements and DFT calculations are also carried out on the ⁷⁷Se enriched complex CpNi(dsit) as well on the two dithiolene analogues CpNi(bdt) and CpNi(dmit). The optimized structures of the isolated CpNi(bds) and CpNi(bdt) complexes have been used to generate the idealized dimers (bds)NiCp···CpNi(bds) and (bdt)NiCp···CpNi(bdt) characterized by Cp···Cp overlap. The exchange parameters J calculated at the DFT level for these systems are in reasonable accord with the experimental values. The influence of the geometry of the dimer on its magnetic properties is assessed by calculating the variation of J as a function of the relative orientation of the two Ni(diselenolene) or Ni(dithiolene) planes.

The synthesis and characterization of two ortho-dimethyltetrathiafulvalene (o-DMTTF)-based rigid dimers containing dimethylsilicon (Me₂Si) or dimethylgermanium (Me₂Ge) linkers are described. Single-crystal X-ray analysis reveals planar geometry for the central 1,4-disilicon or 1,4-digermanium six-membered rings. DFT calculations provide optimized conformations in agreement with the experimental ones, and also emphasize the role of the heteroatomic linkers in the conjugation between the two redox active units. Cyclic voltammetry measurements show sequential oxidation into radical cation, and then dication species. Solution EPR measurements on the radical-cation species indicate full delocalization of the unpaired electron over both electroactive TTF units, with an associated coupling of 0.42 G with twelve equivalent protons. DFT calculations afford fully planar geometry for the radical-cation species and confirm the experimental isotropic coupling constant. Single-crystal X-ray analyses of two charge-transfer compounds obtained upon chemical oxidation, formulated as [(Me₂Si)₂(o-DMTTF)₂]-1/2[TCNQ]·1/2[TCNQF₄] and [(Me₂Ge)₂(o-DMTTF)₂]·[TCNQ], demonstrate the occurrence of genuine mixed-valence radical-cation species, as well as a three-dimensional network of short S···S intermolecular contacts. Temperature-dependent conductivity measurements demonstrate semiconducting behavior for both charge-transfer compounds, with an increase of the absolute value of the conductivity upon applying external pressure. Band structure calculations reveal peculiar pseudo-two-dimensional electronic structures, also confirming electronic interactions through SiMe₂ and GeMe₂ bridges.

Neutral, radical [CpNi(dithiolene)] complexes fused with seven-membered rings, formulated as [CpNi{S₂C₂S₂(CH₂)₂X}] (X = CH₂, CF₂, C=CH₂, S), have been synthesized in 30-60 % yields from the reactions of nickelocene with the corresponding neutral, square-planar, (dithiolene)nickel complexes [Ni{S₂C₂S₂(CH₂)₂X}₂]. [CpNi{S₂C₂S₂(CH₂)₂X}] (X = C=O) was prepared from nickelocene and [1,3]dithiolo[4,5-b][1,4]dithiepine-2,6-dione under thermal or photochemical conditions. All complexes exhibit reversible oxidation and reduction waves to the cation and anion form, respectively. The terminal groups (X) in the seven-membered ring shift their redox potentials to anodic potentials in the following order: CF₂ > C=O > S > C=CH₂ > CH₂. The singlet EPR responses of [CpNi{S₂C₂S₂(CH₂)₂(X)}] appear at g ≈ 2.0514-2.0529 in dichloromethane solution at room temperature. An NIR absorption is observed at λ_max ≈ 798-848 nm (ε ≈ 1700-2400 sup>-1 cm^-1) in dichloromethane solution. The X-ray structures of the five complexes show two-legged piano-stool geometries around the central nickel atom (formally Ni^III) and strong distortions from planarity of the seven-membered C₂S₂(CH₂)₂X rings. In the solid state, those radical (S = 1/2) species adopt either one-dimensional alternating chain-like motifs (X = CH₂, C=CH₂, S) or dimeric entities characterized by a singlet-triplet magnetic behavior (X = CF₂, C=O).

Various preparations of the neutral radical [CpNi(dddt)] complex (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were investigated with CpNi sources, [Cp₂Ni], [Cp₂Ni](BF₄), [CpNi(CO)]₂, and [CpNi(cod)](BF₄), and dithiolene transfer sources, O=C(dddt), the naked dithiolate (dddt^2-), the monoanion of square-planar Ni dithiolene complex (NBu₄)[Ni(dddt)₂], and the neutral complex [Ni(dddt)₂]. The reaction of [CpNi(cod)](BF₄) with (NBu₄)[Ni(dddt)₂] gave the highest yield for the preparation of [CpNi(dddt)] (86%). [CpNi(ddds)] (ddds = 5,6-dihydro-1,4-dithiin-2,3-diselenolate), [CpNi(dsdt)] (dsdt = 5,6-dihydro-1,4-diselenin-2,3-dithiolate), [CpNi(bdt)] (bdt = 1,2-benzenedithiolate), and [CpNi(bds)] (bds = 1,2-benzenediselenolate) were synthesized by the reactions of [Cp₂Ni] with the corresponding neutral Ni dithiolene complexes [Ni(ddds)₂]₂, [Ni(dsdt)₂], [Ni(bdt)₂], and [Ni(bds)₂], respectively. The five, formally Ni^III, radical complexes oxidize and reduce reversibly. They exhibit, in the neutral state, a strong absorption in the NIR region, from 1000 nm in the dddt/ddds/dsdt series to 720 nm in the bdt/bds series with ε values between 2500 and 5000 M^-1 cm^-1. The molecular and solid state structures of the five complexes were determined by X-ray structure analyses. [CpNi(dddt)] and [CpNi(ddds)] are isostructural, while [CpNi(dsdt)] exhibits a closely related structure. Similarly, [CpNi(bdt)] and [CpNi(bds)] are also isostructural. Correlations between structural data and magnetic measurements show the presence of alternated spin chains in [CpNi(dddt)], [CpNi(ddds)], and [CpNi(dsdt)], while a remarkably strong antiferromagnetic interaction in [CpNi(bdt)] and [CpNi(bds)] is attributed to a Cp···Cp face-to-face σ overlap, an original feature in organometallic radical complexes.

The radical cation of the redox active ligand 3,4-dimethyl-3',4'-bis-(diphenylphosphino)-tetrathiafulvalene ( P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S₂C=CS₂ part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2^•+, one-electron oxidation of [Fe( P2)(CO)₃] gives rise to the metal-centered oxidation species [Fe^(I)( P2)(CO)₃], characterized by a coupling with two ³¹P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P–Fe bond is detected. Moreover, in presence of free ligand, [Fe^(I)( P2)(CO)₃] reacts to give the complex [Fe^(I)( P2)₂(CO)] containing two TTF fragments. The two-electron oxidation of [Fe( P2)(CO)₃] leads to decomplexation and to the P2^•+ spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe( P2)(CO)₃] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments.