Mixed-valence Dimers Research Articles

Intervalence involvement of bridging ligand vibrations in hexaruthenium mixed-valence clusters probed by resonance Raman spectroscopy.

Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.

Antisymmetric double exchange and zero-field splittings in mixed-valence clusters

The effect of an antisymmetric double exchange (AS DE) interaction in the mixed-valence (MV) dimeric dn−dn+1 and trimeric dn−dn−dn+1 clusters of orbitally non-degenerate ions is considered. In the dimeric clusters, strong isotropic Anderson–Hasegawa (AH) DE and Heisenberg exchange interactions (t–J model) form isotropic DE states E±0(S). In the MV dimers, the Moriya spin-flop hopping, which is determined by the spin-orbit coupling, is described by the effective Hamiltonian of an AS DE interaction HASDE=2iKabTab(Sb−Sa), where Kab=−Ka is a real AS vector coefficient, Tab is the isotropic transfer operator. The operator HASDE has a form of the spin-transfer interaction. Analytical expressions for the matrix elements of HASDE were obtained for dn−dn+1 clusters. The AS DE matrix elements depend on the projection M of S. AS DE mixes the AH DE states E+0(S) and E−0(S) with the same S of the different parity. The AS DE coupling and the Dzialoshinsky–Moriya (DM) AS exchange (HDM=Gab[Sa×Sb]) mix the AH states with different S of the same parity. AS DE forms the effective spin S′. In the d1−d2 and d9−d8 clusters, the AS DE contributions to the zero-field splitting (ZFS) parameters are different for the AH high-spin states E+(3/2) and E−(3/2). An AS DE leads to non-collinear orientation of spins in the MV pair and anisotropy of g-factors. An anisotropic DE contributes to ZFS. In the trimeric MV clusters, the isotropic DE forms the isotropic trigonal 2S+1Γ terms, Γ=A1, A2, E. The AS DE results in the new effect: the linear fine splittings Δ of the degenerate 2S+1E DE terms. The fine splittings Δ are proportional to the AS DE parameter KZ=(KZab+KZbc+KZca)/3 of the MV trimer. The vector of the AS DE interaction KZ is directed along the trigonal Z-axis of the MV trimer. The AS DE mixes the 2S+1A1 and 2S+1A2, 2S+1E and 2S+1E DE terms (ΔS=0, 1). In the trimeric MV clusters with high individual spins si, the AS DE and DM AS exchange mixing of the DE levels 2S+1Γ determines the contributions of the second order to the ZFS parameters DS, which are different for the Ai and E DE terms. For the [Cu(II)Cu2(I)] delocalized cluster, the AS DE ZFS Δ=2KZ√3of the ground 2E DE term determines strong anisotropy of the Zeeman splittings, anisotropy of g-factors (gZ≠0, gX,Y=0) and magnetic properties.

Antisymmetric double exchange in mixed-valence dimeric clusters

Abstract The model of an antisymmetric double exchange (AS DE) interaction is developed for the mixed-valence (MV) d n –d n +1 clusters of orbitally nondegenerate transition metal ions. The spin–orbit coupling effect is considered for the MV dimers, in which strong isotropic Anderson–Hasegawa (AH) double exchange and Heisenberg exchange interactions ( t – J model) between ions form isotropic states E ± 0 ( S ). The spin–orbit coupling effect in the AH double exchange (the Moriya spin-flop hopping) is described by the effective Hamiltonian of the AS double exchange interaction H ASDE =2 i K ab T ⌢ ab ( S b − S a ) , where K ab =− K ba is a real antisymmetric vector constant, T ⌢ ab is the isotropic transfer operator. Analytical expressions for the matrix elements of the AS DE interaction were obtained in the spin representation for all d n –d n +1 clusters. The AS DE interaction (vector transfer) results in the dependence of the double exchange matrix elements on the projection M of the total spin S . The AS double exchange mixes the AH double exchange states E + 0 ( S ) and E − 0 ( S ) with the same S of the different parity and also the AH states with different S of the same parity. AS DE forms the effective spin S ′ . The AS DE mixing of the AH levels results in the AS DE contributions to the zero-field splittings, which depend on S and parity. In the d 1 –d 2 (d 9 –d 8 ) cluster in the t – J model with an initial ZFS ( H ZFS 0 = D S 0 [ S z 2 − S ( S +1)/3]), the AS DE contributions δ K ± (3/2) to ZFS parameters Δ ZFS ± (3/2)=2 D S ± (3/2)=2[ D S 0 + δ K ± (3/2)] are different for the AH high-spin states E + (3/2) and E − (3/2): δ K ± (3/2)=4[ K z 2 −( K x 2 + K y 2 )/2] J /3 t ( t ±6 J ). The AS DE contributions δ K ± (5/2) to the ZFS parameters D S ± (5/2)= D S 0 + δ K ± (5/2) are different for the E + (5/2) and E − (5/2) states of the d 2 –d 3 cluster: δ K ± (5/2)=2 K z 2 J / t ( t ±15 J ). In the MV clusters, the AS double exchange contribution to the cluster ZFS parameters are more essential than the pure Dzialoshinsky–Moriya AS exchange contribution. The microscopic origin of the AS DE parameters K ab and anisotropy was considered for the d 9 –d 8 (d 1 –d 2 ) and d 1 –d 0 , d 9 –d 10 clusters with the taking into account the detailed crystal field states of d-ions and the spin–orbit coupling admixture of the excited states. An antisymmetric double exchange leads to noncollinear orientation of spins in the MV pair and anisotropy of g -factors. An anisotropic double exchange contributes to zero-field splittings.

Synthesis and Characterization of K38Nb7As24 and Cs9Nb2As6: The First Mixed‐Valence Transition‐Metal Zintl Phases.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Theoretical Studies of Mixed-Valence Transition Metal Complexes for Molecular Computing

Possible molecular implementations of quantum-dot cellular automata (QCA) with mixed-valence ruthenium complexes are discussed. A study of the geometric and electronic structures of three mixed-valence ruthenium dimers has been done using ab initio Hartree-Fock and hybrid density functional methods at the HF/3-21G and B3LYP/3-21G levels of theory. These complexes are representatives of Robin and Day classes I, II, and III. Predicted geometries are compared to experimental data, as well as previous computational studies, where available. The B3LYP method predicts structures in better agreement with experiment than the HF method. The analysis of the orbital energies and localization provides insights into the degree of localization and the Robin-Day classification. They are therefore useful tools for the design of mixed valence compounds for use in molecular QCA.

Interplay of Charge Transfer, Dimensionality, and Amide Hydrogen Bond Network Adaptability in TCNQF 4 Complexes of EDO-TTF-CONH 2 and EDT-TTF-CONH 2

[EDO-TTF-CONH 2][TCNQF 4], triclinic system, space group P-1, a=8.2479(12) Å, b=12.282(2) Å, c=12.6842(18) Å, α=113.850(17)°, β=106.420(17)°, γ=90.284(19)°, V=1116.8(4) Å 3; and [EDT-TTF-CONH 2] 2[TCNQF 4], triclinic system, space group P-1, a=6.5858(9) Å, b=11.699(2) Å, c=12.2281(18) Å, α=104.000(19)°, β=93.611(17)°, γ=98.279(19)°, V=899.9(3) Å 3, whose π-donor molecules, (ethylenedioxo)-carbamoyltetrathiafulvalene and (ethylenedithio)-carbamoyltetrathiafulvalene, respectively, differ solely by the nature of the chalcogen atoms in their outer ethylene dichalcogeno bridge, yet form very different charge-transfer complexes with the same π-acceptor. [EDO-TTF-CONH 2 •+] 2 [TCNQF 4 •−] 2 is a diamagnetic insulating ionic salt with a three-dimensional rock-salt-type structure based on discrete dimers while in the semi-conducting mixed-valence complex, [EDT-TTF-CONH 2] 2 •+[TCNQF 4 •−], the mixed-valence dimers aggregate into infinite chains interspersed within parallel rows of non-interacting radical anions. It is shown how the robust and adaptable supramolecular amide hydrogen bond tweezers-like motifs common to the two solids simply comply to the 3-to-1 dimensionality reduction upon substitution of O for S.

Synthesis and characterization of K(38)Nb(7)As(24) and Cs(9)Nb(2)As(6): the first mixed-valence transition-metal Zintl phases.

The two title compounds were prepared by direct reactions of the corresponding elements at high temperature, and their structures were determined from single-crystal X-ray diffraction data. The structure of K(38)Nb(7)As(24) (orthorhombic; Cmcm; Z = 4; a = 10.4974(6), b = 23.915(2), c = 36.046(2) A) comprises isolated tetrahedra of NbAs(4) and two types of dimers of edge-sharing tetrahedra: dimers containing only Nb(V), [Nb(V)2As(6)](8-), and mixed-valence dimers with both Nb(IV) and Nb(V), [Nb(IV)Nb(V)As(6)](9-). The structure of Cs(9)Nb(2)As(6) (orthorhombic; Pbca; Z = 8; a = 17.5848(7), b = 16.940(2), c = 18.183(4) A) contains only the latter dimers. Magnetic measurements showed Curie-Weiss paramagnetic behavior for both compounds consistent with one unpaired electron/mixed-valence dimer. Cs(9)Nb(2)As(6) exhibits also an antiferromagnetic transition at about 36 K. The two compounds are the first mixed-valence (of class III) transition-metal Zintl phases.

Symmetry-breaking phenomena in metalloporphyrin pi-cation radicals.

Density functional theory (DFT) calculations of the energetics, molecular structures, and spin density profiles of metalloporphyrin pi-cation radicals suggest that the common practice of describing these radicals in terms of a universal A(1u)/A(2u) dichotomy is often not justified, confirming a possibility first foreseen by Prendergast and Spiro (ref 15) over a decade ago on the basis of vibrational spectroscopy and semiempirical calculations. Because of near-degeneracy of the a(1u) and a(2u) HOMOs of many metalloporphyrins, the cation radicals derived from these compounds undergo a pseudo-Jahn-Teller (pJT) distortion and are, therefore, best described as (2)A(u) with reference to the C(4h) point group, rather than as (2)A(1u) (D(4h) or (2)A(2u) (D(4h)). We find that the porphyrin cation radicals undergo a pJT distortion if the energy difference between the (2)A(1u) and (2)A(2u) pi-cation radicals, optimized under D(4h) symmetry constraints, is less than 0.15 eV. According to this criterion, metallo-porphine and metallo-OEP pi-cation radicals should always be pJT-distorted and metallo-meso-tetrahalogenoporphyrin radicals should not. For [Zn(TPP(*))](+), the (2)A(1u)/(2)A(2u) energy difference is almost exactly at the threshold of 0.15 eV, consistent with the experimental observation of both symmetry-broken and undistorted structures for this species. The (2)A(1u)/(2)A(2u) energy difference (when the molecular geometries are optimized under a D(4h) symmetry constraint) also appears to govern whether the real pJT-distorted cation radical is more A(1u)- or A(2u)-like in terms of its spin density profile. Because many metalloporphyrin pi-cation radicals exist as cofacial dimers in the crystalline phase, we examined the symmetries and structures of the model compounds [[Zn(P)](2)](+,2+) by means of DFT geometry optimizations. The results showed that dimerization has relatively little impact on the bond length alternation in the individual rings. A final interesting result, consistent with experiment, is that the bond length alternation in the delocalized mixed-valence dimer [[Zn(P)](2)](+) is about half that found for [[Zn(P)](2)](2+).

Manifestation of the double-exchange interaction in the Mössbauer spectrum of mixed-valence systems

The dependence of the Mössbauer line shape on the double-exchange interaction is studied for dimeric mixed-valence systems in the framework of a model that treats the entire nuclear-electron system quantum-mechanically. The coherent tunneling motion of the “excess” electron between the metal sites produces a resonance splitting of the localized states. In the case of weak double-exchange interaction, this motion shows up as a splitting of Mössbauer lines. This effect however, is readily suppressed by an asymmetry distortion, which destroys the resonance conditions. The origin of different patterns in the temperature dependence of the Mössbauer line shape is discussed.

Oxidation state dependence of the geometry, electronic structure, and magnetic coupling in mixed oxo- and carboxylato-bridged manganese dimers.

Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)and [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)in the Mn(IV)Mn(IV), Mn(III)Mn(IV), and Mn(III)Mn(III) oxidation states. The magnetic coupling in the dimer is profoundly affected by changes in both the bridging ligands and Mn oxidation state. In particular, change in the bridging structure has a dramatic effect on the nature of the Jahn-Teller distortion observed for the Mn(III) centers in the III/III and III/IV dimers. The principal magnetic interactions in [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)() involve the J(xz/xz)and J(yz/yz) pathways but due to the tilt of the Mn(2)O(2) core, they are less efficient than in the planar di-mu-oxo structure and, consequently, the calculated exchange coupling constants are generally smaller. In both the III/III and III/IV dimers, the Mn(III) centers are high-spin, and the Jahn-Teller effect gives rise to axially elongated Mn(III) geometries with the distortion axis along the Mn-O(c) bonds. In the III/IV dimer, the tilt of the Mn(2)O(2) core enhances the crossed exchange J(x)()()2(-)(y)()()2(/)(z)()()2 pathway relative to the planar di-mu-oxo counterpart, leading to significant delocalization of the odd electron. Since this delocalization pathway partially converts the Mn(IV) ion into low-spin Mn(III), the magnetic exchange in the ground state can be considered to arise from two interacting spin ladders, one is the result of coupling between Mn(IV) (S = 3/2) and high-spin Mn(III) (S = 2), the other is the result of coupling between Mn(IV) (S = 3/2) and low-spin Mn(III) (S = 1). In [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)(), both the III/III dimer and the lowest energy structure for the III/IV dimer involve high-spin Mn(III), but the Jahn-Teller axis is now orientated along the Mn-oxo bond, giving rise to axially compressed Mn(III) geometries with long Mn-O(c) equatorial bonds. In the IV/IV dimer, the ferromagnetic crossed exchange J(yz)()(/)(z)()()2 pathway partially cancels J(yz/yz) and, as a consequence, the antiferromagnetic J(xz/xz) pathway dominates the magnetic coupling. In the III/III dimer, the J(yz/yz) pathway is minimized due to the smaller Mn-O-Mn angle, and since the ferromagnetic J(yz)()(/)(z)()()2 pathway largely negates J(xz/xz), relatively weak overall antiferromagnetic coupling results. In the III/IV dimer, the structures involving high-spin and low-spin Mn(III) are almost degenerate. In the high-spin case, the odd electron is localized on the Mn(III) center, and the resulting antiferromagnetic coupling is similar to that found for the IV/IV dimer. In the alternative low-spin structure, the odd electron is significantly delocalized due to the crossed J(yz)()(/)(z)()()2 pathway, and cancellation between ferromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling. The delocalization partially converts the Mn(IV) ion into high-spin Mn(III), and consequently, the spin ladders arising from coupling of Mn(IV) (S = 3/2) with high-spin (S = 2) and low-spin (S = 1) Mn(III) are configurationally mixed. Thus, in principle, the ground-state magnetic coupling in the mixed-valence dimer will involve contributions from three spin-ladders, two associated with the delocalized low-spin structure and the third arising from the localized high-spin structure.

High-frequency EPR study of a new mononuclear manganese(III) complex:. [(terpy)Mn(N3)3] (terpy = 2,2':6',2''-terpyridine).

The isolation and structural characterization of [(terpy)Mn(III)(N3)3], complex 1, is reported (terpy = 2,2':6',2' '-terpyridine). Complex 1, a product of the reaction between the mixed-valence dimer [(terpy)(H2O)Mn(III)(O)2Mn(IV)(OH2)(terpy)](NO3)3 and NaN3, crystallizes in a triclinic system, space group P1, a = 8.480(1) A, b = 8.9007(2) A, c = 12.109(2) A, alpha = 93.79(1) degrees, beta = 103.17(1) degrees, gamma = 103.11(1) degrees, and Z = 2. Complex 1 exhibits a Jahn-Teller distortion of the octahedron characteristic of a six-coordinated high-spin Mn(III). A vibrational spectroscopic study was performed. The nu(asym)(N3) mode of complex 1 appears in the IR as a strong band at 2035 cm(-1) with a less intense feature at 2072 cm(-1), and in the FT-Raman as a strong band at 2071 cm(-1) with a weaker broad band at 2046 cm(-1). The electronic properties of complex 1 were investigated using a high-field and high-frequency EPR study (190-475 GHz). The different spin Hamiltonian parameters have been determined (D = -3.29 (+/-0.01) cm(-1), E = 0.48 (+/-0.01) cm(-1), E '= 0.53 (+/-0.01) cm(-1), g(x) = 2.00 (+/-0.005), g(y) = 1.98 (+/-0.005), g(z) = 2.01 (+/-0.005)). These parameters are in agreement with the geometry of complex 1 observed in the crystal structure, a D < 0 related to the elongated distortion, and a value of E/D close to 0.2 as expected from the highly distorted octahedron. The two values of the E-parameter are explained by the presence of two slightly different structural forms of complex 1 in the crystal lattice. A second hypothesis was explored to explain the experimental data. The calculation for the simulation was done taking into account that the g and D tensors are not collinear due to the low symmetry of complex 1. In that case, the spin Hamiltonian parameters found are D = -3.29 (+/-0.01) cm(-1), E = 0.51 (+/-0.01) cm(-1), g(x) = 2.00 (+/-0.005), g(y) = 1.98 (+/-0.005), and g(z) = 2.01 (+/-0.005).

S K-edge X-ray absorption studies of tetranuclear iron-sulfur clusters: mu-sulfide bonding and its contribution to electron delocalization.

X-ray absorption spectroscopy (XAS) at the sulfur ( approximately 2470 eV) and chlorine ( approximately 2822 eV) K-edges has been applied to a series of 4Fe-4S model complexes. These are compared to 2Fe-2S model complexes to obtain insight into the localized ground state in the mixed-valence dimer versus the delocalized ground state in the mixed-valence tetramer. The preedges of hypothetical delocalized mixed-valence dimers [Fe(2)S(2)](+) are estimated using trends from experimental data and density functional calculations, for comparison to the delocalized mixed-valence tetramer [Fe(4)S(4)](2+). The differences between these two mixed-valence sites are due to the change of the sulfide-bridging mode from micro(2) to micro(3). The terminal chloride and thiolate ligands are used as spectator ligands for the electron density of the iron center. From the intensity of the preedge, the covalency of the terminal ligands is found to increase in the tetramer as compared to the dimer. This is associated with a higher effective nuclear charge on the iron in the tetramer (derived from the energies of the preedge). The micro(3)-bridging sulfide in the tetramer has a reduced covalency per bond (39%) as compared to the micro(2)-bridging sulfide in the dimer (51%). A simple perturbation model is used to derive a quadratic dependence of the superexchange coupling constant J on the covalency of the metal ions with the bridging ligands. This relationship is used to estimate the superexchange contribution in the tetramer (J = -156 cm(-)(1)) as compared to the mixed-valence dimer (J = -360 cm(-)(1)). These results, combined with estimates for the double exchange and the vibronic coupling contributions of the dimer sub-site of the tetramer, lead to a delocalized S(t) = (9)/(2) spin ground state for the mixed-valence dimer in the tetramer. Thus, the decrease in the covalency, hence the superexchange pathway associated with changing the bridging mode of the sulfides from micro(2) to micro(3) on going from the dimer to the tetramer, significantly contributes to the delocalization of the excess electron over the dimer sub-site in the tetramer.

Electron localization–delocalization in mixed-valence iron dimers

Two mixed-valence iron dimers, [L 1 Fe 2 (μ-OAc) 2](ClO 4) and [L 1 Fe 2 (μ-OBz)(OBz)(H 2O)](ClO 4), were investigated. The temperature dependence of the effective magnetic moments and the position and shape of the intervalence absorption bands were calculated using a Hamiltonian taking into account isotropic exchange interaction, double exchange, Zeeman interaction, zero-field splitting for the ground state and vibronic coupling with the PKS out-of-phase mode. The similarities and differences of the investigated compounds were explained on the basis of analysis of the adiabatic potentials. The behaviour of both systems is determined by a strong competition between two main processes: double exchange interaction and vibronic coupling with PKS out-of-phase mode. Degree of delocalization of the itinerant extra electron was calculated at different values of temperature.

Anisotropic double exchange in orbitally degenerate mixed valence systems

The problem of the double exchange is considered for the mixed valence dimers in which one or both transition metal ions possess orbitally degenerate ground states. In the pseudo-angular momentum representation, the general formula is deduced for the matrix elements of double exchange involving the transfer integrals and all spin and orbital quantum numbers. The pairs 3 T 1 t 2 2 – 2 T 2 t 2 1 and 3 T 1 t 2 2 – 4 A 2 t 2 3 are considered in three high-symmetric topologies: edge-shared D 2h, corner-shared D 4h, and face-shared D 3h bioctahedra. The double exchange in orbitally degenerate systems is shown to produce strong magnetic anisotropy of an orbital nature. The character of the anisotropy proved to be dependent on the overall symmetry of the system and on the ground terms of the metal ions.

Vibronic model of hyperfine interaction in dimeric mixed-valence clusters

The problem of “vibronic” hyperfine coupling parameters in spin-coupled states of the dimeric mixed-valence clusters is considered in the framework of the adiabatic approach. The employed generalised vibronic model involves the interaction of the “extra” electron with the local (“breathing” modes) and molecular (intermetallic) vibrations. The first type of vibrations produces a localisation effect while the second one promotes delocalisation. The explicit expressions for the hyperfine parameters in terms of the double exchange and two vibronic parameters are given, and the conditions are found under which the effect of partial delocalisation can manifest itself in the hyperfine structure of EPR for different spin states of the system.

ELECTROCHEMICAL BEHAVIOR OF SOME INORGANIC ESTERS OF OXOVANADIUM(V)

The electrochemical behavior of ethylic esters of the type Q2VO(OEt), derived from the oxo-vanadium(V) complex Q2VO(OH) (Q = 8-hydroxyquinoline, 5-chloro and 5, 7-dichloro-8-hy-droxyquinoline or 2-methyl-8-hydroxyquinoline), was investigated by cyclic voltamperometry in organic solvents. The detailed analysis of the results confirmed the generation of different complex species in solution. The presence of water in the solvents modifies the electrochemical behavior, as a consequence of the formation of a mixed valence dimer of the type [Q2VO-O-OVQ2]−. The somewhat different behavior of the 2-methyl-8-hydroxyquinoline derivative could also be explained. The analysis of the electronic spectra of the investigated solutions, after and during the electrochemical measurements, gave additional support to some of the proposed reactions, steps and mechanisms.

DFT Description of Mixed Valence Magnetic Systems. Mn(III)-Mn(IV) and Fe(II)-Fe(III) Complexes

Density functional theory has been successfully applied to characterize the electronic structure and the magnetic properties of the mixed-valence dinuclear complex [Fe2(OH)3(tmtacn)2]2±[1],[2], modeled by replacing the tmtacn ligand with 3 ammonia molecules, and the complex [Mn2(O)2(NH3)8]3±[3], where the ammonia modeled two Tren ligands. Spectroscopic and magnetic properties have been computed in nice agreement with the experimental values. Minimum energy path calculations allowed us to compute the frequencies v. associated to the normal coordinate Q. responsible of the delocalization of the extra electron and we present here a procedure for the full characterization of mixed-valence transition metal dimers.

Theory for electron transfer from a mixed-valence dimer with paramagnetic sites to a mononuclear acceptor

Polynuclear transition-metal complexes, such as Fe–S clusters, are the prosthetic groups in a large number of metalloproteins and serve as temporary electron storage units in a number of important redox-based biological processes. Polynuclearity distinguishes clusters from mononuclear centers and confers upon them unique properties, such as spin ordering and the presence of thermally accessible excited spin states in clusters with paramagnetic sites, and fractional valencies in clusters of the mixed-valence type. In an earlier study we presented an effective-mode (EM) analysis of electron transfer from a binuclear mixed-valence donor with paramagnetic sites to a mononuclear acceptor which revealed that the cluster-specific attributes have an important impact on the kinetics of long-range electron transfer. In the present study, the validity of these results is tested in the framework of more detailed theories which we have termed the multimode semiclassical (SC) model and the quantum-mechanical (QM) model. It is found that the qualitative trends in the rate constant are the same in all treatments and that the semiclassical models provide a good approximation of the more rigorous quantum-mechanical description of electron transfer under physiologically relevant conditions. In particular, the present results corroborate the importance of electron transfer via excited spin states in reactions with a low driving force and justify the use of semiclassical theory in cases in which the QM model is computationally too demanding. We consider cases in which either one or two donor sites of a dimer are electronically coupled to the acceptor. In the case of multiconnectivity, the rate constant for electron transfer from a valence-delocalized (class-III) donor is nonadditive with respect to transfer from individual metal sites of the donor and undergoes an order-of-magnitude change by reversing the sign of the intradimer metal–metal resonance parameter (β). In the case of single connectivity, the rate constant for electron transfer from a valence-localized (class-II) donor can readily be tuned over several orders of magnitude by introducing differences in the electronic potentials at the two metal sites of the donor. These results indicate that theories of cluster-based electron transfer, in order to be realistic, need to consider both intrinsic electronic structure and extrinsic interactions of the cluster with the protein environment.

Chain Compound of Mixed-Valent Ruthenium Dimers Linked by Hydrogen-Bonding between the Axially Coordinated Nitronyl Nitroxide and Water Molecules

An adduct of ruthenium(II,III) dimer with nitronyl nitroxide, [{Ru2(O2CCMe3)4(nitme)2}{Ru2(O2CCMe3)4(H2O)2}]n(BF4)2n (nitme = 2,4,4,5,5-pentamethyl-4,5-dihydro-1H-imidazolyl-1-oxyl 3-oxide), which has a chain structure formed by hydrogen bonds between the axial nitme and water molecules, has been prepared and characterized by the X-ray crystallography, magnetic susceptibility, and EPR measurements.

Distortional Isomers of a Mixed-Valence Binuclear Cu Complex

The resonance Raman spectra of a mixed-valence copper dimer [Cu2(L)]3+ encapsulated by the ligand L = N(CH2CH2N(H)CH2CH2N(H)CH2CH2)3N are reported. Copper isotope data (63/65Cu) of solution spectra of [Cu2(L)]3+ allow identification of vibrational bands at 289 and 172 cm-1 as predominately Cu−Cu stretching and Cu−Cu−Neq bending modes. The results indicate that the Cu−Cu bond is sufficiently weak that ligand conformations, influenced by hydrogen bonding, can give rise to distortional isomers of the Cu−Cu bond.