On the linear mixed valence compounds obtained by partial oxidation ofcis-diamminedichloroplatinum(II)

Summary The partial oxidation products ofcis-[Pt(NH₃)₂Cl₂] (Cisplatin) by (NH₄)₂S₂O₈ and by K₂PtCl₆, as well as the recrystallization of the former product in various solutions (HClO₄, NaClO₄, NaBF₄, NaHSO₄, H₂SO₄) have been re-examined. Contrary to the conclusions of previous works, the general formulation of these compounds iscis-[Pt²⁺(NH₃)₂Cl₂]X_x·yH₂O ( = 0.3–0.4, X = SO ₄ ^2– , PtCl ₄ ^2– , ClO ₄ ^2– , BF ₄ ^2– , HSO ₄ ^2– , ..., 0y 1). These materials are all linear chain Pt—Pt compounds belonging to the class III of Robin-Day compounds with Pt—Pt distances in the range 3.00–3.06 Å. The oxidation of Cisplatin by persulphate gives two sulphate compounds of the same formulation (x = 0.2), which differ in aspect, metallic character and spectral properties. Structural information was obtained from polarized micro-Raman spectra of a needle-like sample of the perchlorate and from X-ray powder diffraction spectra. The cell of the perchlorate was orthorhombic while the two sulphate forms had a monoclinic cell with slightly different parameters. Expansion of the Cisplatin interchain distance occurred only along one crystallographic direction. The two sulphate forms differed in the degree of order along this direction. An approximate structure is proposed and discussed in relation to the Cisplatin precursor.     

A model for two-electron mixed valence in metal-metal bonded dirhodium compounds

We propose that two-electron mixed valency in bimetallic complexes possessing three bridging ligands results from a second-order Jahn-Teller instability of the redox-symmetric state.     

A mixed valence manganese triangle in a trigonal lattice: structure and magnetism

The synthesis, structural and magnetic characterisation of trinuclear manganese cluster, [Mn(3)O(O(2)C-anth)(6)(HOCH(3))(3)] 1 (where O(2)C-anth = 9-anthracenecarboxylate), with crystallographic three-fold (C(3)) symmetry, are described. The cluster was prepared by a carboxylate exchange reaction between HO(2)C-anth and [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)] with concomitant fragmentation of the dodecanuclear Mn core of the starting material to form a trinuclear Mn(3)(μ(3)-O) cluster capped by six carboxylate ligands. Bond valence sum calculations and SQUID magnetometric measurements establish the oxidation states of the metal ions as Mn(II)·2 Mn(III) which are antiferromagnetically coupled.     

Photoswitchable mixed valence

For a molecular electronics technology to be fully serviceable, switching functions will be indispensable. Specifically, it will be desirable to control the conductivity of a given molecule using an external stimulus. This tutorial review discusses photoswitchable mixed valence systems that are comprised of a reversibly photoisomerizable bridging unit connecting two redox-active moieties, and as such represent some of the most simple chemical systems in which switching of charge delocalization can be explored. As photoisomerizable units, dithienylethenes have received much attention in the context of photoswitchable mixed valence, but there are also more exotic examples such as norbornadiene- and dimethyldihydropyrene-based switchable systems. As redox-active units responsible for the mixed valence phenomenon, both metal-containing as well as purely organic moieties have been employed. Typical investigations in this area involve the comparison of cyclic voltammograms and (near-infrared) optical absorption spectra of the two isomeric forms of a given system. The magnitude of the comproportionation constant and evaluation of intervalence absorption bands using appropriate theoretical models yield information regarding the extent of charge delocalization in the two isomeric forms. In several of the compounds investigated so far, the light stimulus induces a substantial increase of charge delocalization, or in the terminology commonly used in mixed valence chemistry, a changeover from class I to class II or even class III behavior.     

Electrochromic devices based on binuclear mixed valence compounds adsorbed on nanocrystalline semiconductors

A series of cyano-bridged binuclear mixed valence complexes of the general formula M-Ru(III)(NH(3))(4)pyCOOH [pyCOOH = isonicotinic acid; M = cis-Ru(bpy)(2)(CN)(2), 1 (bpy = 2,2' bipyridine); trans-Ru(py)(4)(CN)(2), 2 (py = pyridine); [Ru(CN)(6)](4)(-), 3; [Fe(CN)(6)](4)(-), 4] have been prepared and anchored through the carboxylic function to nanocrystalline TiO(2) or SnO(2) electrodes. The complexes display a reversible electrochromic behavior in the range of applied potential from -0.5 to +0.5 V, versus SCE. Tuning of the electronic transitions in the visible and near-infrared spectral regions is achieved through changes of the solvent and of the cyano-bridged metal moiety M.     

Effect of asymmetry on the electronic delocalization in diiron and iron-cobalt mixed valence metallocenic compounds

In this work, we report the synthesis and a study on the degree of electronic delocalization in the asymmetric mixed valence complexes [CpFe(C(8)H(6))Fe(C(8)H(7))](+), 3a(+), and [CpCo(C(8)H(6))Fe(C(8)H(7))](+), 3b(+), (Cp = C(5)Me(5), C(8)H(6) = pentalenyde, C(8)H(7) = hydropentalenyde, and = ((3,5(CF(3))(2)C(6)H(3))(4)B(-))). Electrochemical methods, (57)Fe M?ssbauer spectroscopy, electronic spectroscopy, and electron paramagnetic resonance were used for this purpose. Although the anti conformation of the complexes precludes any metal-metal interaction, all the techniques employed show that 3a(+) is a electronic delocalized system, while 3b(+) behaves as two individual metallic centers with localized electron density.     

Excited-state mixed valence in transition metal complexes

Mixed valence in the lowest-energy metal-to-ligand charge-transfer excited state of di-(4-acetylpyridine)tetraammineruthenium(II) complexes is defined and analyzed. The excited state has two interchangeably equivalent ligands with different oxidation states. The electronic absorption band energies, selection rules, and bandwidths are analyzed quantitatively in terms of the signs and orientations of the transition dipole moments, sign and magnitude of the coupling, and resonance Raman analysis of displaced normal modes.     

Statistical thermodynamics of polynuclear linear complexes with mixed valence states by means of the correlated walk

Expressions for the equilibrium electrode potential of linear N-nuclear complexes with homoredox centers were derived by the theory of the correlated walk as a function of the molar fraction of the oxidized moiety, the nearest-neighbor interaction energies and N. When the interaction energy was large in two-, three- and four-nuclear complexes, the expressions predicted two, three and four voltammetric peaks respectively owing to the formation of mixed valence states. The intuitive extension that the N-nuclear complex might exhibit N peaks was invalid. There were three peaks for any odd number of N. In contrast, four peaks appeared for any even number of N more than 4. For a polynuclear complex with N → ∞, the number of the peaks was reduced to two, as if the complex might be a binucleus. The log plot for the fraction vs. potential curve at large values of N deviated from a straight line. The averaged inverse slope was ca. 90 mV at 25°C. From the concentration distribution of a predominant species varying with the potential, the deviation of the log plot was ascribed to the coexistence of various isomers with different interaction energies. The difference in the voltammetric peak potential was approximately linear with the interaction energy for any N. Approximate equations for the potential difference for N= 2, 3 and 4 were obtained, and were applied to the experimental data available for polyferrocenes.     

Spin lattice relaxation in linear polyethylene

The proton spin-lattice relaxation times (T₁) of linear polyethylenes (PE) of varying morphology were measured as a function of temperature. The T₁ of oriented bulkcrystallized and solution-crystallized material was isotropic, in disagreement with calculations based on a sample dipole pair model. Motion in the non-crystalline regions of the samples is shown to be responsible for the T₁ minimum occurring around ?20°C. The dependence of T₁ at the minimum on the long period reinforces the model of an amorphous fraction composed of disordered lamellar surface layers. The temperature of the T₁ minimum and the dependence of T₁ on the long period imply that the mobility of these amorphous regions is reduced in cold-drawn and solution-crystallized samples. This mobility irreversibly increases with annealing. In highly relaxed samples the motion of a small portion of the disordered regions is almost liquidlike, as indicated by the presence of a second shorter T₁.     

X-ray photoelectron study of mixed valence metatungstate anions

X-ray photoelectron spectra of metatungstate H₂W₁₂O^6?₄₀ and of reduced derivatives with 5, 12, and 24 electrons have been recorded. W(4f) signals are consistent with the presence of tetravalent tungsten in the reduced species. In particular the 12e^? derivative does not contain W^V but W^IV and W^VI in equal amounts. Valence band spectra show the W(5d) levels near 2 eV in reduced forms, this energy being appreciably lower than in W bronzes and WO₂. This can be correlated with the relative inertness of reduced metatungstates towards oxidation.     

Semiclassical model of a trinuclear mixed valence system

Potential energy surfaces are calculated for a trinuclear mixed valence system in which a mobile electron can exchange between three chemically equival localization/delocalization conditions which are found different from the bicentric case. Furthermore, they depend on the electronic interaction mechan or indirect overlap through a closed shell ligand such as O^2?. The height of the activation energy barrier for thermal electron transfer is calcu metal atom can either assist or hinder the electron transfer process. Finally the energies of the two optical (intervalence) transitions are given.     

Origins of cooperative noncovalent host-guest chemistry in mixed valence complexes

The electronic effects resulting from noncovalent host-guest interactions between calix[6]arene and a ruthenium dimer, [Ru3O(OAc)6(CO)(ppy)]2-mu-pz (ppy=4-phenyl pyridine, pz=pyrazine), are presented. The noncovalent interaction is between the calix[6]arene and the ppy ligands of the dimer. The dimer can bind 2 equiv of calix[6]arene. The complex [Ru3O(OAc)6(CO)(ppy)]2-mu-pz forms a highly stable mixed valence ion with strong electronic coupling between the two Ru3 clusters. The strength of the electronic interaction is found to be moderated by calix[6]arene binding. Addition of calix[6]arene to the mixed valence ion causes the electronic coupling to decrease. The binding of calix[6]arene is found to be cooperative. The origins of cooperative binding are developed in terms of the potential energy surfaces associated with the symmetric and asymmetric mixed valence ion. In particular, it is found that symmetry breaking (through the binding of a single calix[6]arene) destabilizes the mixed valence state. Restoration of symmetry (through the binding of a second calix[6]arene) increases the stability of the mixed valence ion and provides an additional driving force for the binding of the second calix[6]arene.     

Longitudinal acoustic mode in linear aliphatic polyesters

The longitudinal acoustic mode has been observed in a series of linear aliphatic polyesters. In some cases, the vibrational frequencies observed do not strictly obey the expected inverse chain-length relationship. Both the periodicity measured in small-angle x-ray diffraction and the chain length deduced from Raman spectroscopy are highly dependent on thermal history.     

Double-stranded metal-organic networks for one-dimensional mixed valence coordination polymers

The design of new types of metal-organic networks and the search for unusual crystal architecture represents an important task for modern inorganic and materials chemistry research. A group of new monosubstituted phenylcyanoximes, containing F, Cl, and Br atoms at the 2, 3, or 4 positions, were synthesized using the high yield nitrosation reaction with CH3-ONO and were spectroscopically (1H NMR, 13C NMR, UV-visible, IR, mass spectrometry) and structurally characterized. Results of X-ray analysis revealed nonplanar trans-anti geometry for 2-chlorophenyl(oximino)acetonitrile, H(2Cl-PhCO); a nonplanar anti configuration for 4-chlorophenyl(oximino)acetonitrile, H(4Cl-PhCO); and planar cis-syn geometry for 3-fluorophenyl(oximino)acetonitrile, H(3F-PhCO). All arylcyanoximes undergo deprotonation in solutions with the formation of colored anions exhibiting pronounced negative solvatochromism in a series of polar protic and aprotic solvents. Nine thallium(I) cyanoximates were obtained using the reaction between hot (approximately 95 degrees C) aqueous solutions of Tl2CO3 and solid powdery monohalogenated arylcyanoximes HL. Crystal structures of two Tl(I) cyanoximates [Tl(2Cl-PhCO) and Tl(4Br-PhCO)] contained centrosymmetric dimeric units (TlL)2 that are connected to a coordination polymer by means of an oxygen atom of the oxime group of the neighboring molecule. Cyanoxime anions act as bridging ligands in both structures where the polymeric motif consists of double-stranded Tl-O chains interconnected with the formation of zigzagging Tl2O2 planar rhombes. Thallium atoms form infinite linear arrays with close intermetallic separations. The nearest Tl(I)...Tl(I) distances are 3.838 and 4.058 angstroms in the Tl(2Cl-PhCO) and Tl(4Br-PhCO) structures, respectively, close to that in metallic thallium (3.456 angstroms). Monosubstituted phenyl groups are well aligned in pi-stacking columns that are perpendicular to the array of Tl(I) atoms and stabilize formed structures. Coordination polyhedrons of thallium(I) in these complexes represent distorted trigonal pyramids with stereoactive lone pair.     

Liquid lattice modes studied by far infrared adsorption

The far infrared spectra of many polar liquids show features which can be interpreted as showing that within these liquids relatively stable clusters of molecules are formed. Both temperature and pressure can be expected to influence the size and stability of these clusters. Far infrared spectra of acetone recorded as functions of temperature and pressure show a band (72.5 cm⁻¹ at zero P and 290K) that behaves in a way that is consistent with its assignment as an intermolecular vibrational mode of such a cluster.     

Resonance Raman de-enhancement caused by excited state mixed valence

Resonance Raman and absorption spectra of 9,10-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-anthracene (2) are measured and analyzed. The contribution of the individual vibrational normal modes to the reorganization energy is investigated. Excited-state mixed valence in this system is analyzed using density functional theory electronic structure calculations. The resonance Raman excitation profiles exhibit a resonance de-enhancement effect around 20 725 cm-1, but a corresponding feature is not observed in the absorption spectrum. This unusual observation is attributed to the presence of a dipole-forbidden, vibronically allowed component of the split mixed valence excited state. The de-enhancement dip is calculated quantitatively and explained in terms of the real and imaginary components of the polarizabilities of the two overlapping excited states.     

A study of the cerium valence in multinary garnet compounds

The valence state of cerium ions in the ferrite garnets {Gd_1.96BiCe_0.04}Ga_1.2Fe_3.8O₁₂, {GdBiCe_0.5Ca_0.5}Ga_1.2Fe_3.8O₁₂, and {GdBiCe_0.25Ca_0.25}Ga_1.2Fe_3.8O₁₂ was studied by L ₃ X-ray absorption spectroscopy. Information on the change in the chemical bonding in ferrite garnets upon the violation of lattice completeness was obtained from the shifts of absorption maxima.