Chemical constitution of 3d complexes and rate constants of ortho-para positronium conversion reactions

The positronium spin exchange (SE) reactions promoted by the 3d complexes Mn _aq ²⁺ , Co _aq ²⁺ , [Cr(NH₃)₆]³⁺, [Cr(NH₃)₅H₂O]³⁺, and [Cr(H₂O)₅Cl²⁺] were investigated at different temperatures in order to ascertain whether they were diffusion controlled like those promoted by Cr _aq ³⁺ , Fe _aq ²⁺ , Ni _aq ²⁺ and [Ni(NH₃)₆] _aq ²⁺ ions studied previously.It was found that the reactions are diffusion controlled and that the effective reaction radii deduced from the Smoluchowski equation are smaller than the compound radii calculated from bond lengths and angles. It was also found that the rate constants, and therefore the reaction radii, are correlated with the spin—orbit coupling of 3d unpaired electrons and ligand capabilities to cause expansion of the d-electron cloud (nephelauxetic effect). In particular the effective reaction radii of SE reactions promoted by 3d complexes increase as the electron delocalization from 3d atoms increases.Work supported by Ministero Università e Ricerca Scientifica e Tecnologica (M.U.R.S.T.) and by Consiglio Nazionale Ricerche (C.N.R.).     

Antiferromagnetic complexes involving metalmetal bonds IV. Synthesis,molecular structure and magnetic properties of the heterotrinuclear cluster, (C5H5Cr)2(μ2-SCMe3)(μ3-S)2Fe(CO)3, with direct and indirect exchange between CrIII and FeI centers

The photochemical reaction between the antiferromagnetic complex (C₅H₅-CrSCMe₃)₂S (I) (containing a CrCr bond 2.689 Å long) and Fe(CO)₅ results in the elimination of two carbonyl groups and one tert-butyl radical to give (C₅H₅Cr)₂(μ²-SCMe₃)(μ³-S)₂ · Fe(CO)₃ (III). As determined by X-ray diffraction, III contains a CrCr bond of almost the same length as in I (2.707 Å), together with one thiolate and two sulphide bridges. The latter are also linked with the Fe atom of the Fe(CO)₃ moiety (average FeS bond length 2.300 Å). Fe also forms a direct bond, 2.726 Å long, with one of the Cr atoms, whereas its distance from the other Cr atom (3.110 Å) is characteristic for non-bonded interactions. Complex III is antiferromagnetic, the exchange parameter, ?2J, values for CrCr, Cr(1)Fe and Cr(2)…Fe are 380, 2600 and 170 cm^?1, respectively. The magnetic properties of III are discussed in terms of the “exchange channel model”. The contributions from indirect interactions through bridging ligands are shown to be insignificant compared with direct exchange involving metalmetal bonds. The effects of steric factors and of the nature of the M(CO)_n fragments on the chemical transformations of (C₅H₅CrSCMe₃)₂S · M(CO)_n are discussed.     

Study of the [Cr(H2O)5NO]2+ Complex via Density Functional Theory

On the basis of density functional theory, the spin ground state of chromium‐nitrosyl complex [Cr(H₂O)₅NO]²⁺ (S = 1/2) is studied via B3LYP hybrid method. Its vibrational frequencies, atomic charges, and spin densities are analyzed. The excitation energies are evaluated using the CIS method. Our calculated N‐O stretching frequency and excitation energies are in good agreement with the IR and UV‐vis data. The related Cr^I(H₂O)₆⁺, Cr^II(H₂O)₆²⁺, and Cr^III(H₂O)₆³⁺ complexes are employed as the reference compounds to determine the characteristics of the central Cr. Results indicate that the effective Cr oxidation state is close to Cr(I).     

Zur koordination mehrfunktioneller thioether in cyclopentadienyleisen-komplexen

[C₅H₅Fe(CO)₂thf]⁺ reacts with the ligands LL and LLL to give the cations [C₅H₅Fe(CO)₂LL]⁺ (LL = RS(CH₂)_nSR, 1,4-dithiane) and [C₅H₅Fe(CO)₂LLL]⁺ (LLL = 1,3,5-trithiane, tris(methylmercapto)methane) containing monodentate coordinated sulfur ligands. In a similar way, sulfur ligand bridged dinuclear dications [(C₅H₅Fe(CO)₂)₂(μ-LL)]²⁺ and [(C₅H₅Fe(CO)₂(μ-LLL)]²⁺ and tri-nuclear trications [(C₅H₅Fe(CO)₂)₃(μ-LLL)]³⁺ are formed. Irradiation of the mononuclear cations gives the chelate complexes [C₅H₅Fe(CO)(η²-LL)]⁺.     

Kinetic study of the reactions Br + IBr→I + Br2 and I + Br2→Br + IBr

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBr→I+Br₂ (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k₁=(2.7±0.4)×10⁻¹¹ cm³ molecule⁻¹s⁻¹. The rate constant of the reverse reaction: I+Br₂→Br+IBr (−1) has been obtained from the Br₂ consumption rate (with an excess of I atoms) and the IBr formation rate: k₋₁=(1.65±0.2)×10⁻¹³ cm³molecule⁻¹s⁻¹. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k₁ and k₋₁ and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br₂, is: K₁=k₁/k₋₁=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH₂₉₈°=−(3.6±0.1) kcal mol⁻¹ and the heat of formation of the IBr molecule, ΔH_f,298°(IBr)=(9.8±0.1) kcal mol⁻¹. © 1998 John Wiley & sons, Inc. Int J Chem Kinet 30: 933–940, 1998     

Studies of layered uranium (VI) compounds. VI. Ionic conductivities and thermal stabilities of MUO2PO4 · nH2O,where M = H,Li,Na,K,NH4 or 12Ca, and where n is between 0 and 4

We have measured the ionic conductivities of pressed pellets of the layered compounds MUO₂PO₄ · nH₂O, and correlated the results with TGA data. The conductivities (in ohm^?1 m^?1), at temperatures increasing with decreasing water content over the range 20 to 200°C, were approximately as follows: Li⁺4H₂O, 10^?4; Li⁺, Na⁺, K⁺, and NH₄⁺3H₂O, 10^?4, 10^?2, 10^?4, and 10^?4; H⁺, Li⁺, and Na⁺1.5H₂O, 10^?2, 10^?4, and 10^?4; Na⁺1H₂O, 10^?5; H⁺, K⁺, and NH₄⁺0.5H₂O, all 10^?5; and H⁺, Li⁺, Na⁺, K⁺, NH⁺₄, and $\text{1}\text{2}\text{Ca}{}^{\mathrm{2+}}\text{}\text{OH}{}_2\text{O}$, 10^?5, 10^?5, 10^?4, 10^?5, 10^?5, and 10^?6. A ring mechanism is proposed to account for the high conductivity found in NaUO₂PO₄ · 3.1H₂O. The accurate TGA data showed that most of the hydrates had water vacancies of the Schottky type, and should be represented as MUO₂PO₄(A ? x)H₂O, where x can be between 0 and 0.3.     

A dichromium(II) compound with an elongated metal-metal bond: Cr2(oxindolate)4(oxindole)2

The title compound has been prepared by reaction of (C₅H₅)₂Cr with oxindole (indole with CO in place of CH₂ at the 2-position). Red single crystals belong to space group P2₁/c with a = 10.107(4) Å, b = 22.496(7) Å, c = 9.210(3) Å, β = 93.26(3)°, V = 2091(2), and Z = 2. The centrosymmetric molecule has a CrCr distance of 2.495(4) Å. The mean CrO and CrN distances for the bonds to bridging oxindolate anions are 2.024(7) and 2.065(8) Å, respectively. There is an oxindole molecule bound at each end with a CrO axial bond of length 2.341(8) Å and a hydrogen bond from the oxindole NH group to an equatorial oxygen atom of length 2.83(1) Å. The significance of this compound with respect to CrCr bonding is discussed.     

Antiferromagnetic complexes involving metalmetal bonds : I. Synthesis and molecular structure of an antiferromagnetic dimer with a CrCr bond

The binuclear complex (C₅H₅)₂Cr₂(S)(SCMe₃)₂ was prepared by refluxing a solution of chromocene and t-butylmercaptane in heptane. The structure of the product was determined by single crystal X-ray diffraction. The chronium atoms are linked by a sulphide bridge (SCr 2.24 Å;, <CrSCr 74.1° and two SCMe₃ bridges (CrS 2.38 Å;, <CrSCr 68.3–69.3°). The two cyclopentadienyl ligands (CC 1.41 Å;, CrC 2.23 Å;) are in apical positions, their ring planes being parallel to each other. The complex is an antiferromagnet (?2J cm^?1) despite the small CrSCr angles and short chromiumchromium distance (2.689 Å;) indicative of strong CrCr bonding.     

Mehrfachbindungen zwischen hauptgruppenelementen und übergangsmetallen: XXXII. Insertions- und metathesereaktionen von metallorganischen eisen-selen-komplexen

The compound (μ-Se)[η⁵-C₅H₅)Fe(CO)₂]₂ (1) has labile metalselenium bonds and therefore undergoes insertion and metathesis reactions. Thus, reaction with elemental selenium gives the novel FeSeSeFe chain-type complex (μ,η¹:η¹-Se₂)[(η⁵-C₅H₅)Fe(CO)₂]₂ (2). Both compounds 1 and 2 react with the homodinuclear chromium complex [(η⁵-C₅H₅)Cr(CO)₃]₂ with formation of the heterodinuclear derivatives of composition (η⁵-C₅H₅)₂CrFe(CO)₅. (3; single-crystal X-ray structure: d(CrFe) 290.1(1) pm) and (μ-Se₂)[(η⁵-C₅H₅)₂CrFe(CO)₄] (4). The latter compound exhibits a diselenido bridge ligand in η¹:η²-coordination (single-crystal X-ray structure: d(CrSe) 249.5(2) and 255.5(2) pm, d(FeSe) 238.9(2), d(SeSe) 229.7(1) pm) and can also be obtained by treatment of the chromium complex 3 with elemental selenium.     

Kinetics of anation of chromium(III) by L-phenylalanine

Summary The kinetics of anation of chromium(III) species, [Cr(H₂O)₆]⁴⁺ and [Cr(H₂O)₅OH]²⁺, by L-phenylalanine in aqueous acid has been studied spectrophotometrically. Effects of varying [substrate], [ligand], [H⁺], , % ethanol and temperature were investigated. The kinetic data suggest a mechanism where outersphere-associations [between chromium(III) species and phenylalanine in the zwitterionic form] precede anation. Comparison of the results with published data suggest an I_a path for the [Cr(H₂O)₆]³⁺ reaction and I_d path for the [Cr(H₂O)₅OH]²⁺ reaction.     

Separation and kinetic study of chromium(III) chloro complexes by capillary electrophoresis

Summary Three Cr(III) species (dichlorotetraaquachromium (III), [CrCl₂(H₂O)₄]⁺; monochloropentaaquachromium(III), [CrCl(H₂O)₅]²⁺; and hexaaquachromium(III), [Cr(H₂O)₆]³⁺) have been separated and determined by capillary electrophoresis. The first two complexes could be detected in direct mode in phosphate buffer, but because the absorption of complex [Cr(H₂O)₆]³⁺ is poor in the UV range, indirect UV detection had to be used. For indirect detection 5 mM imidazole was added to the buffer solution. The formation and decomposition of the different Cr(III) complexes were monitored in time after the preparation of solutions of CrCl₃.6H₂O. The slowest process was the decomposition of [CrCl(H₂O)₅]²⁺; 300 h after preparation of a solution of CrCl₃.6H₂O of pH 1 the solution contained only [Cr(H₂O)₆]³⁺. The effects of pH and the content of some matrix ions on the rates of conversion of the complexes were studied. The kinetic characteristics of this complex system could be investigated adequately by means of capillary electrophoresis. Presented at Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Kinetics and mechanism of oxidation of the binary and ternary complexes of chromium(III) involving inosine and glycine by N -bromosuccinimide

The kinetics of oxidation of the chromium(III) complexes, [Cr(Ino)(H₂O)₅]³⁺ and [Cr(Ino)(Gly)(H₂O)₃]²⁺ (Ino?=?Inosine and Gly?=?Glycine) involving a ligands of biological significance by N-bromosuccinimide (NBS) in aqueous solution to chromium(VI) have been studied spectrophotometrically over the 25–45°C range. The reaction is first order with respect to both [NBS] and [Cr], and increases with pH over the 6.64–7.73 range in both cases. The experimental rate law is consistent with a mechanism in which the hydroxy complexes [Cr(Ino)(H₂O)₄(OH)]²⁺ and [Cr(Ino)(Gly)(H₂O)₂(OH)]⁺ are significantly more reactive than their conjugate acids. The value of the intramolecular electron transfer rate constant, k ₁, for the oxidation of the [Cr(Ino)(H₂O)₅]³⁺ (6.90?×?10^?4?s^?1) is lower than the value of k ₂ (9.66?×?10^?2?s^?1) for the oxidation of [Cr(Ino)(Gly)(H₂O)₂]²⁺ at 35°C and I?=?0.2?mol?dm^?3. The activation parameters have been calculated. Electron transfer apparently takes place via an inner-sphere mechanism.     

A kinetic study of the complex formation between aquachromium(III) ions and L-iso-leucine

Summary The kinetics of complex formation between aquachromium(III) ions and L-iso-leucine have been studied spectrophotometrically. Effects of varying the total chromium(III), total amino acid and H⁺ concentrations, ionic-strength, temperature and % EtOH on the k_ohs were determined. The results are best accounted for by outer-sphere complexation equilibria involving HL (the amino acid zwitterion) and [Cr(H₂O)₆]³⁺/[Cr(H₂O)₅OH]²⁺ which precede anations. A rate-equation is established which involves K_os1, K_os2, k₁, k₂ (the respective outer-sphere complexation and interchange rate constants with [Cr(H₂O)₆]³⁺ and [Cr(H₂O)₅OH]²⁺), K_a and K_h (the acid-dissociation constants of H₂L⁺HL and [Cr(H₂O)₆]³⁺ [Cr(H₂O)₅OH]²⁺ pairs). The proposed mechanism is I_a for the path involving hexaaqua- and I_d for that involving hydroxopentaaquachromium(III).     

The effect of ammine,ethanoate, trichloro- and trifluoroethanoate ligands on tris(2,4-pentanedionato) chromium(III) formation in high pH aqueous solution

The yields of tris(2,4-pentanedionato)chrvmium(III) (Cr(acac)₃) formed in the presence of either the ammonia (Am), ethanoate (ET), trichloroethanoate (TCE), or trifluoroethanoate (TFE) ligand in high pH aqueous solution, were compared with those from a medium containing only hydroxyl and water as the principal ligands besides the acac. The presence of Am, ET, and TCE drastically reduced the yields at pH’s 9.5–10.5, 7.5–9.0/9.5–11.0, and 9.0–12.0, respectively in increasing order ET > Am < TCE. The role of Am is attributed mainly to the oxo-bridged species ((OH)_m(H₂O)_nAm_5-(m+n)Cr(O)(OH)CrAm_5-(m+n)(H₂O)_n(OH)_m)^3-2m (1), ((OH)_m(H₂O)_nAm_5-(m+n)Cr(O)₂CrAm_5-(m+n)(H₂O)_n(OH)_m)^2-2m (2), and ((OH)_m(H₂O)_nAm_5-(m+n)Cr(O)(OH)CrAm_4(m+n)(H₂O)_n(OH)_m+1)^2-2m (3). 2 is the most deactivating species mainly on the basis that the Cr-O bond of the oxo-bridge is suggested as being stronger than the Cr-O bond of the hydroxo-bridge. As for ET and TCE, oxo-bridged polymeric ethanoato- and trichloroethanoatochromium(III) species are also proposed as the main origin of the drastic deactivation of the reaction not observed for TFE due possibly, to the insignificance of oxo-bridges in tnfluoroethanoatochromium(III) species.     

Bis(fluorbenzoyloxy)methylphosphanoxide

Bis(fluorbenzoyloxy)methyl phosphane oxides CH₃P(O)[OC(O)R]₂ [R = C₆H₄2F (1), C₆H₄3F (2), C₆H₄4F (3), C₆H₃2,6F₂ (4), C₆H2,3,5,6F₄ (5)] were prepared by treating silver salts of carboxylic acids AgOC(O)R with CH₃P(O)C?₂ (IR-, ¹H-, ¹⁹?F-and ³¹P{¹H}-NMR-data). The mixed anhydrides 1–5 show unusual thermal stability at room temperature. Stability against hydrolysis decreases with increasing number of fluorine-atoms. The reaction of R′P(O)C?₂ [R′ = CH₃, C₆H₅, (CH₃)₃C] with M^IOC(O)R_F [R_F = CF₃, C₂F₅, C₆F₅; M^I = Ag^I, Na^I T?^I] was investigated.     

Crystal structure of oxo(diperoxo)bipyridylmolybdenum(VI)

Yellow oxo(diperoxo)bipyridylmolybdenum(VI), C₁₀H₈MoN₂O₅, M_r = 332.1 crystallizes in the monoclinic space group P2₁/n, a = 6.261(3), b = 12.726(1) c = 13.752(3)A, β = 91.84(2)°, V = 1095.2(5)A³, Z = 4, D_c = 2.014(1) g/cm³, MoKα(λ = 0.7107A), μ = 11.8 cm^?1, T = 22(1)°C, R = 0.034, ωR = 0.040, number of reflections in least squares (F₀ > 2σ(F₀)) = 1125. The molybdenum coordination (distorted trigonal bipyramidal) is as in the corresponding chromium complex, C₁₀H₈CrN₂O₅ which has closely similar bond angles but is not isomorphous. The difference in MoN_apex and MoN_eq bond distances (2.312(5) and 2.199(5)A) is similar to that in the CrN_apex and CrN_eq distances (2.23(2) and 2.11(2)A). The MO distances for each peroxo ligand (ave. 1.910(2) and 1.950(2)A) are significantly different and slightly longer than those in the chromium complex as is the OO distance of 1.459(6)A. The latter is indicative of greater negative charge on the O₂ ligands, approaching that of O^2?₂ in the molybdenum complex.     

Basische metalle : IX. Synthese und kristallstruktur von C5H5Co(PMe3)CS2. Reaktionen zu zweikernkomplexen mit Co(SCS)Cr- und Co(SCS)Mn-Brückenbindungen

C₅H₅Co(PMe₃)CS₂ (IV) is formed in practically quantitative yield in the reaction of C₅H₅Co(PMe₃)₂ (I) or the heterobinuclear complex C₅H₅(PMe₃)Co(CO)₂Mn(CO)C₅H₄Me (III) with CS₂. The crystal structure shows that the carbon disulfide bonds as a dihapto ligand through the carbon and one sulfur atom (S(2)) (CoC = 1.89, CoS(2) = 2.24 Å, S(2)CS(1) = 141.2°). The two CS bond lengths in IV (CS(2) = 1.68, CS(1) =1.60 Å) are greater than in free CS₂ (1.554Å) which is in agreement with the strong π-acceptor character of h²-CS₂ as shown in the spectroscopic data. IV reacts with Cr(CO)₅THF and C₅H₅Mn(CO)₂THF to give the complexes C₅H₅(PMe₃)Co(SCS)Cr(CO)₅ (V) and C₅H₅(PMe₃)Co(SCS)Mn(CO)₂C₅H₅ (VI) respectively, in which the sulfur atom S(1) that is not bound to cobalt coordinates to the 16-electron fragments Cr(CO)₅ and Mn(CO)₂C₅H₅. The spectroscopic data of IV, V and VI are discussed.     

Spectres infrarouges et raman des organomagnesiens EtMgX·2Et2O (X = Br,I) cristallises et dissous dans l'ether ethylique

The infrared and Raman spectra of the low-temperature polycrystalline organomagnesium compounds, C₂H₅MgI·2(C₂H₅)₂O and C₂H₅MgBr·2(C₂H₅)₂O and of some deuterated derivatives, C₂H₅MgBr·2(C₂D₅)₂O, CD₃CH₂MgBr·2(C₂H₅)₂O, CH₃CD₂MgBr·2(C₂H₅)₂O and C₂D₅MgBr·2(C₂H₅)₂O have been investigated in the 4000-100 cm^?1 region. An assignment is given for most intramolecular fundamentals of the gauche—gauche isomer of coordinated ether molecules and the effect of coordination on ether frequencies is discussed. The fifteen fundamentals of the “isolated” ethyl group are identified and their frequencies are correlated with those of some hydrocarbons and similar organometallic compounds. The magnesium—ligand stretching vibrations, ν(MgC), ν_asym(MgO₂), ν_sym(MgO₂), ν(MgBr), and ν(MgI) are observed at 485, 317, 301, 248 and 224 cm^?1, respectively; the MgC, MgO and MgBr bond force constants are estimated and correlated with some other structural parameters. The structure of the mixed organomagnesium compound C₂H₅MgBr·2(C₂H₅)₂O in pure liquid state and in ether solution is discussed on the basis of the corresponding Raman spectra. Ethylmagnesium bromide is found to be monomeric in diethyl ether even at high concentrations.     

Syntheses of σ-cyclobut-1-en-3-onyl complexes by cycloaddition of ketenes to some transition metal alkynyl complexes

Reactions of ketenes (R¹R²CCO) with (η⁵-C₅H₅)Ni(PPh₃)CCR (I) and (η⁵-C₅H₅)Fe(CO)(L)CCR (III, L = CO and PPh₃) give σ-cyclobut-1-en-3-onyl complexes, {(η⁵-C₅H₅)Ni(PPh₃)$\text{CC(R)COC}$}R¹R² (VI) and (η⁵-C₅H₅)Fe(CO)(L)$\text{CC(R)COC}$R¹R² (IX)}, (2 + 2) cycloaddition products, in good yields. The σ-cyclobutenonyl complexes also can be prepared by the reaction of I and III with acyl chlorides in the presence of triethylamine.     

Preparation and structural characterization of [(η5-C5H5)2Zr(SC6H5)]2O. A qualitative description of the bonding in oxo-bridged dicyclopentadienyl-transition metal dimers

The hydrolysis of (η⁵-C₅H₅)₂Zr(SC₆H₅)₂ was shown previously by IR spectroscopy to produce an oxo-bridged complex. The molecular structure of this material has been determined by X-ray diffraction methods and consists of two (η²-C₅H₅)₂Zr(SC₆H₅) units linked by an oxo bridge. The ZrOZr bond is nonlinear at 165.8(2)° with a Zr?Zr interatomic separation of 3.902(1)Å. The two independent SZrO bond angles of 98.7(1) and 103.3(1)° are consistent with a d° electronic structure for each zirconium atom. The relatively short ZrO distances of 1.968(3) and 1.964(3) Å support the presence of partial double-bond character arising from the donation of electron density from filled p_π-orbitals on the oxygen atom to unfilled d-orbitals on the electron deficient d⁰ metal atoms. This bonding feature requires based upon orbital symmetry arguments that the (ML)₂O molecular core in [(η⁵-C₅H₅)₂ML]₂O complexes must be nonplanar with a dihedral angle between the two LMO planes less than 90°. For [(η⁵-C₅H₅)₂Zr(SC₆H₅)]₂O, dihedral angle of 61.7° was observed. The compound crystallizes in an orthorhombic space group, Pbca, with refined lattices parameters a 16.458(4), b 20.281(5), and c 17.016(4) Å. Full-matrix least-squares refinement of 2613 diffractometry data I > σ(I) led to a final discrepancy index R(F₀²) = 0.044.