Transition metal–carbene complexes XCVII. π-cyclopentadienyldicarbonylmethylphenylcarbenerhenium

Addition of P(OMe)₃ to [Rh(η³?C₃H₅) (CO)₂] gives [Rh(η³?C₃H₅){P(OMe)₃}₃]; this reacts with hydrogen or silanes to form a species which is an effective hydrogenation catalyst for olefins and a hydrosilylation catalyst for terminal olefins, aldehydes and ketones.     

Alkylation reactions with organometallic compounds : IV. The reaction of tris(triphenylphosphine)methylcobalt with diphenylacetylene

Tris(triphenylphosphine)methylcobalt;(Ph₃P)₃CoCH₃, reacts with diphenyl-acetylene to give α-methylstilbene as the sole addition product.(Ph₃P)₃CoCH₃ can be used as an alkylation reagent in ethers or in aromatic solvents.     

Analytical properties of the amidoxime group: Part VI. Electroanalytical behaviour of ethanediamidoxime

The electroanalytical behaviour of ethanediamidoxime (EDA) has been studied at 0.1 mol l^?1 ionic strength and several pH values (1.75–8.62), using several techniques (dc and DP polarography, chronocoulometry, dc and DP cyclic voltammetry). EDA undergoes an irreversible diffusion-controlled reduction in the entire pH range considered. The best conditions for identification and determination of EDA with the techniques mentioned are also studied.     

Reduction of dicarbonyl compounds on a DME: Part VI. A comparative study of the electrochemical behaviour of aromatic dicarbonyl compounds

Polarographic and voltammetric studies have been carried out on the reduction of benzyl and acetylbenzoyl in a basic medium.The effect of pH, reactant concentration, drop time and ethanol content on the polarographic and kinetic parameters is shown. The occurrence of adsorption phenomena is inferred from C - E curves. The degree of adsorption decreases as the ethanol content increases. Tafel slopes and reaction orders have been obtained at potentials corresponding to the foot of the wave. On the basis of these experimental data, reduction mechanisms are proposed in the zone where Tafel's law is obeyed.The results obtained here and those corresponding to the acidic medium, together with those concerning phenylglyoxal and phenylglyoxylic acid reported in the literature, are thoroughly discussed. A parallelism in the electrochemical behaviour of these compounds is made evident.     

Thermal studies on metal dithiocarbamato complexes. A review

In the present review, the work done on thermal studies of metal dithiocarbamato complexes during the period 1969–1982 are given. The thermal studies include thermogravimetry, differential thermogravimetry, differential thermal analysis, differential scanning calorimetry and mass spectrometry. Fifty-two references are included.     

Reactions of platinum complexes with spiro compounds. formation of platinospiroalkanes

Spiro[2.nalkanes (n = 2, 4, 5) react with platinum complexes to form compounds of composition PtCl₂(spiroalkane). These materials have been characterized by infrared and nuclear magnetic resonance spectra, and the point of insertion has been shown to be the cyclopropyl carbon—carbon bond opposite the spiro carbon.     

N-alkyl- and N-aryl-thioformamido complexes of the platinum group metals

Alkyl- and aryl-isothiocyanates undergo insertion reactions with platinum metal hydrides to yield the corresponding N-alkyl- and N-aryl-thioformamido

derivatives, the structure and stereochemistry of which have been deduced using ¹H NMR data.     

Stepwise reduction of the thiocarbonyl ligand: hydride transfer to CS: thioformyl,thioformaldehyde, methylthiolato,secondary carbene,formyl and iminoformyl complexes of osmium

The complexes OsHX(CS)L(PPh₃)₂ (X  Cl, Br; L  CO and X  Cl; L  CN-p-tolyl), which contain mutually cis hydrido and thiocarbonyl ligands, undergo transfer of the hydrido ligand to CS when treated with CO to give blue complexes containing the thioformyl ligand [OsCHS]. OsCl(CHS)(CO)₂(PPh₃)₂ reacts with borohydride to give the first metal complex of the thioformaldehyde monomer, viz. Os(η²-CH₂S)(CO)₂(PPh₃)₂, which reacts rapidly with HCl to give OsCl(SCH₃)(CO)₂(PPh₃)₂ and then, by a slower reaction, OsCl₂(CO)₂(PPh₃)₂ and CH₃SH. The ligands produced in this stepwise reduction have possible relevance as models for postulated intermediates in the Fischer—Tropsch synthesis. Synthetic routes to formyl [OsCHO], iminoformyl [OsCHNMe] and secondary carbene complexes [OsCHSMe, OsCHNMe₂, OsCHOMe] are also demonstrated.     

The reactivity of methylnickel, -palladium and -platinum compounds towards alkenes and alkynes

The reactivity of alkyl-transition metals towards alkenes and alkynes falls in the sequence Et₂Ni(bipy) > Et₂Pt(bipy) > Me₂Pt(bipy) > MePtCl(bipy) > MePdCl(bipy), which is also the order of decreasing energy of the filled metal d-orbitals.     

Polymerization by alkaline earth metal compounds—I. Studies on the polymerization of methylmethacrylate by triphenylmethyl metal derivatives and related compounds of calcium,strontium and barium

A series of related organo-alkaline earth metal compounds of formula (Ph₃C)MX(THF)_η, (M = Ca, Sr, Ba; X = Cl: M = Ca, Sr; X = Br: M = Ba; X = CPh₃), has been isolated and characterized using analytical and spectroscopic techniques. The polymerization of methylmethacrylate (MMA) has been studied using these organometallic compounds as initiators. Investigations of the tacticity variations of polymethylmethacrylate (PMMA) produced in 1,2-dimethoxyethane or tetrahydrofuran (THF) at 210 K have shown that the syndiotacticity is higher in the former solvent, decreases with the following changes in initiator composition: Ca > Sr ? Ba; Cl > Br; in THF, it is dependent on the monomer concentration. The molecular weight distributions of the PMMA samples are broad and have distinct bi- or poly-modal features. The polymerization process appears to be anionic in nature but the evidence suggests that more than one type of propagation occurs and mechanistic aspects are discussed. Polymerization of bulk MMA by these same initiators is also reported. The initiators RMX(THF)_n (M = Sr, R = Me, n = 3 or R = Et, n = 2) and (C₃H₅)SrBr(THF)₂ are shown to be inefficient for polymerization of MMA. The glass transition temperatures of a range of PMMA samples of differing tacticity are presented and discussed.     

Electrochemical reduction of α-unsaturated carbonyl compounds: Part VI. Effect of electrogenerated bases in very weakly protic solvents

Unsaturated carbonyl compounds were reduced electrochemically in carefully dried aprotic solvents. In the absence of protonating impurities, the i—V curves could be modified due to the de-activation of a part of the substrate by the action of the electrogenerated bases. In DMF solutions, the electrogenerated base which is able to deprotonate the ketone is mainly the dianion and discrepancies occur with, classical polarograms, of this class of reactants as observed at the level of the second step. For more conjugated ketones, a very cathodic step is assumed to correspond to the reduction of the conjugated anion of the ketone but the instability of this anion (inducing polymerisation reactions) prevents the acquisition of careful analytical evidence.     

Electrode kinetics of the metal complexes with aminopolycarboxylic acids: Part II. Cadmium-ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) complexes

The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH₃L⁺, CdH₂L, CdHL^? and CdL^2?, where CdH_pL^(p?2)+ and L^4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH₃L⁺?Cd²⁺+H₃L^? with k₃^d=6.3×10² s^?1 and k₃^f=3.3×10⁶ s^?1M^?1, where k₃^d and k₃^f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship.     

Transition metal methylene complexes : III. Methylene (CH2), a carbonyl analogous ligand; crystal structure of μ-methylenebis(carbonyl-η5-cyclopentadienylrhodium)(Rh—Rh

μ-Carbonylbis(carbonyl-ν⁵-cycopentadienylrhodium)(Rh—Rh) reacts with N-methyl-and N-ethyl-N-nitrosourea in boiling benzene to yield the dinuclear, diamagnetic, neutral rhodium complexes μ-methylene- (A) and μ-ethylidenebis(carbonyl-η⁵-cyclopentadienylrhodium)(Rh—Rh) (B), respectively. Deuterium labelled experiments prove the origin of the metal-stabilized methylene ligand to be the alkyl group of the organic precursor. This new method of preparation of transition metal—methylene complexes may be used as an alternative to the commonly used diazo method; the latter method was shown to work with diazodiethylmalonate and dicarbonyl-η⁵-cyclopentadienylrhodium, the reaction yielding μ-bis(ethoxycarbonyl)methylenebis(carbonyl-η⁵-cyclopentadienylrhodium)(Rh—Rh).Compound A crystallizes in the triclinic system, P$\text{1}$, and with cell constants of a 803.42(5), b 909.98(6), c 938.81(2) pm, α 74.402(3), β81.923(3), and γ 83.685(6)°. The unit cell volume and the calculated density are 651.6 ų and 2.069 g cm^-3, for one molecule in the asymmetric unit. The molecular geometry of μ-CH₂[η⁵-C₅H₅Rh(CO)]₂ was established from 2718 unique reflections collected with a computer-controlled diffractometer and refined to a final R(F) = 0.0379. The molecular parameters derived from the single-crystal X-ray study conform to a remarkable degree with those found for μ-CO[η⁵-C₅H₅Rh(CO)]₂. Thus, the bridging ligands CH₂ and CO seem to be analogous in their effects on the structural characteristics of the molecular framework of the two molecules.     

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.     

Boron hydride reduction of transition metal—acetyl complexes

Acetyl complexes of iron(II) and ruthenium(II) of the type (π-C₅H₅)(CO)LM(COCH₃), where L = PPh₃, P(OPh)₃, P(cyclohexyl)₃, PMe₂Ph or CO for M = Fe, and PPh₃ for M = Ru, are rapidly reduced to the corresponding ethyl complexes by BH₃ · THF or B₂H₆/C₆H₆. In some cases hydrido complexes of the type (π-C₅H₅)(CO)LMH are also formed. The reaction has been studied by use of ¹H NMR and the spectrum of (π-C₅H₅)(CO)(PPh₃)FeC₂H₅, which shows several unusual features, is discussed in detail. It is suggested that the rate of reduction increases with increasing electron density at the metal centre.Acetyl complexes of other transition metals, i.e. Ir, Pt, Pd, Co and Mo, are also reduced to the corresponding ethyl compounds by B₂H₆/C₆H₆.     

Hydrothermal reactions of lactic acid catalysed by group: VIII Metal complexes

At 220–250°C, in the presence of [PtH(PEt₃)₃]⁺ in aqueous solution, α-hydroxypropanoic acid (lactic acid) is converted into propanoic, propenoic, β-hydroxypropanoic, pyruvic and acetic acids. The reactions catalysed by [PtH(PEt₃)₃]⁺ appear to be double bond hydration and hydrogenation, alcohol dehydration, transfer hydrogenation, decarbonylation and the water-gas shift reaction. Propenoic acid under similar conditions gives similar products. Attempts to catalyse the dehydrocarboxylation of lactic acid using PdCl₂, [PtCl₂(PPh₃)₂], [IrH(CO)(PPh₃)₃] and [Ni(CO)₂(PPh₃)₂] were partially successful, although competing disproportionation gave a wide range of products.     

The effect of the central metal charge on the reduction half-wave potentials of d6 metal complexes

A SCF MO calculation is made to obtain the energies of the lowest vacant π-orbitals of tris-2,2′-bipyridine complexes of d⁶ transition metals in various oxidation states. Any overlap of a metal t_2g-orbital and a ligand π-orbital is neglected and the metal ion is considered as a source of an electrostatic potential field. The π-electron system of the three ligand molecules is treated as a whole by taking account of the overlap of 2pπ-AO's belonging to different ligand molecules. When it is assumed that a ligand π^*-orbital is occupied by the electron added in the course of reduction, the results of the calculation and the Born equation for solvation energy together lead to a linear relation between the reduction half-wave potentials of the complexes and the sum of the charges on the central metal ion and the ligand nitrogen atoms. This linear relation is confirmed experimentally by using the available data on the reduction half-wave potentials of the tris-bipyridine complexes of the following d⁶ metals: Ir(III), Fe(II), Ru(II), Os(II), Cr(0), Mo(0), V(?I) and Ti(?II).     

Studies of layered uranium(VI) compounds. I. High proton conductivity in polycrystalline hydrogen uranyl phosphate tetrahydrate

We have found that hydrogen uranyl phosphate tetrahydrate HUO₂PO₄·4H₂O has a high proton conductivity. The ac conductivity was 0.4 ohm^?1 m^?1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole^?1. The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H⁺ conductivity thus inferred is attributed primarily to the high concentration of H⁺, which exists as H₃O⁺ in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H₃O⁺ ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.