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The synthesis of a new Costa-type ligand (H2mdo), in which the propylene bridge has been replaced by a methylenedioxy bridge has been accomplished: two molecules of butanedione monoxime are coupled via a CH2 group by reaction with CH2Cl2 and 50% aqueous NaOH in the presence of a phase-transfer reagent. Dihalidocobalt and alkyl(halido)cobalt complexes of the deprotonated ligand have been prepared and structurally characterized by NMR spectroscopy.

The effect of pressure on the rates of anaerobic thermal homolytic scission of the CoC bond in alkylcobalamins (RB12, R = adenosyl, methyl and neopentyl) has been determined in different solvents of varying viscosities. Homolytic fission is compared to bond breaking using βH elimination (isopropylcobalamin in ethylene-glycol + water mixtures) and heterolysis. These two mechanisms are characterized by a volume of activation that is close to zero, whereas homolysis shows a substantial positive value (18.5 cm3 ṁ mol−1 for AdoB12, 17.0 cm3 ṁ mol−1 for MeB12 and 7.9 cm3 ṁ mol−1 for neopentylB12, all in ethylene glycol) for this parameter. Consequently, pressure effects can be used, with caution, as mechanistic probes to identify CoC bond fission mechanisms for the reactions studied.

CoII(salen) derivatives (salen = {N,N′-ethylenebis[salicylideneaminato]}) whose ethanediyl moiety carries an alkenyl side-chain R [R = prop-2-en-1-yl (6a), 2-methylprop-2-en-1-yl (6b), but-2-en-1-yl (6c), but-3-en-1-yl (6d)] react with oxygen and alcohols to give organocobalt(III) complexes containing a β-alkoxy-substituted three- or four-carbon bridge between cobalt and the equatorial ligand. NMR and UV–VIS spectroscopic studies show that product formation is a three-stage process involving (1) oxidation of cobalt(II) to produce an (alkoxo)cobalt(III) complex, (2) intramolecular interaction of cobalt(III) with the alkenyl double bond to yield a carbocationic intermediate, and (3) nucleophilic attack by the alcohol. In the case of cobalt(II) complex 6e (R = 3-methylbut-3-en-1-yl), the major product is bridged β-methylene organocobalt(III) complex 10, demonstrating that proton loss competes with addition of alcohols when the intermediate organocobalt(III) species has a substantial degree of tertiary carbocation character. Application of the alkoxycobaltation reaction to 6d and ethane-1,2-diol afforded bridged [β-(2-hydroxyethoxy)alkyl]Co(salen) complex 20, a simple model for coenzyme B12 with a built-in substrate. The molecular structure of 20 has been determined by X-ray diffraction methods.

Product and kinetic studies on the anaerobic thermolysis and photolysis of the cobalt-carbon bond in intramolecularly bridged organoCosalen compounds are presented. NMR, UV-Vis and electron spin resonance spectroscopic investigations reveal the formation of paramagnetic Co(II) complexes upon thermolysis or photolysis in solution. Solid state thermolysis (studied by differential thermal analysis, thermogravimetry, mass spectrometry and magnetochemistry) also leads to cobalt (II) products. The fate of the carbon radical produced by homolytic cleavage of the Co C bond in solution depends on the reaction conditions used. In the presence of a radical trap (tempo, pbn), carbon radical spin trapping is the major pathway. On the other hand, a Co(II) salen complex carrying an ω-alkenyl side chain was shown to be the major product when Cosalen(CH 2 ) 4 ( 2 ) was subjected to prolonged photolysis in the absence of a radical trap. While the chemistries of intramolecularly bridged organoCosalen and open chain alkylCosalen complexes are qualitatively quite similar, kinetic studies using the radical trap method reveal large quantitative differences. The bridged complex Cosalen(CH 2 ) 3 ( 1 ) is quite unreactive towards thermolysis and photolysis, even at high trap concentration. Quantum yields (Φ) of Cosalen(CH 2 ) 4 ( 2 ) and butylCosalen ( 4 ) were determined in toluene as a function of the concentration of tempo. For 2 , Φ max is reached at 0.5 M of tempo, while for 4 Φ is already maximal at a very small excess of tempo (≈0.0005 M); in both cases the complex concentration is ca. 10 4 M. These results are discussed in terms of the solvent-caged radical pair concept.

The synthesis of H2salen {2,2′-[ethane-1,2-diylbis(nitrilomethylidyne)]diphenol} derived models 1 for coenzyme B12 with a carbon bridge between the ligand and cobalt has been accomplished by condensation of salicylaldehyde and ω-substituted 1,2-diamines 8 followed by complexation with CoII, reduction to CoI complexes, and intramolecular alkylation. The structures of intramolecularly alkylated Co(salen) complexes with a bridge of three, 1b, and of four methylene groups, 1c, have been investigated by NMR spectroscopy and determined by X-ray crystallography.

Solvational changes on the transfer of coenzyme B12 from water to 1-propanol/water mixtures were studied from the temperature dependence of the measured solubilities. These results were combined with calculations using the scaled particle theory. The strong decrease of the Gibbs energy of cavity formation on going from water to 1-propanol is for the smaller mole fractions of 1-propanol (x < 0.3) nearly balanced by a decreasing preferential (peripheral) solvation of the hydrophilic macromolecule by water. Thermodynamic and activation parameters were derived for the thermal homolytic Co-C bond breaking of coenzyme B12 in 50wt% 1-propanol/water and compared with earlier values for the homolysis at 380.6 K in the mixtures. An initial state-transition state dissection of solvent effects for the homolysis reveals that desolvation does not play an important role in activation. The slightly exceptional position of water stems from a stabilization of the initial state.

Air oxidation of a CoII(salen) derivative, whose ethanediyl moiety carries a methylene-linked 4-hydroxypent-1-en-3-yl substituent, yields an intramolecularly bridged (tetrahydrofurfuryl)CoIII(salen) complex of which the crystal structure has been determined; this B12 model is very resistant to Co–C bond homolysis, even in the presence of a large excess of the radical trap TEMPO.

CoII(salen) derivatives (salen = {N,N′-ethylenebis[salicylideneaminato]}) whose ethanediyl moiety carries an alkenyl side-chain R [R = prop-2-en-1-yl (6a), 2-methylprop-2-en-1-yl (6b), but-2-en-1-yl (6c), but-3-en-1-yl (6d)] react with oxygen and alcohols to give organocobalt(III) complexes containing a β-alkoxy-substituted three- or four-carbon bridge between cobalt and the equatorial ligand. NMR and UV–VIS spectroscopic studies show that product formation is a three-stage process involving (1) oxidation of cobalt(II) to produce an (alkoxo)cobalt(III) complex, (2) intramolecular interaction of cobalt(III) with the alkenyl double bond to yield a carbocationic intermediate, and (3) nucleophilic attack by the alcohol. In the case of cobalt(II) complex 6e (R = 3-methylbut-3-en-1-yl), the major product is bridged β-methylene organocobalt(III) complex 10, demonstrating that proton loss competes with addition of alcohols when the intermediate organocobalt(III) species has a substantial degree of tertiary carbocation character. Application of the alkoxycobaltation reaction to 6d and ethane-1,2-diol afforded bridged [β-(2-hydroxyethoxy)alkyl]Co(salen) complex 20, a simple model for coenzyme B12 with a built-in substrate. The molecular structure of 20 has been determined by X-ray diffraction methods.

The effect of pressure on the rate of the anaerobic thermal fission of the CoC bond in alkylcobalamins (RB12, R = adenosyl, methyl, isopropyl and neopentyl) and [adenosylcobinamide]+OH− has been determined in water and 1-propanol/water mixtures. The volumes of activation for homolysis are discussed in terms of the cage model and solvent dependence. It is shown that homolysis at high temperatures (370–390 K) can be described by a negligible cage efficiency and a large volume of activation. At lower temperatures the cage efficiency increases. The type of group R has hardly any influence on the size of the volume of activation for homolysis, which is constant at about 20 cm3 mol−1. The results are compared with those obtained earlier for organocobalt models of coenzyme B12. It is shown that pressure effects can be used as mechanistic probes to identify reactions in which the radicals have the opportunity to escape from the cage.

The effect of viscosity on the rates of anaerobic thermal and photolytical scission of the cobalt-carbon bond in coenzyme B 12 (AdoB 12 ) has been determined in different water/glycerol mixtures. The photolysis of the coenzyme is compared with [adenosylcobinamide] + OH − . Using a simple diffusion model, the fractional cage efficiency and the rate of recombination of the radicals trapped in the solvent cage could be determined from the viscosity dependence of the reaction rates. The activation parameters for the cage recombination were derived from the temperature dependence of the recombination reaction giving Δ H ‡ = 17.9±0.8 kJ mol −1 and Δ S ‡ = 21±2 J K −1 mol −1 . The results obtained are shown to be relevant to the discussion about the CoC bond dissociation energy of AdoB 12 , for which a combination of our results with published data gives 119±8 kJ mol −1 .

The effects of pressure and temperature on the rates of the thermal homolytic fission of the cobalt-carbon bond in some organocobalt model compounds for co-enzyme B 12 has been determined. The investigated model systems comprise six-coordinated benzyl complexes of dimethylglyoxime with nitrogen and phosphorus coordinated axial ligands and five- and six-coordinated butyl complexes of the Schiff base bis(salicylidene)ethylenediamine. The observed activation volumes, reported for the first time for this type of compound, are discussed in terms of the cage model and solvent dependence. The observed activation enthalpies, a measure for the cobalt-carbon bond dissociation energies, are discussed in terms of steric and electronic factors of the trans -ligand. Finally, an isokinetic relationship and the infiuence of the coordination number are mentioned.

The preparation and oxygenation of CoII(salen) complexes 2a–d whose ethanediyl moieties carry an ω-hydroxyalkyl pendant arm is reported. Air oxidation of 2a gives bridged (alkoxo)CoIII(salen) complex 1a with a planar salen ligand. Upon aeration of 2b in MeOH, monomeric bridged complex 1b with a planar salen ligand precipitates, whereas in CH2Cl2 a dimeric bridged complex 1b′ with twisted-salen ligands is obtained and characterized by X-ray crystallography. Formation of a bridged (alkoxo)CoIII(salen) complex upon aeration of 2c is probably followed by β-elimination. The tert-butyl substituted oxidation product of 2d is paramagnetic in non-coordinating and diamagnetic in coordinating solvents, supporting the concept that the salen moiety of (alkoxo)CoIII(salen) complexes can have either a planar or a twisted geometry depending on the solvent and the solid state properties.

CoII(salen) derivatives whose ethanediyl moiety carries an ω-alkenyl side chain react with oxygen and methanol or ethanol to give, via interaction between intermediate CoIII species and the pendant olefin, alkylated products with a β-alkoxy substituted carbon bridge between cobalt and the equatorial ligand; the corresponding intermolecular reaction between CoII(salen) and hex-1-ene does not take place.

A new cobaloxime-type model for coenzyme B12 has been synthesized. For this model strong steric repulsion between its dioxymethylene bridge and (one of) the axial ligands on cobalt presages enhanced rates of axial ligand exchange.

The synthesis and structure of the first H2salen {2,2′-[1,2-ethanediylbis(nitrilomethylidyne)]diphenol} derived model for coenzyme B12 with an intramolecular carbon bridge between the ligand and cobalt are reported.

Air oxidation of a CoII(salen) derivative, whose ethanediyl moiety carries a methylene-linked 4-hydroxypent-1-en-3-yl substituent, yields an intramolecularly bridged (tetrahydrofurfuryl)CoIII(salen) complex of which the crystal structure has been determined; this B12 model is very resistant to Co–C bond homolysis, even in the presence of a large excess of the radical trap TEMPO.

The ammoniation ofcis-[Rh(en)2Cl2] · (ClO4) in liquid NH3 was studied at constant ionic medium of 0.20 m perchlorate in the 0 to 35° range. The complex reacts in two distinct steps to givecis-[Rh(en)2(NH3)2] · (ClO4)3, with the intermediate formation ofcis-[Rh(en)2(NH3)Cl] · (ClO4)2. Both steps follow a conjugate-base mechanism. Activation parameters were obtained for the acid-base preequilibrium and the rate-determining step. The entropies of activation for the rate-determining step are 0 and −42 JK−1mol−1 for the first and second ammoniations respectively. These values are considerably lower than those found for the cobalt(III) analogues. The entropy changes for the acid-base equilibria are −84 and −36 JK−1mol−1 respectively, which is less negative than those values found for the cobalt(III) analogues. Trans-[Rh(en)2I2] · (ClO4) ammoniates totrans-[Rh(en)2(NH3)I] · (ClO4)2. The contribution of spontaneous ammoniation to the overall reaction oftrans-[Rh(en)2I2] · (ClO4) is negligible, so the uniqueness oftrans-[Co(en)2Cl2] · (ClO4) among cobalt(III) complexes in this respect is not reproduced for thetrans-dihalotetraamine structure in rhodium(III) complexes. A comparison of cobalt(III) and rhodium(III) amines with respect to activation parameters and the influence of formal charge of the metal complex on reactivity indicates a more associative type of activation for rhodium(III).

A kinetic study is reported on the reaction between copper(II) perchlorate and the tridentate ligand 1-(2-hydroxyphenyl)-3,5-diphenylformazan in ammoniacal ethanol–water mixtures at constant ionic strength. A detailed analysis of medium effects has resulted in the elucidation of the mechanism of the reaction. Coordination of an amminecopper(II) complex to the formazan is shown to occur in discrete steps. A rather stable bicoordinated intermediate could be studied in solution. A comparison with earlier results has clarified some of the factors that determine the attack of metal ions on the formazans.

Etude des reactions d'ammoniation des complexes du titre ou X, Y=Cl − , Br − , DMSO, DMT ou X et Y sont des coordinats substitues rapidement. Spectres RMN des produits. Le taux de substitution semble dependre de la nature du coordinat X ou Y le plus fortement lie

Etude de la 1ere etape de l'ammoniation de trans-[Co(en) 2 YX]ClO 4 (avec Y=N 3 , NCS; X=Cl, Br) dans NH 3 liquide. La deprotonation limite la vitesse. Parametres d'activation

The kinetics of the final bromide-chloride substitution in the formation of FeCl 4 − from FeBr 4 − have been studied photometrically in six dipolar aprotic solvents. The role of the counterion could be evaluated with the aid of conductometrically-determined ion-pair association constants. Whether an ion-pair, ion-pair mechanism, an ion-pair, ion mechanism or an ion, ion mechanism is operative depends mainly on the dielectric constant of the solvent. The activation parameters (ΔH ≠ between 17 and 36 kJ mol − ; ΔS ≠ between −83 and −27 J K −1 mol − ) indicate an associative mode of activation. An initial-state transition-state dissection reveals the importance of donor and acceptor properties of the solvents for the ion-pair, ion-pair mechanism and the prime importance of the acceptor properties for the ion-pair, ion mechanism.

Nucleophilic Substitution, Palladium(II), Formazan, 1-(2-hydroxyphenyl)-3,5-diphenylformazan, Solvent Effects, Substitution Reactions Nucleophilic substitution in different non-aqueous solvents at square planar complexes of palladium(II) with 1-(2-hydroxyphenyl)-3,5-diphenylformazan (H2Fo) according to FoPdX + Y → FoPdY + X, with X = ammonia and pyridine and Y = triphenylphos-phine, thiourea and the thiocyanate ion, obeys the rate law: rate = k1[FoPdX] + k2[FoPdX][Y]. Values of k1 and k2 were obtained at 25.0 °C. From measured solubilities and literature values of transfer functions the transfer chemical potentials for initial and transition state were calculated. A comparison of these quantities in different solvents leads to the conclusion that non-specific solvation effects are dominant in these compounds. The mutual dependence of the effects of leaving and entering ligands and solvent on the substitution rate indicates a level of synchronicity of bond-breaking and bond-making that is unusually high for palladium(II) complexes.

Rate constants for the reaction of aquanitrocobaloxime with thiourea were measured as a function of pH, solvent composition, pressure and temperature, in dioxane-water mixtures. With the aid of solubility measurements a complete quantitative analysis of solvent effects on the initial state and transition state transfer parameters could be made. It was found that the activation enthalpy and entropy vary strongly with solvent composition. This is in contrast to the variations found for vitamin B12, for which this cobaloxime is a model compound. The rate constants increase strongly after 50 vol.% dioxane in the dioxane-water mixtures, another difference with vitamin B12. The volumes of activation are small and positive, in accordance with a dissociative mode of activation.

Etude de l'influence du milieu et des concentrations sur les mecanismes des reactions. Cinetique

The preparation of the series ofcis- andtrans-[Co(NH3)4(RNH2)Cl]2+ complexes (withcis, R = Me orn-Pr andtrans, R = Me, Et,n-Pr,n-Bu ori-Bu) is described. The u.v-visible spectra indicate a decrease of the ligand field on increasing chain length. Infrared spectra show an enhanced Co-Cl bond strength compared to the pentaammine. Partial molar volumes of the complex cations do not reveal steric compression. From proton exchange studies in D2O it follows that [Co(NH3)5Cl]2+ and thecis- andtrans-[Co(NH3)4-(CH3NH2)C1]2+ complexes exchange the amine protons on the grouptrans to the chloro faster than those on thecis. A coordinated methylamine group exchanges its amine protons slower than a corresponding NH3 group in the parent pentaammine, but the methyl introduction accelerates the exchange of the other NH3 groups. The aquation of thetrans-alkylamine complexes (studied at 52° C) is acceleratedca. 10 times compared to the parent pentaammine, irrespective of the nature of the alkyl group. Thecis complexes do not show this acceleration of aquation. In base hydrolysis (studied at 25° C) thecis complexes are the most reactive (a factor 20 over the parent ion). Thecis/trans product ratio in base hydrolysis and the competition ratio in the presence of azide ions were calculated from the 500 MHz1H n.m.r. spectra, which display distinctly different alkyl resonances for each individual complex. Thecis ions react under stereochemical retention of configuration; thetrans compounds give 10±1%trans tocis rearrangement. The ionic strength (⩽4 mol dm−3) and the pH do not affect this result. The same product ratio is obtained in methanol-water and DMSO-water mixtures. Ammoniation in liquid ammonia gives the same ratios as in base hydrolysis, base-catalyzed solvolysis in neat methylamine gives stereochemical retention for both thecis- andtrans-methylamine ion. The product competition ratio (Co-N3)/(Co-OH2) for thecis compounds and the bulkier amines (R =n- andi-Bu), 15–25% at 1 mol dm−3N 3 − , isca. twice that of thetrans compounds and the pentaammine. The results are interpreted in the classical conjugate base mechanism, and discussed in the context of current ideas about stereochemistry of base hydrolysis.

The preparation is described of [CoCO3(NH3)5]ClO4 · H2O, trans-[CoCO3(NH3)4(15NH3)]ClO4, and trans-[CoCO3(NH3)4(NH2CH3)]ClO4. The transformation reactions of these complexes, in which a chelate carbonate ligand is formed and one NH3 is eliminated, were studied in solution and in the solid state. 1H NMR spectroscopy is used for the identification of the products. It is shown that the transformation reactions are not stereospecific. Bildung und Umwandlung von Ammincarbonatocobalt(III)-Komplexen. Die Darstellung von [CoCO3(NH3)5]ClO4 · H2O, trans-[CoCO3(NH3)4(15NH3)]ClO4 und trans-[CoCO3(NH3)4(NH2CH3)]ClO4 wird beschrieben. Die Umwandlungsreaktionen dieser Komplexe, in denen ein Chelat-Carbonat-Ligand gebildet und ein NH3 abgespalten wird, wurden in Losung und im festen Zustand untersucht. Die Identifizierung der Verbindungen erfolgte mittels 1H-NMR-Spektroskopie. Es wird gezeigt, das die Umwandlungs-reaktionen nicht stereospezifisch sind.

The preparation is described of cis-tetramminebis(dimethylformamide)cobalt(III) and cis-tetramminebis(trifluoromethanesulfonato)cobalt(III) complexes. The compounds were studied as starting materials for the synthesis of other tetrammine compounds formed by substitution processes which are based on the reactivity of the ligands. Darstellung von cis-Tetrammincobalt(III)-Komplexen Die Darstellung der cis-Tetramminbis(dimethylformamid)cobalt(III)- und cis-Tetramminbis(trifluormethansulfonat)cobalt(III)-Komplexe wird beschrieben. Die Verbindungen werden als Ausgangsmaterialien fur die Herstellung von anderen Tetrammin-Verbindungen unter sucht, die durch Substitutionsreaktionen, basierend auf der Reaktivitat der Liganden, gebildet werden.

A kinetic study is reported on the substitutions: FoPdX+Y→FoPdY+X (H2Fo=l-(2-hydroxyphenyl)-3,5-diphenylformazan) with X=NH3 and py and Y = thiourea, tetramethylthiourea, triphenylphosphine and the thiocyanate ion, in seven nonaqueous pure solvents. Under pseudo first-order conditions a two-term rate-law is obeyed: k(obs)= k1+k2 [Y]. The results in terms of reactivity pattern and solvent effects on initial and transition states are very similar to the ones found for the analogous substitutions at platinum(II). The rate of solvolysis (k1) is determined by the donor number of the solvent and is not related to the transfer free energy of solvation of the substrate. Nonspecific solvation effects dominate. The entropy of activation for the direct nucleophilic displacement, FoPdNH3+Ph3P, is found to lie between −110 and −20 JK−1 mol−1, indicating the associative character of the substitutions. The named reaction exhibits an isokinetic relationship in the various solvents. In spite of that, an initial state — transition state — final state comparison shows the position of the transition state on the reaction coordinate to be solvent dependent. The importance of charge transfer from the donor solvent to the metal ion in determining the Gibbs free energy of the transition state is emphasized.

The kinetics of water–ammonia exchange at trans-[Rh(NH3)4(OH)2]+ and [Rh(NH3)5(OH)]2+ have been studied in aqueous ammoniacal solution. The reaction of trans-[Rh(NH3)4(OH)2]+ with ammonia to form [Rh(NH3)5(OH)]2+ has been studied in the temperature range 68–90 °C. The pH dependence of the observed rate constant indicates H2O to be the leaving group. The kinetics could be interpreted in terms of the usual Eigen–Wilkins interchange mechanism, with a more associative character than cobalt(III). The reaction of [Rh(NH3)5(OH)]2+ with ammonia has been studied in the temperature range 112–142 °C, where the reaction results in an equilibrium mixture of [Rh(NH3)5(OH)]2+ and [Rh(NH3)6]3+. The equilibrium constant has been determined spectrophotometrically and kinetically. The forward reaction is pH-independent. The reverse reaction, which gives a small contribution to the overall rate, is base-catalysed. These results can be explained by a direct interchange of OH– and NH3 or by the conjugate-base mechanism.

The kinetics substitution of the reversible reactions of SCN − , Cl − and Br − ligands at tetrahedral tetrahaloferrates(III) were studied photometrically in dichloromethane. Infrared spectra show thiocyanate to be N-bonded in these complexes. With the aid of conductometrically-determined ion-pair association constants, evidence was found for an ion-pair, ion-pair mechanism. The activation parameters (ΔH ≠ between 3 and 30 kJ mol −1 , ΔS≠ between −77 and −155 J K −1 mol t-1 ) indicate an associative mode of activation. A linear enthalpy relationship was found (slope 0.57), implying substantial bond-breaking and bond-making in the transition stare (I a mechanism). The relative bond strengths of the (FeX 3 )Y − combinations, with X − = Cl − or Br − and Y − = Br − , Cl − or NCS − , vary in the order Br − − ⪡ NCS − .

At high pH, polyelectrolytes like chondroitin sulphate form soluble complexes in water with a number of di- and trivalent cations probably through binding at the carboxylate groups. This interaction was investigated by ligand-field and ESR spectroscopy, H + and Cu 2+ potentiometry, and viscometry.

The rate constants for the reaction of aquocobalamin with thiosulphate were measured as a function of ionic strength and solvent composition in dioxane-water mixtures.

1H n.m.r. spectra of coordinated ethylenediamine HN< hydrogens for 23trans-[Co(en)2XY]n+ complexes and1H-2H exchange rates for some selected compounds are reported in the solvent liquid ammonia. Assignments are presented for asymmetric complexes. These are based on the chemical shift order of the symmetric complexes. The assignments are confirmed by independent experimental findings including the effect of specific ion-association and the observed exchange rates. The exchange rates follow the relation kobsd=k0+k1 [N2H4ClO4]−1, illustrative of general base-catalysis; in this case rate-determining proton abstraction by N2H3 and N2H2−, respectively. Chemical shifts and exchange rates in liquid ammonia follow the published order of the aqueous parameters.

in full text.

The rate constants for the reaction of aquocobalamin with the thiocyanate ion were measured as a function of ionic strength and solvent composition in acetonitrile-water mixtures. The reaction is described by a two-step mechanism: the ligation reaction, where the most stable isomer (S-bonded) is formed and the isomerisation reaction (S-bonded to N-bonded thiocyanate). For the ligation reaction a full quantitative analysis of solvent effects could be performed, whereas for the isomerisation reaction only qualitative observations were made. The equilibrium constant for the isomerisation (S-bonded/N-bonded) is large and does not change with the solvent composition. It is found that the transfer Gibbs energies of activation for the ligation reaction are the same as found for the ligand thiourea. The absence of a solvent effect on the isomerisation reaction is a further example of the ability of vitamin B 12 to create its own micro environment.

The cis–trans product ratio for the base-catalyzed solvolysis of the 15NH3 labelled trans-[Co(NH3)4(15NH3)Cl]2+ ion has been determined as a function of the solvent. The solvents used were H2O–CH3OH and H2O–dmso (dimethyl sulphoxide) mixtures, as well as anhydrous CH3OH and CH3NH2. The trans-[Co(NH3)4(15NH3)(dmso)]3+ ion was included also in the study. The base hydrolysis results do not show any solvent or leaving-group dependence, within experimental error (2%). All trans systems give (44 ± 1)% rearrangement. Small, significant differences are shown by the penta-amminechloro-complex in CH3NH2(50% rearrangement) and by the dmso complex (48% rearrangement in H2O and 46% in CH3OH). The results are interpreted as indicating the adequacy of a mechanistic model involving a five-co-ordinate intermediate.

A new cobaloxime-type model for coenzyme B12 has been synthesized. For this model strong steric repulsion between its dioxymethylene bridge and (one of) the axial ligands on cobalt presages enhanced rates of axial ligand exchange.

A kinetic study is reported on the reaction between nickel(II) perchlorate and the tridentate ligand 1-(2-hydroxyphenyl)-3,5-diphenylformazan, resulting in two linkage isomers, in ammonia ammonium perchlorate buffers in an ethanolwater (50 wt.%) mixture. The medium dependence of the rate law and the isomer product ratio were analysed on the basis of the Eigen-Wilkins mechanism. The first step of the two-ring chelate coordination seems to be the formation of the Ni(II)-phenoxide bond, followed by a rate-determining Ni(II)N ring closure. The isomer rearrangement reaction gave information on the decomposition of the complex. The pH dependence of the rate of decomposition indicates the closure of the first ring to be rate-determining

The preparation is described of [CoCO3(NH3)5]ClO4 · H2O, trans-[CoCO3(NH3)4(15NH3)]ClO4, and trans-[CoCO3(NH3)4(NH2CH3)]ClO4. The transformation reactions of these complexes, in which a chelate carbonate ligand is formed and one NH3 is eliminated, were studied in solution and in the solid state. 1H NMR spectroscopy is used for the identification of the products. It is shown that the transformation reactions are not stereospecific. Bildung und Umwandlung von Ammincarbonatocobalt(III)-Komplexen. Die Darstellung von [CoCO3(NH3)5]ClO4 · H2O, trans-[CoCO3(NH3)4(15NH3)]ClO4 und trans-[CoCO3(NH3)4(NH2CH3)]ClO4 wird beschrieben. Die Umwandlungsreaktionen dieser Komplexe, in denen ein Chelat-Carbonat-Ligand gebildet und ein NH3 abgespalten wird, wurden in Losung und im festen Zustand untersucht. Die Identifizierung der Verbindungen erfolgte mittels 1H-NMR-Spektroskopie. Es wird gezeigt, das die Umwandlungs-reaktionen nicht stereospezifisch sind.

Apparent molar volumes φ v , were measured at 25 °C in 0.1 molal NaClO 4 for aquocobalamin chloride, methylcobalamin, 5′-deoxyadenosylcobalamin and aquanitrocobaloxime. For the organocobalamins the pH dependence of φ v was studied, and for aquocobalamin and aquanitrocobaloxime the dependence on solvent composition was studied. The base-off forms of the organocobalamins have the same volumes as the base-on forms. The apparent molar volume of aquocobalamin chloride is almost independent of solvent composition in dioxane-water mixtures, but increases dramatically in acetonitrile-water mixtures.

Sowohl cis- als auch trans- Dichlorotetraammincoba t(III)-Salze liefern bei Behandlung mit AgClO4 in DMF bei Raumtemp. den cis-Bis-DMF-Komplex (I).

The kinetics of water–ammonia exchange at trans-[Rh(NH3)4(OH)2]+ and [Rh(NH3)5(OH)]2+ have been studied in aqueous ammoniacal solution. The reaction of trans-[Rh(NH3)4(OH)2]+ with ammonia to form [Rh(NH3)5(OH)]2+ has been studied in the temperature range 68–90 °C. The pH dependence of the observed rate constant indicates H2O to be the leaving group. The kinetics could be interpreted in terms of the usual Eigen–Wilkins interchange mechanism, with a more associative character than cobalt(III). The reaction of [Rh(NH3)5(OH)]2+ with ammonia has been studied in the temperature range 112–142 °C, where the reaction results in an equilibrium mixture of [Rh(NH3)5(OH)]2+ and [Rh(NH3)6]3+. The equilibrium constant has been determined spectrophotometrically and kinetically. The forward reaction is pH-independent. The reverse reaction, which gives a small contribution to the overall rate, is base-catalysed. These results can be explained by a direct interchange of OH– and NH3 or by the conjugate-base mechanism.

The cis–trans product ratio for the base-catalyzed solvolysis of the 15NH3 labelled trans-[Co(NH3)4(15NH3)Cl]2+ ion has been determined as a function of the solvent. The solvents used were H2O–CH3OH and H2O–dmso (dimethyl sulphoxide) mixtures, as well as anhydrous CH3OH and CH3NH2. The trans-[Co(NH3)4(15NH3)(dmso)]3+ ion was included also in the study. The base hydrolysis results do not show any solvent or leaving-group dependence, within experimental error (2%). All trans systems give (44 ± 1)% rearrangement. Small, significant differences are shown by the penta-amminechloro-complex in CH3NH2(50% rearrangement) and by the dmso complex (48% rearrangement in H2O and 46% in CH3OH). The results are interpreted as indicating the adequacy of a mechanistic model involving a five-co-ordinate intermediate.

The preparation is described of trans-[Co(NH3)4(15NH3)(OSO2CF3)][ClO4]2 as a precursor for the synthesis of 15NH3-labelled complexes trans-[Co(NH3)4(15NH3)X]3+ with weakly co-ordinating neutral ligands X. The availability of these compounds makes it possible to study the stereochemistry of base-catalysed solvolysis of penta-amminecobalt(III) complexes in weakly co-ordinating dipolar aprotic solvents. For a number of cases the observed sterochemistry differs significantly from the results reported earlier for more strongly co-ordinating and protic solvents (around 50% rentention). This difference in sterochemistry cannot be exclusively explained by sovational effects. Specific effects of the entering and leaving groups are snown to be important. The results are compatible with a mechanistic model involving a five-co-ordinate intermediate.

The solubilities of aquocobalamin chloride and eight other cobalt(III) complexes have been measured in dioxane–water mixtures. For aquocobalamin chloride, aquanitrocobaloxime and aquamethylcobaloxime solubilities have also been measured as a function of temperature. The transfer thermodynamic functions ∂mG°, ∂mH° and ∂mS° have been calculated. The results may be accounted for in terms of the cavity energy and specific interactions. A difference has been found between the transfer functions of the aquocobalamin cation in dioxane–water mixtures and those in acetonitrile–water mixtures.