1,721,109 research outputs found
Reaction of di-μ-phenylthio-bis(tricarbonyliron)(Fe-Fe) with triphenyl-phosphine: A detailed kinetic and mechanistic study
No full textDi-μ-phenylthio-bis(tricarbonyliron)(Fe-Fe) undergoes a two-step CO substitution reaction with triphenyl-phosphine in decalin. The substitution does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. The unsubstituted complex undergoes direct attack by PPh3 either on the predominant anti-form or on the very reactive syn-form which is produced in the rate-determining anti-syn-isomerisation. The monosubstituted complex, which is also present mainly in the anti-form in equilibrium with a reactive syn-form, reacts with carbon monoxide through an SN2 mechanism, but a CO-dissociative mechanism is involved in its reaction with the bulkier PPh3. The bis(phosphine) complex so obtained is unable to undergo SN2 displacement even by carbon monoxide and must previously lose one molecule of phosphine. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediate by carbon monoxide and triphenylphosphine have been obtained. Equilibrium and activation parameters for these reactions are reported
Kinetics and substitution mechanism in the reaction of the seven-coordinate complexes bis-mu-diphenylphosphido- and bis-mu-dimethylphosphido-octacarbonyldimolybdenum (Mo-Mo) with tri-n-butyl-phosphine
No full textThe dinuclear phosphido-bridged molybdenum complexes [(CO)(4)Mo(mu -PR2)(2)Mo(CO)(4)] (R = Ph or Me) react with P(n-Bu)(3) to give the corresponding mono and bis-phosphine derivatives. A kinetic study of the first substitution in decalin indicates a CO-dissociative mechanism involving the coordinatively unsaturated intermediate [(CO4)Mo(mu -PR2)(2)Mo(CO)(3)], The overall substitution rate depends on the rate of CO dissociation, k(1), and on the rate of bimolecular attack by CO, k(-1), and by P(n-Bu)(3), k(2). on the reactive intermediate. The nature of the substituents at the phosphido bridge markedly affects the value of k(1) which is higher for the phenyl compared with the methyl group. This is mainly due to a lower activation enthalpy (DeltaH(1)* = 125 (Ph) versus 141 (Me) kJ mol(-1)), which may reflect a weakening of the Mo-CO bond in the presence of a more electron-withdrawing ligand in trans position. The values of the competition rate ratio k (- 1)/k(2), always largely greater than unity, show that attack of the small CO is favoured with respect to the large P(n-Bu)3; this suggests that the steric crowding observed on the molecular structure of the starting seven-coordinate complexes should play an important role also on the reactivity of their six-coordinate intermediates
Solution dynamics of the reversible carbonyl?phosphorus ligand exchange in [(OC)4Mo(�-PEt2)2Mo(CO)4]: kinetic parameters as a measure of the relative steric hindrance of phosphines and phosphites
No full textReaction of di-μ-diethylphosphido-bis(tetracarbonylmolybdenum) (Mo-Mo) with tri-n-butylphosphine: Kinetics and mechanism of a reaction involving seven-co-ordinate complexes
No full textIn the absence of light, di-μ-diethylphosphido-bis(tetracarbonylmolybdenum) (Mo-Mo), (I), undergoes a two-step carbonyl substitution reaction with tri-n-butylphosphine in decalin, giving [(Bu3P)(OC)3Mo(μ-PEt2) 2Mo(CO)4], (II), and [(Bu3P)(OC)3Mo(μ-PEt2) 2Mo(CO)3(PBu3)], (III). The substitution reaction does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. All the substitutions occur by a dissociative mechanism involving the reactive intermediate [L(OC)3Mo(μ-PEt2)2Mo(CO)3] (L = CO or PBu3) which has a co-ordinatively unsaturated six-co-ordinate molybdenum atom. Values of the competition ratio kco/kPBus for bimolecular attack on this intermediate, at 80 °C, range from 67.0 for L = CO to 2.62 × 104 for L = PBu3. Thus the co-ordinatively unsaturated metal centre shows an unexpected high sensitivity to the nature of the incoming ligand and to steric and/or electronic variations on the adjacent metal atom. The substitution of one CO group by PBu3 in (I) does not affect the rate of dissociation of CO, whereas (II) has a different rate of dissociation of PBu3 compared to (III). Activation parameters for the rate constants and competition ratios, together with equilibrium data, for these reactions are reported
Reaction of bis-�-diethylphosphido-bis(tetracarbonylmetal)(M?M)(M = Cr or W) with tri-n-butylphosphine : kinetics and mechanism of a reaction involving seven-co-ordinate complexes
No full textKinetics and mechanism of the reaction of bis-μ-diethylphosphidobis(tetracarbonylmanganese) with tri-n-butylphosphine
No full textRedox reaction mechanisms in non-complementary processes. Part II. Kinetics of platinum(II)-iron(III) and iron(II)-platinum(IV) interconversions
No full textThe kinetics of the equilibration reactions (i) (L = NH3, NH2Me, NH2Et, NH2Pr, or 1/2en; X = Cl or Br) have been cis- or trans-[PtL2X2] + 2Fe3+ + 2X- ⇌ cis- or trans-[PtL2X4] + 2Fe2+ (i) studied in water in the presence of 0·5M-perchloric acid at 1M ionic strength. The rate law for the reduction of the platinum(IV) complexes has the form: rate = kr[PtIV][Fe2+]. The rate law for the oxidation of the platinum(II) bromo-complexes, when no iron(II) is added to the reacting mixture, is: rate = kf[PtII] [Fe3+][Br-] + kf′[PtII][Fe3+][Br-] 2. When an excess of iron(II) is added to the reacting mixtures the reactions do not go to completion and the rates of approach to equilibrium strongly depend on the amount of iron(II) added. The forms of the rate laws are consistent with a mechanism involving a platinum(III) intermediate and two one-electron redox steps. A five-co-ordinate platinum(II) complex, [PtL2Br3]-, is postulated to be responsible for the kf′ oxidation path. The free energies of activation for both oxidation of platinum(II) and reduction of platinum(IV) complexes are found to parallel the overall free-energy changes of the reactions. A change of the geometric form of the complexes has no effect on the reaction rate or the equlibrium constant of the redox reaction. trans-[PtL2Br4] is 5-15 times less reactive than the corresponding chloro-complexes, mainly as a consequence of the formation of a more stable oxidation product, FeX2+, when X is Cl rather than Br
Redox reaction mechanisms in non-complementary processes. Part I. Redox reactions between the tetrabromobismethylaminoplatinum(IV)- dibromobismethylaminoplatinum(II) and iron(III)-iron(II) systems in the presence of bromide ions
No full textThe mechanism of the reversible redox reaction (i) has been studied in the presence of 0·5M-perchloric acid at trans-[Pt(NH2Me) 2Br2] + 2Fe3+ + 2Br- ⇌ trans-[Pt(NH2Me)2Br4] + 2Fe2+ (i) 1M ionic strength. The rate law for the reduction of trans-[Pt(NH 2Me)2Br4] by iron(II) in the presence of bromide is: rate = kr[PtIV][Fe2+]. The rate law for the oxidation of trans-[Pt(NH2Me)2Br2] by iron(III) in the presence of bromide, when no iron(II) is added to the reacting mixture, is: rate = kf[PtII] [Fe3+] [Br -]. The rate of oxidation of the platinum(II) complex is lowered by an excess of iron(II) in the reacting mixture. Moreover, in these conditions the rate is not strictly linearly dependent on the bromide concentration. In such cases the pseudo-first-order rate constant of approach to equilibrium at 50°C, when iron(III), iron(II), and bromide are all present in the reacting mixture in concentrations considerably higher than those of the platinum complexes, is Kobs/S-1 = 4·1[Fe3+] [Br-]1 + 1·25 × 10-2 [Fe2+] [Fe3+] [Br-] + 4·1 × 10-3[Fe 2+]1 + 80 [Fe32+] [Br-][Fe2+] (ii) as shown in equation (ii). The form of the rate constant is quantitatively consistent with a mechanism involving a platinum(III) intermediate and two, one-electron redox steps. Values for the equilibrium constant of the reaction calculated from the concentrations of the various species present at equilibrium (6·2 × 104 l2 mol-2 at 50°C), and from the specific rate constants derived on the basis of the suggested mechanism (8 × 104 l2 mol-2 at 50°C) agree reasonably well and provide additional confirmation for the suggested mechanism
Reaction mechanisms of metal-metal bonded carbonyls. Part IV. The substitution reaction of μ-diphenylacetylene-bis(tetracarbonylcobalt) with tri-n-butylphosphine
No full textμ-Diphenylacetylene-bis(tetracarbonylcobalt) undergoes two successive substitution reactions with tri-n-butylphosphine in decalin and the kinetics of these reactions have been studied. Each step shows pseudo-first-order behaviour with kobs = k1 + k2[PBu3] under an atmosphere of argon. Activation parameters have been obtained for each path in each step and the effect of carbon monoxide on the paths governed by k1 is consistent with a CO-dissociative mechanism. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediates by carbon monoxide and tributylphosphine have been obtained. The rate parameters for the two dissociative paths are quite similar but the second bimolecular path is governed by a much higher value of ΔH‡, and a much less negative value of ΔS‡, than the first
Reaction mechanisms of metal-metal bonded carbonyls. Part V. A kinetic study of the reaction of diphenylacetylene with hexacarbonylbis(tri-n-butylphosphine)dicobalt
No full textThe kinetics of reaction of diphenylacetylene with hexacarbonylbis(tri-n-butylphosphine)dicobalt in decalin at 100 °C have been studied. The reaction proceeds in two stages, the initial product being the acetylene-bridged complex [(OC)3Co(μ-C2Ph2)Co(CO)2PBu 3]. This then undergoes a substitution reaction with tributylphosphine, released in the first stage, to form (μ-C2Ph2){Co(CO)3PBu3} 2. The first stage proceeds by two main paths, one of which rises to a limiting rate with increasing [C2Ph2], the other being first order in [C2Ph2]. Both paths are retarded by carbon monoxide and by tributylphosphine but quantitative study of the latter effect is made difficult by the occurrence of direct attack by phosphine on the complex, apparently to form a more highly substituted cobalt carbonyl. Nevertheless, it can be concluded that the two paths are probably distinguished by whether the acetylene attacks a reactive intermediate before or after reversible dissociation of both a phosphine and a carbon monoxide ligand. When dissociation occurs before attack by acetylene it appears that carbon-monoxide dissociation followed by phosphine dissociation occurs at least ten times more frequently than the reverse order. The reactive intermediate involved when attack by the acetylene occurs before dissociation can be formulated as a carbonyl-bridged complex, formed by metal migration, and the sequence of dissociation is again predominantly CO followed by phosphine
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