The limiting dissociative mechanism for substitution at thed 6 centres molybdenum(O), iron(II), and cobalt(III): Kinetics of substitution at the pentacyano-4-Cyanopyridineferrate(II) anion and at 4-cyanopyridinemolybdenum(I) pentacarbonyl

Summary Rate constants are reported for reaction of the 4-cyanopyridine complexes [Fe(CN)₅(4CNpy)]^3– and [Mo(CO)₅(4CNpy)] with a variety of incoming ligands, in aqueous methanol (40 vol % MeOH) and in toluene respectively, at 298.2 K (ambient pressure). The dependence of rate constants on the nature and concentration of the incoming ligand is discussed in terms of the operation of the limiting dissociative,D, mechanism for substitution; the operation of this mechanism here, and in analogous pentacyanoferrate(II), pentacarbonylmolybdenum(I), and penta- and tetra-cyanocobaltate(III) complexes is reviewed. The effect of pressure on rate constants for replacement of 4-cyanopyridine in [Mo(CO)₅(4CNpy)], in toluene solution at 298.2 K, indicates an activation volume of +3 cm³ mol^–1.     

Anchoring of hydrophobically modified poly(sodium acrylate)s into DDP vesicle bilayers: hydrophobic match and mismatch

 Differential scanning microcalorimetric thermograms have been recorded for aqueous solutions containing vesicles formed by sodium di-n-dodecyl phosphate, in the presence of different concentrations of poly(sodium acrylate-co-n-alkyl methacrylate), where n-alkyl= C₉H₁₉, C₁₂H₂₅, C₁₈H₃₇. The mole fraction of hydrophobic moieties in the copolymer is 0.04. The main phase transition temperature (T _m) is hardly affected by the presence of poly(sodium acrylate)s bearing n-dodecyl chains, whereas the anchoring of polymers bearing n-nonyl or n-octadecyl groups reduces the main phase transition temperature significantly from ca. 34 °C to ca. 32 °C. In parallel, the enthalpy of transition per mole of DDP monomer (Δ_m H _int) is lowered upon adding polymer. Again, the polymer containing n-dodecyl moieties hardly affects Δ_m H _int. These patterns are explained by the notion that the extent of the disruptive effect of alkyl chains incorporated into the bilayer depends on the extent of the mismatch between the chain lengths of the intruding alkyl chains and the hydrophobic moieties composing the vesicle bilayer. Added hydrophobically modified polymers increase the cooperativity of the melting process, as shown by the increase of n _DDP. We suggest that the anchoring poly(sodium acrylate-co-n-alkyl methacrylate) relieves the strain in the curved outer monolayer of a pure DDP bilayer by allowing the presence of larger “patches” characterized by low curvature. Received: 12 May 1997 Accepted: 20 October 1997     

Reactivity and solvatochromism of 2,2′-bi(4H-5,6-dihydrothiazine)-tetracarbonylmolybdenum(O)

Summary The dependence of the charge-transfer frequency for [Mo(CO)₄(btz)], btz = 2,2-bi(4H-5,6-dihydrothiazine), on solvent is described, and the solvatochromic behaviour of this compound compared with that of other [Mo(CO)₄(LL)] species, with LL = 2,2-bipyrimidine or 2,2-bipyridine, and of iron(II) analogues [Fe(btz)₂(CN)₂] and [Fe(bipy)₂(CN)₂]. Kinetics of solvolysis (k, H, S) are reported for [Mo(CO)₄(btz)] in methanol, acetonitrile, and dimethyl sulphoxide. These kinetic results are analysed into initial state and transition state contributions. A parallel analysis of the solvatochromic results for [Mo(CO)₄(btz)] into ground state and excited state solvation contributions is compared with similar analyses for the solvatochromic organic compoundsp-nitroanisole and dimethylindoaniline.     

The reaction of nickel(II) with 2,2′-bipyridyl in methanol- and in dimethyl sulphoxide-water mixtures: Solvent effects on reactivity

Summary Enthalpies of solution (298 K) are reported for NiCl₂ · 6H₂O, CoCl₂ · 6H₂O, and FeCl₂ · 4H₂O in aqueous methanol (0–40% methanol) and for NiCl₂ · 6H₂O, 2,2-bipyridyl, and water in aqueous dimethyl sulphoxide (0–60% DMSO). Solubilities (298 K) of 2,2-bipyridyl are reported for aqueous dimethyl sulphoxide (0–60% DMSO). From these, from appropriate published thermodynamic data, and from published rate constants and activation enthalpies, we have analysed solvent effects on reactivities ( and H) into initial state and transition state contributions (the latter are in fact composite ion-association and interchange quantities). In all systems examined the observed kinetic trends represent small differences between large and almost identical solvent effects on the respective initial and transition states.     

Kinetic and spectroscopic intimations of intermediates in nucleophilic attack at diimine complexes of iron(II) and of molybdenum(O)

Summary Kinetics of nucleophilic substitution at a range of low-spin iron(II)diimine complexes have been examined in the presence of a variety of salts, to probe the role of hydroxide and cyanide as nucleophiles and of other ions in ion association equilibria. Equilibrium constants for interaction of hydroxide and of cyanide with many of these ligands, free or complexed with iron(II) or molybdenum(0), have been measured, in water and in binary aqueous solvent mixtures. Effects of solvent, temperature, and pressure on rate constants and on equilibrium constants have been monitored for selected systems. In the light of these results, and of ancillary qualitative observations, we discuss the role and nature of intermediates in nucleophilic substitution reactions of transition metaldiimine complexes.     

Initial state and transition state contributions to reactivity in mercury(II)-catalysed aquation of thetrans-[Rh(en)2Cl2]+, [Cr(NH3)5Cl]2+, andcis-[Cr(NH3)4(OH2)Cl]2+ cations in binary aqueous solvent mixtures

Summary Rate constants are reported for mercury(II)-catalysed aquation of thetrans-[Rh(en)₂Cl₂]⁺, [Cr(NH₃)₅Cl]²⁺, andcis-[Cr(NH₃)₄(OH₂)Cl]²⁺ cations in water and in methanol-, ethanol-, and acetonitrile-water solvent mixtures. In the case oftrans-[Rh(en)₂Cl₂]⁺, the dependence of rate constants on mercury(II) concentration indicates reaction through a binuclear (Rh-Cl-Hg bridged) intermediate. The dependence of the equilibrium constant for the formation of this intermediate and of its rate constant for dissociation (loss of HgCl⁺) on solvent composition have been established. With the aid of measured solubilities, published ancillary thermodynamic data, and suitable extrathermodynamic assumptions, the observed reactivity trends for these mercury(II)-catalysed aquations are dissected into initial state and transition state components. The reactivity patterns for these three complexes are compared with those for mercury(II)-catalysed aquation of other chloro-transition metal complexes, particularlycis-[Rh(en)₂Cl₂]⁺, [Co(NH₃)₅Cl]²⁺, and [ReCl₆]^2–.     

Kinetics of reactions of 2,2′-bipyridyltetracarbonylmolybdenum(0)

Summary Kinetic results are presented for reaction of Mo(CO)₄(bipy) with cyanide in several nonaqueous solvents and in DMSO-H₂O mixtures, with methoxide, and with azide; for reaction of Mo(CO)₄(5-NO₂phen) with cyanide and with methoxide; and for reaction of Mo(CO)₄(phen) and of W(CO)₄(bipy) with cyanide. Solvent effects on the reaction of Mo(CO)₄(bipy) with cyanide are dissected into their initial state and transition state components. Here, and in the dependence of the activation volume for this and related reactions on solvent, the important role played by cyanide solvation is apparent. Preliminary investigations on reactions of compounds of this M(CO)₄(diimine) type with tertiary phosphines, diethyldithiocarbamate, and ether peroxides are described.     

Solvation of transition metal complexes: thermochemical approaches

Conclusion and extensions We hope that this Review has made readers more aware of solvation of inorganic complexes, and of the importance of such knowledge in understanding their chemistryperhaps particularly their reactivity. The approach just set out for inorganic complexes should be of considerable value in the field or organometallic chemistry. In particular, informed use of solvation characteristics should help in optimising conditions for organometallic reactions and in homogeneous catalysis. Unfortunately, solvation data on reactants are too sparse (the subject index ofComprehensive Organometallic Chemistry contains justthree entries under solubility!) for serious examination of reactivity trends in terms of initial state and transition state contributions to be possible in almost all areas. Moreover, there are some fundamental problems over transfer parameters. Thus, a favourite electrochemical assumption is that the ferrocene/ferrocinium redox potential is independent of solvent. Yet, the dependence of rate constants on medium for outer-sphere electron transfer in the ferrocene/ferrocinium system can only be understood⁽⁶⁶⁾ in terms of specific solvation effects which are incompatible with the parallel solvation changes of these two substrates implicit in the redox potential assumption. The solvation of organometallic species should prove a most rewarding area for continued study, but it will be some time before the overall picture becomes as clear as in the more limited area of classical transition metal complexes considered in the present Review.     

Transfer chemical potentials for iron(II)-diimine complexes from water into aqueous methanol

Summary Solubilities of perchlorate and thiocyanate salts of several iron(II)-diimine complexes of Schiff base ligands in methanolwater mixtures are reported. From these results and published values for transfer chemical potentials of the perchlorate and thiocyanate anions, transfer chemical potentials for the complex cations are calculated. Trends with solvent composition suggest that preferential solvation varies from negligible to very strongly by methanol, depending on the size and hydrophilic-hydrophobic character of the complex.On leave from the Faculty of Science, Sohag, Egypt.