摘要:

For Co(III) amine complexes, there is a large body of evidence that points to spontaneous substitution reactions occurring via interchange mechanisms in which bond breaking substantially precedes bond making in the transition state. The mechanisms of substitution of the Rh(III), Ir(III), Ru(III), Os(III) and Cr(III) analogues depend very much on a number of factors, such as the nature of the leaving group and steric factors, as well as the electronic configuration of the metal ion. On the balance of evidence, substitution reactions of the d3and d5complexes can proceed via interchange mechanisms in which either bond breaking precedes bond making orvice versa. For the spontaneous substitution reactions of [M(NH3)5Cl]2+ complexes, the very large ionic solvation terms of the [M(NH3)5]3+ and Cl- components with respect to [M(NH3)5Cl]2+ strongly favor a transition state in which there is substantial charge separation, i.e., dissociative interchange mechanisms are energetically more accessible than associative interchange mechanisms. New crystallographic evidence indicates strongly that the different kinetic behavior observed for Co(III), Rh(III) and Cr(III) on the addition of steric bulk into [M(NH2R)5Cl]2+ complexes is not due to previously argued changes in mechanism with the nature of the metal ion. Rather, the kinetic differences reflect differences in the influence of π-bonding on the ground state and all of these complexes are believed to undergo substitution reactions via mechanisms in which bond breaking substantially precedes bond making. By contrast, the mechanisms of aquation reactions of the hexaammine complexes depend mainly on the electronic configuration and size of the central metal ion, since solvation terms in the transition state are similar to those in the ground state for either a dissociative or associative interchange mechanism. For these complexes, the preferred mechanism for substitution is an associative interchange for [M(NH3)6]3+ M = Cr, Ru and Os, and a dissociative interchange mechanism for M = Co. All of the mechanisms of base-catalysed aquations of metal amine complexes appear to involve five-coordinate intermediates in a dissociative conjugate-base mechanism (Dcbor SN1cb), independent of the metal ion or the leaving group. However, the geometries of municating the results contained in Ref. 23 prior to publication. Helpful discussions with James Beattie, Alan Sargeson and Henry Taube on a number of aspects are gratefully acknowledged. Some aspects of this work have been supported by grants from the Australian Research Council.

ISSN:0260-3594    

DOI:10.1080/02603599108035828

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Macrocyclic polyamine ligands have been used in studying intrinsic acid properties of Zn(II) ion to help elucidate the role of Zn(II) in Zn-enzymes such as carbonic anhydrase (CA), carboxypeptidase, etc. Among macrocyclic tri- and tetraamines, [12]aneN3is the most appropriate ligand that mimics the ligand field surrounding the Zn(II) in CA. In its 1:1 [ZnIIL]2+ complex, the H2O bound at the fourth coordination site deprotonates with a pKavalue of 7.30 at 25°C andI= 0.1, almost the same value reported for CA. Anion binding affinity to the [ZnIIL]2+ is determined by pH-metric titration and inhibition kinetics of 4-nitrophenyl acetate hydrolysis. The order and magnitude, OH–≫ HCO–3> CH3COO–> I–> Br–> Cl–> F–are almost comparable with the anion inhibition for CA. The pH-metric determination of the interaction of Zn(II) and Cd(II) with dissociable (acidic) hydrogen-containing macrocyclic polyamines has served to distinguish acid and coordination properties of these two metal ions. Thanks to macrocyclic stabilities the metal-promoted amide proton dissociations were observed for the first time, particular with the more acidic Zn(II). To yield the same complexes with less acidic Cd(II), higher pH was required. Thus, monooxocyclam is proven to be the first Zn(II)-selective chelating agent. Taking all of these equilibrium data (from our model complexes) into consideration, we find that the pKavalue for Zn-OH2Zn-OH, 1:1 anion affinity constants, blood pH, and the pKavalues for H2CO3HCO–3CO2–3are all ideally consistent for CA activities.

ISSN:0260-3594    

DOI:10.1080/02603599108035829

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The thermal reaction chemistry and electrochemistry of oxidation and reduction reactions occupies a prominent position in the development of modern inorganic chemistry. The thermodynamic parameters associated with redox reactions have been extensively studied via electrochemical as well as calorimetric techniques, and these parameters provide a convenient basis for establishing equilibrium constants and spontaneity for a wide variety of redox processes. Development of theories describing electron transfer reactions1-4has provided, with the help of structural information, an important means of correlating these thermodynamic paraineters with the kinetics of redox reactions. Thermal electron transfer reactions continue to play a central role in current inorganic chemical research in such diverse areas as theoretical and synthetic inorganic chemistry, inorganic polymer chemistry. and bioinorganic chemistry.

ISSN:0260-3594    

DOI:10.1080/02603599108035830

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The present article intends to propose electrochemistry as a tool to distinguish classical hydride coordination to metal centers from nonclassical dihydrogen complexation. The successful application to a series of Group VIII metal-polyhydride complexes of the type [(XP3)MH2] + (XP3= X(CH2CH2PPh2)3(X = P, N); M = Co, Rh, Ir), in which classical and nonclassical hydrides are present, induces us to think that such a technique, notably less demanding of sophisticated instrumentations than established crystallographic, NMR and IR techniques, might play some role in the future for what concerns the yet elusive characterization of dihydrogen complexes. The limitations of such an electrochemical approach are put in evidence. The scattered and sometimes disputed data in the literature are also discussed.

ISSN:0260-3594    

DOI:10.1080/02603599108035831

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0260-3594    

DOI:10.1080/02603599108035827

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor