Activation parameters and reaction mechanism in octahedral substitution

Swaddle, Thomas W.

doi:10.1016/s0010-8545(00)80306-9

Cited by 201 publications

(65 citation statements)

References 184 publications

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“…The mechanisms of the reactions between the positive metal ion and ligands in aqueous solution are generally classified to be of three types, as proposed by Lanford and Gray in 1965 (6); associative, dissociative, and interchange. Based on the kinetic studies of activation volume (14), the substitution in octahedral aqua cobalt complexes is generally believed to favor a dissociative interchange activation (15)(16)(17); bond breakage is involved in the formation of the activated complex of decreased coordination number. The rates and the activation energies reported here indicate that association is an essential process; the incoming ligand is significantly involved in bond formation and the activated complex, as discussed below.…”

Section: Resultsmentioning

confidence: 99%

“…3 (10) and is significant for solutions of high concentrate (Ͼ5 M). It was anticipated that the substitution in octahedral complexes would follow a dissociative interchange pathway (15)(16)(17) in which the rate of leaving water molecule(s) becomes a determining step. However, the observed linearity studied over a wide range and up to 13 M, suggests that the chloride anation is the rate-limiting step of the bimolecular process.…”

Section: A and B)mentioning

confidence: 99%

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Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump

Wan

Zewail

2008

Proc. Natl. Acad. Sci. U.S.A.

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Ligand exchange of hydrated metal complexes is common in chemical and biological systems. Using the ultrafast T-jump, we examined this process, specifically the transformation of aqua cobalt (II) complexes to their fully halogenated species. The results reveal a stepwise mechanism with time scales varying from hundreds of picoseconds to nanoseconds. The dynamics are significantly faster when the structure is retained but becomes ratelimited when the octahedral-to-tetrahedral structural change bottlenecks the transformation. Evidence is presented, from bimolecular kinetics and energetics (enthalpic and entropic), for a reaction in which the ligand assists the displacement of water molecules, with the retention of the entering ligand in the activated state. The reaction time scale deviates by one to two orders of magnitude from that of ionic diffusion, suggesting the involvement of a collisional barrier between the ion and the much larger complex. C oordinated metal ions, besides being of fundamental interest in the chemical literature, are central to biological catalysis and function, as in, e.g., metabolism (1, 2). Among these, cobalt complexes are structurally prototypical systems that are biologically relevant in, e.g., vitamin B 12 function (3) and biomedical applications of cancer therapy (4, 5). Ligand substitution reactions (6-8), which involve the replacement of the ligand coordinated to the metal ion by a free one(s) from solution, are multistep processes and part of the hydration mechanism in aqueous media. The exchange, which is temperature-or ligand concentration-sensitive, in many cases is accompanied by dramatic color changes.The studies reported here focus on the aquachloro complexes of cobalt(II). Steady-state, optical, and x-ray absorption studies (9, 10) have revealed that the complex in aqueous halide solutionϪ , and [CoX 4 ] 2Ϫ ; remaining charge compensation comes from the added chloride salt, in our case LiCl. The first three species have an octahedron configuration, whereas the latter two adopt a tetrahedron configuration because of the crystal field stabilization energy (9, 11). The striking spectroscopic shift from pink to dark blue when increasing [Cl Ϫ ] or temperature is due to the shift of equilibrium from the octahedral to the tetrahedral structure, each with a characteristic absorption. Upon sudden temperature (e.g., T-jump) or concentration change, the equilibrium changes, and coordinated water molecules gradually get replaced by free chloride ions. Accompanying this change is the structural interconversion from the octahedral to the tetrahedral configuration:[1]For this complex reaction, it is important to elucidate the nature of the dynamics and the free energy surface.Here, using the ultrafast T-jump (12, 13), we systematically probed the substitution processes involved over a wide range of concentration, temperature, and time scale. By carefully selecting the reactant concentration and the initial temperature, it was possible to capture the elusive intermediates. The experiment...

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Section: Resultsmentioning

confidence: 99%

Section: A and B)mentioning

confidence: 99%

Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump

Wan

Zewail

2008

Proc. Natl. Acad. Sci. U.S.A.

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“…Such studies are very common in solution chemistry to elucidate the mechanisms of the reactions, e.g. to distinguish between the intra-and intermolecular mechanisms of the reactions of the coordination compounds and to study the role of the solvent in the reaction (Sinn, 1974;Stranks, 1974;Swaddle, 1974;Isaacs, 1981;Palmer & Kelm, 1981;van Eldik, 1986van Eldik, , 1999. In relation to the solid-state reactions, high pressure can be used as a tool of a continuous compression of the 'reaction cavity' (Boldyreva, 1996(Boldyreva, , 1997.…”

Section: Chemical Reactionsmentioning

confidence: 99%

High-pressure diffraction studies of molecular organic solids. A personal view

2007

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“…Equations (18) and (19) point out that values are k obs independent of the initial concentration of Hdhd as it was found experimentally. These equations now only contain two unknown parameters, k 1 and k 2 .…”

Section: Obsmentioning

confidence: 60%

Thermodynamic analysis of 1,3-dicarbonylic monochelates of iron(III) from equilibrium and kinetic measurements

Romero

Verdú

Blanco

1998

Int. J. Chem. Kinet.

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From a study of the complexation of with 2,6-dimethyl-3,5-heptanedione in III Fe aqueous solution, thermodynamic and kinetic parameters have been obtained. Results obtained for this system and a series of structurally similar iron(III) diketonates at different temperatures, establishes isokinetic behaviors of these kinds of reactions.Analytical correlations obtained may be useful to predict, for analogous ligands in similar experimental conditions, an essential mechanism, that pathways involving reaction of metalhydrolyzed species with the enol tautomer are faster than pathways involving hexa-aqua species. Additionally, thermodynamic parameters associated with the monochelated complexations may be predicted.

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Activation parameters and reaction mechanism in octahedral substitution

Cited by 201 publications

References 184 publications

Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump

Dynamics of ligand substitution in labile cobalt complexes resolved by ultrafast T-jump

High-pressure diffraction studies of molecular organic solids. A personal view

Thermodynamic analysis of 1,3-dicarbonylic monochelates of iron(III) from equilibrium and kinetic measurements

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