Ligand Substitution Processes

From the preface: The subject of the mechanistic study of ligand substitution reactions is currently undergoing an exciting growth. New fast-reaction techniques have removed the upper limit on rates that can be measured, and extension to less familiar central metal atoms has begun in earnest. This might seem the wrong moment for review of the field. As yet, definitive treatment is possible only for those complexes involving monodentate ligands with cobalt(III) and platinurn(II). But, because information is so extensive for these systems, it is clear that they are functioning as models from which concepts and experiments are generated for application over the fast-growing range of the subject. We believe that this is an important moment to reopen debate on fundamentals so that concepts will be most felicitously formulated to aid growth of understanding. This monograph is centrally concerned with three aspects of those fundamentals. We have attempted to develop an approach to classification of ligand substitution reactions that is adapted to what seem to have emerged as the characteristic features of these reactions and is susceptible to operational tests. (We do recognize that any such scheme of ideas is necessarily obsolescent once it is formulated since new experiments will certainly follow immediately.) We have tried to evaluate the basis for making generalizations about ligand substitution processes and to formulate tests to show whether new reactions fall within familiar patterns. Finally, we have sought to base the models of ligand substitution processes in the language of molecular-orbital theory. We believe that MO theory is most useful, because it may be used to correlate rate data on complexes with the extensive information available from spectral and magnetic studies, yet differs from crystal-field theory in providing a natural place for consideration of the bonding electrons, which must be a principal determinant of reaction processes. To keep this essay within bounds, we assume familiarity with the elements of experimental kinetics, transition-state theory, and the simple molecular-orbital theory of complexes. Introductory physical chemistry, some familiarity with the study of reaction mechanisms, and mastery of one of the qualitative treatments of MO theory as applied to transition-metal complexes should provide sufficient background. Thus, we hope that this book will be useful to students, relatively early in their careers, who wish to explore this field. Our debts to very many workers will be obvious throughout. We want to record here our special personal debt to Professors Ralph G. Pearson and Fred Basolo and to Dr. Martin Tobe. We particularly thank Professor George S. Hammond for his interest and enthusiasm in this project. Professor Hammond carefully read and criticized the entire manuscript in the final drafts. We received many other valuable criticisms at various stages of this project from Professors R. D. Archer, F. Basolo, J. O. Edwards, J. Finholt, P. Haake, J. Halpern, A. Kropf, R. G. Pearson, S. I. Shupack, M. S. Silver, and C. Walling, and Dr. U. Belluco and Dr. L. Cattalini. We very much appreciate their help and probably should have followed their suggestions more closely. We warmly acknowledge expert assistance from Mrs. Madeline deFriesse, Miss Jan Denby, and Mrs. Diane Celeste in preparation of the manuscript. COOPER H. LANGFORD HARRY B. GRAY Amherst, Massachusetts New York, New York October 196

Heterogeneous catalysis of inorganic substitution reactions

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Controlling platinum, ruthenium, and osmium reactivity for anticancer drug design

The main task of the medicinal chemist is to design molecules that interact specifically with derailed or degenerating processes in a diseased organism, translating the available knowledge of pathobiochemical and physiological data into chemically useful information and structures. Current knowledge of the biological and chemical processes underlying diseases is vast and rapidly expanding. In particular the unraveling of the genome in combination with, for instance, the rapid development of structural biology has led to an explosion in available information and identification of new targets for chemotherapy. The task of translating this wealth of data into active and selective new drugs is an enormous, but realistic, challenge. It requires knowledge from many different fields, including molecular biology, chemistry, pharmacology, physiology, and medicine and as such requires a truly interdisciplinary approach. Ultimately, the goal is to design molecules that satisfy all the requirements for a candidate drug to function therapeutically. Therapeutic activity can then be achieved by an understanding of and control over structure and reactivity of the candidate drug through molecular manipulation

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

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 rate-limited 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

Formation And Kinetics Of Dissociation Of Some Pentaaquotrihalomethyl Chromium(Iii) Ions.

The present.study was an investigation of the formation and kinetic stability of store complexes contain.ir)ing a chromium-carbon bond in an aqueous medium. An orange-brown pentaaquotrifluoromethylchromium(III) ion was obtained by the reduction of trifluororrethyl iodide with Cr(II) and is the most inert of any organochrotnium(III) complex known. The initial aquation rates of trifluoromethylchromium(III) ion were described by the differential rate law -dln [(H2O)5CrCF32+]/dt = ko + k1(H+) The products of the initial reaction were hexaaquochromium (III), carbon monoxide and HF. The values of ko and k1 at 45° were 5.21 x 10-1 and 9.38 x 10M-1sec-1 respectively. The activation parameters for the acid independent and acid dependent pathways were: ΔH+ = 19.7 kcal/more, ΔS+ = -27.4 cal-deg-1mole-1; and ΔHǂ1 = 23.4 kcal/mole and ΔSǂ1 = -12.7 cal-deg-1mole-1. The acceleration in the aquation with time was observed indicating autocatalysis by a product of the reaction. An intermediate, (Cr( OH2)4FCrCF31+ ), was separated from the aquation reaction solution; this same ion was also formed immediately when fluoride ion was added to the (H2)5CrCF32+ ion in 0.05M perchloric acid. The added fluoride ion also increased the rate of aquation. Rapid formation of (Cr(OH2)4FCrCF31+) ion was attributed to the strong stabilizing effect of the trifluoromethyl ligand on water molecules in the inner coordination sphere of chromium( III). A solvent-assisted mechanism for the aquation of pentaaquotrifluoromethyl-chromium(III) ion was proposed in which an activated complex was formed which dissociated in a concerted manner to give the products. Also, a reaction sequence was proposed for the complete aquation reaction which was consistent with the rate laws, the effect of added fluoride ion and the activation parameters. Pink organochromium(III) species of 2+ charge was isolated in the reduction of carbon tetrachloride and carbon tetrabromide with chromium( II). he pink species have ·chromium:carbon:halide ratios of 1:1:1 and their uv-visible spectra showed unusually high absorbance values in the 500 nm region. The pink organochromium(III) reacted very rapidly with oxygen. The products of dissociation were carbon monoxide, formic acid, formaldehyde, bexaaquochromium(III), chromium( II) halide ions. The kinetics of aquation were studied, and initial rate coefficients and activation parameters were obtained. The identity dt these organochromium(III) species was not established but most possible structures were proposed

Mechanistic investigation of halopentaaquachromium(III) complexes: Comparison of computational and experimental results

The mechanism for the substitution reactions of halopentaaquachromium(III) complexes in the series, [CrX(H2O)5]2+, where X = F−, Cl−, Br−, or I− have been investigated using density functional theory. Several different mechanistic pathways were explored including associative interchange (Ia), dissociative (D) and the associatively activated dissociation mechanisms (Da). The lowest overall activation enthalpy (ΔH‡) obtained for the fluoride system is for the Ia pathway, with immediate proton abstraction leading to the formation of HF and the conjugate base. For the chloride, bromide and iodide systems the Da pathway has the lowest ΔH‡ values. Activation enthalpies determined at the PBE0/cc-pVDZ level, in aqueous solution (PCM), are in excellent agreement with the experimental results (MAD is 1.0 kJ mol−1). Reaction profiles were analyzed in terms of activation volumes to explain the observed trends

Kinetic and thermodynamic studies of some rhodium cyclam complexes.

Dept. of Chemistry and Biochemistry. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1972 .C37. Source: Dissertation Abstracts International, Volume: 62-13, Section: A. Thesis (Ph.D.)--University of Windsor (Canada), 1972