Noninnocence in Metal Complexes: A Dithiolene Dawn

Noninnocence in inorganic chemistry traces its roots back half a century to work that was done on metal complexes containing unsaturated dithiolate ligands. In a flurry of activity in the early 1960s by three different research groups, homoleptic bis and tris complexes of these ligands, which came to be known as dithiolenes, were synthesized, and their structural, electrochemical, spectroscopic, and magnetic properties were investigated. The complexes were notable for facile one-electron transfers and intense colors in solution, and conventional oxidation-state descriptions could not account for their electronic structures. The bis complexes were, in general, found to be square-planar, including the first examples of this geometry for paramagnetic complexes and different formal dn configurations. Several of the neutral and monoanionic tris complexes were found to have trigonal-prismatic coordination, the first time that this geometry had been observed in molecular metal complexes. Electronic structural calculations employing extended Hückel and other semiempirical computational methods revealed extensive ligand–metal mixing in the frontier orbitals of these systems, including the observation of structures in which filled metal-based orbitals were more stable than ligand-based orbitals of the same type, suggesting that the one-electron changes upon oxidation or reduction were occurring on the ligand rather than on the metal center. A summary of this early work is followed with a brief section on the current interpretations of these systems based on more advanced spectroscopic and computational methods. The take home message is that the early work did indeed provide a solid foundation for what was to follow in investigations of metal complexes containing redox-active ligands

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

Ligand Substitution Dynamics

Substitution of a ligand in an inner sphere complex by an outside group is the most fundamental reaction in metal ion chemistr

Electron Transfer in Proteins

Electron-transfer (ET) reactions are key steps in a diverse array of biological transformations ranging from photosynthesis to aerobic respiration. A powerful theoretical formalism has been developed that describes ET rates in terms of two parameters: the nuclear reorganization [lambda] energy (1) and the electronic-coupling strength (HAB). Studies of ET reactions in ruthenium-modified proteins have probed [lambda] and HAB in several metalloproteins (cytochrome c, myoglobin, azurin). This work has shown that protein reorganization energies are sensitive to the medium surrounding the redox sites and that an aqueous environment, in particular, leads to large reorganization energies. Analyses of electronic-coupling strengths suggest that the efficiency of long-range ET depends on the protein secondary structure: [beta]sheets appear to mediate coupling more efficiently than [alpha]-helical structures, and hydrogen bonds play a critical role in both

Molecular electronic structures : an introduction

The present book is an introduction to molecular electronic structural theory. It is aimed at students who have reasonable familiarity with differential and integral calculus and are beginning a study of the physical description of chemical systems. We have decided to concentrate on the description of ground state electronic structures, or, in other words, the principles of chemical bonding in molecules. In this important respect the present volume differs from our earlier book "Molecular Orbital Theory" (Benjamin, 1964), which included a strong emphasis on the description of electronic excited states. In our treatment of molecular wave functions we make use of "symmetry operators", the latter being operators that leave the Hamiltonian unchanged. By using such symmetry operators, it is possible to characterize the electronic structures of molecules. In our opinion, this approach provides good preparation for later studies that may be undertaken in which formal group theory is employed. The heart of the book is Chapter 4, where we discuss in some detail the bonding in several selected molecules. Examples from both organic and inorganic chemistry are included in an attempt to make the coverage as general as possible. Our objective here is to provide an introduction to molecular bonding that will serve as a foundation for more advanced study of electronic structures. Suggested reading and problems are included in each chapter. Some of the problems are challenging, but working them will give the student a much better feeling for the principles involved. The suggested reading is of two types, books (and reviews) and original papers. And we urge students to examine at least some of the older papers in the field, as muck can be learned from them

Chemical dynamics

CHEMICAL EDUCATION is changing rapidly, not only because of the explosive growth of knowledge but also because the new knowledge has stimulated reformulation of working principles in the science. Undergraduate curricula and individual courses are in constant flux. Nowhere is the change and challenge greater than in freshman chemistry. Teachers of freshmen must meet the intellectual needs of students who have had more sophisticated and stimulating high school courses than those given a decade ago. At the same time, the freshman teacher must be aware of the constant modification of the more advanced courses in chemistry and other fields that his students will study later. Continuous reformulation of courses sometimes results in the inclusion of valuable new material at the expense of other equally valuable material. We believe that this has happened in some of the sophisticated courses in freshman chemistry. Structural chemistry often receives far greater emphasis than chemical dynamics. In 1965, the Westheimer Report (Chemistry: Opportunities and Needs, National Academy of Sciences, 1965) identified the three major fields of chemistry as structure, dynamics, and synthesis. We firmly believe that a balanced course in general chemistry should reflect the outlook of this report. The study of modern chemical synthesis is too demanding to be covered in depth in an introductory course. However, chemical dynamics -- the systematic study of reactions and reactivity -- can and should be studied at the freshman level. The study of changing chemical systems is the most fascinating part of the field for many students, and its early introduction forms a solid foundation for later study. This small volume is our attempt to answer the need. The book is intended for students who have had introductory stoichiometry, energetics, and structure at the level of a modern freshman textbook (for example, Basic Principles of Chemistry, by H. B. Gray and G. P. Haight, Jr., W. A. Benjamin, Inc., New York, 1961). Chemical Dynamics is designed to accompany approximately 20-25 lectures to be given as the concluding section of a freshman chemistry course. We have chosen topics for their fundamental importance in dynamics and then tried to develop a presentation suitable for freshman classes. Discussion of each topic is limited, because chemistry majors will inevitably return to all the subject matter in more advanced courses. We hope that the following ideas have been introduced with a firm conceptual basis and in enough detail for the student to apply them to chemical reality. 1. Thermodynamics and kinetics are two useful measures of reactivity. 2. Characteristic patterns of reactivity are systematically related to molecular geometry and electronic structure. 3. Reaction mechanisms are fascinating in their own right and indispensable for identification of significant problems in reaction rate theory. 4. The concepts underlying experiments with elementary reaction processes (molecular beams) are simple, even though the engineering of the experiments is complicated. 5. Application of theories of elementary reaction rates to most reactions (slow reactions, condensed media, etc.) provides enough challenge to satisfy the most ambitious young scientist. The book includes exercises at the end of each chapter except the last. Their purpose is didactic, inasmuch as most have been written with the aim of strengthening a particular point emphasized in the chapter, or of introducing an important topic which was not developed in the text for reasons of space and which would normally be taken up in greater detail in later courses. The material in this volume has been adapted primarily from a portion of the lectures given by H.B.G. and G.S.H. to the Chemistry 2 students at the California Institute of Technology during the academic years 1966-1967 and 1967-1968. These lectures were taped, written up by J.B.D., and distributed to the students in the form of class notes. The final manuscript was written after class-testing of the notes. Our decision to revise the Chemistry 2 notes in the form of an introductory text was made after H.B.G. and G.S.H. participated in the San Clemente Chemical Dynamics Conference, held in December 1966 under the sponsorship of the Advisory Council of College Chemistry. At San Clemente we found we were not the only group concerned over the exclusion of significant reference to chemical reactions and reactivity relationships in freshman courses. In addition to their general encouragement, which provided the necessary additional impetus, these colleagues prepared a series of papers for publication in an issue of the Journal of Chemical Education. It is a pleasure to acknowledge here the direct contribution these papers made in shaping the final form of our volume; specifically, in preparing Chapter 6, we have drawn examples from the San Clemente papers of Professors R. Marcus, A. Kuppermann and E. F. Greene, and J. Halpern. The concluding chapter of this book was developed from the lectures given by Professors E. F. Greene (dynamics in simple systems), Richard Wolfgang (atomic carbon), John D. Roberts (nuclear magnetic resonance), and F. C. Anson (electrochemical dynamics) to the students of Chemistry 2 in May 1967. These colleagues have kindly given us permission to use their material. We are grateful to Professors Ralph G. Pearson and Paul Haake, who read the entire manuscript and offered valuable criticism. It is a special pleasure to acknowledge the enormous contribution our students in Chemistry 2 made to the project. Their enthusiastic, critical attitude helped us make many improvements in the manuscript. Thanks are also due to four very special members of the staff of W. A. Benjamin, Inc., for seeing this project through with infectious vigor. Finally, and not the least, we acknowledge the role Susan Brittenham and Eileen McKoy played in preparing the final manuscript. JOSEPH B. DENCE HARRY B. GRAY GEORGE S. HAMMOND Pasadena, California January 196

Mentoring: Reflections and Suggestions

I have been enormously lucky in my 58 years of working with students and postdocs. A great many members of the Gray Nation have been successful in careers in academia, industry, and government. Mark Wrighton, one of my first Caltech students, just retired from his position as Chancellor at Washington University St. Louis. (See the accompanying editorial, DOI 10.1021/acscentsci.9b00841.) Mark made transformational changes at WUSTL during his 24-year tenure. Five others also have led major universities, and over a hundred are provosts, deans, and professors in the USA, Canada, UK, Denmark, Sweden, Germany, the Czech Republic, Italy, South Korea, Japan, Australia, Taiwan, Hong Kong, and China. Can I take credit for their contributions to the science enterprise? Did my mentoring make a difference? I would like to think so

Molecules, materials, and mechanisms for solar fuel production

Collaborative research efforts in the NSF CCI Solar Fuels Program are focused on developing new fundamental understanding of mols. and materials that efficiently generate renewable hydrogen fuel using the energy of sunlight. Emphasis in these efforts is placed on a mechanistic understanding of reactions relevant to achieving fuel formation. Catalysis of water oxidn., the anodic half reaction of overall water splitting, is being intensely studied. Pulsed laser ablation has proven to be a valuable technique for synthesis of small, surfactant-free, mixed-metal nanomaterials with size and compn. control. Deposition of these materials on electrodes results in assemblies that are highly active for water oxidn. In-situ spectroscopic studies of these assemblies are providing new insights into possible mechanisms of oxygen evolution. In-situ spectroscopies are also being applied to investigate new trimetallic water-oxidn. catalysts. Metal oxides contg. Ni, Fe, and a third metal (M = Al, Ga, Mo, Cr) have been found to be superior in catalytic performance to the Ni-only or Ni-Fe analogs. Understanding the role of the third metal promises better understanding of the mechanism of catalysis in these materials. Exptl. cyclic voltammetry has demonstrated that fluorinated iron glyoxime complexes act as hydrogen evolving catalysts at modest overpotentials. Our objective is to use d. functional theory (DFT) calcns. and CV simulations to identify the mechanisms that are consistent with the obsd. activity. The calcd. redn. potentials and pKa's have allowed us to propose mechanisms for two catalyst derivs. In one case, the mechanism involves a single pathway through an Fe(0) intermediate and a subsequent Fe(II)-hydride intermediate. In a second case, a parallel pathway involving protonation of the ligand gives rise to a qual. different electrochem. response. These mechanistic insights are guiding the synthesis of more active mol. electrocatalysts

Electrons and Chemical Bonding

THIS BOOK WAS DEVELOPED from my lectures on chemical bonding in Chemistry 10 at Columbia in in the spring of 1962, and is mainly intended for the undergraduate student in chemistry who desires an introduction to the modern theories of chemical bonding. The material is designed for a one-semester course in bonding, hut it may have greater use as a supplementary text in the undergraduate chemistry curriculum. The book starts with a discussion of atomic structure and proceeds to the principal subject of chemical bonding. The material in the first chapter is necessarily quite condensed and is intended as a review. (For more details, the student is referred to R. M. Hochstrasser, Behavior of Electrons in Atoms, Benjamin, New York, 1964). Each chapter in the bonding discussion is devoted to an important family of molecules. Chapters II through VII take up, in order, the principal molecular structures encountered as one proceeds from hydrogen through the second row of the periodic table. Thus, this part of the book discusses bonding in diatomic, linear triatomic, trigona1 planar, tetrahedral, trigonal pyramidal, and angular triatomic molecules. Chapters VIII and IX present an introduction to modern ideas of bonding in organic molecules and transition metal complexes. Throughout, our artist has used small dots in drawing the boundary-surface pictures of orbitals. The dots are intended only to give a pleasing three-dimensional effect. Our drawings are not intended to be charge-cloud pictures. Charge-cloud pictures attempt to show the electronic charge density in an orbital as a function of the distance from the nucleus by varying the "dot concentration." The discussion of atomic structure does not start with the Schrödinger equation, hut with the Bohr theory. I believe most students appreciate the opportunity of learning the development of atomic theory in this century and can make the transition from orbits to orbitals without much difficulty. The student can also calculate several important physical quantities from the simple Bohr theory. At the end of the first chapter, there is a discussion of atomic-term symbols in the Russell-Saunders LSMLMs approximation. In this book the molecular orbital theory is used to describe bonding in molecules. Where appropriate, the general molecular orbitals are compared with valence-bond and crystal-field descriptions. I have written this book for students who have had no training in group theory. Although symmetry principles are used throughout in the molecular orbital treatment, the formal group-theoretical methods are not employed, and only in Chapter IX are group-theoretical symbols used. Professor Carl Ballhausen and I are publishing an introductory lecture-note volume on molecular orbital theory, which was written as a slightly higher level than the present book. The lecture notes emphasize the application of group theory to electronic structural problems. The present material includes problems integrated in the text; most of these are accompanied by the worked-out solutions. There are also a substantial number of problems and questions at the end of each chapter. It is a great pleasure to acknowledge the unfailing support, encouragement, and devotion of the seventy-seven fellows who took the Columbia College course called Chemistry 10 in the spring of 1962. I doubt I shall ever have the privilege of working with a finer group. The class notes, written by Stephen Steinig and Robert Price, were of considerable help to me in preparing the first draft. I would like to thank Professors Ralph G. Pearson, John D. Roberts, and Arlen Viste for reading the manuscript and offering many helpful suggestions. Particularly I wish to thank one of my students, James Halper, who critically read the manuscript in every draft. Finally, a large vote of thanks goes to Diane Celeste