Mixed-Valence Compounds

Theory and Applications in Chemistry, Physics, Geology,and Biology

Author: D.B. Brown

Publisher: Springer Science & Business Media

ISBN:

Category: Science

Page: 520

View: 587

It has been a decade since two seminal reviews demonstrated that mixed-valence compounds share many unique and fascinating features. The insight pro vided by those early works has promoted a great deal of both experimental and theoretical study. As a result of extensive efforts, our understanding of the bonding and properties of mixed-valence compounds has advanced substantially. There has been no compre hensive treatment of mixed-valence compounds since 1967, and the meeting convened at Oxford in September, 1979, provided a unique opportunity to examine the subject and its many ramifications. Mixed-valence compounds play an important role in many fields. Although the major impact of the subject has been in chemistry, its importance has become increasingly clear in solid state physics, geology, and biology. Extensive interest and effort in the field of molecular metals has demonstrated that mixed-valency is a prerequisite for high elec trical conductivity. The intense colors of many minerals have been shown to be due to mixed-valency, and the electron-transfer properties of certain mixed-valence metalloproteins are important in biological processes. Experts from all of these areas participated in this meeting, and the truly interdisciplinary nature of the subject made it a unique learning experience for all in attendance.

Mixed Valency Systems: Applications in Chemistry, Physics and Biology

Author: K. Prassides

Publisher: Springer Science & Business Media

ISBN:

Category: Science

Page: 451

View: 205

Mixed valency is one of various names used to describe compounds which contain ions of the same element in two different formal states of oxidation. The existence of mixed valency systems goes far back into the geological evolutionary history of the earth and other planets, while a plethora of mixed valency minerals has attracted attention since antiquity. Indeed, control of the oxidation states of Fe in its oxides (FeO, Fe304' Fe203) was elegantly used in vase painting by the ancient Greeks to produce the characteristic black and red Attic ceramics (Z. Goffer, "Archaeological Chemistry", Wiley, New York, 1980). It was, however, only 25 years ago that two reviews of mixed valency appeared in the literature almost simultaneously, signalling the first attempt to treat mixed valency systems as a separate class of compounds whose properties can be correlated with the molecular and the electronic structure of their members. Then mixed valency phenomena attracted the interest of disparate classes of scientists, ranging from synthetic chemists to solid state physicists and from biologists to geologists. This activity culminated with the NATO ASI meeting in Oxford in 1979. The 1980's saw again a continuing upsurge of interest in mixed valency. Its presence is a necessary factor in the search for highly conducting materials, including molecular metals and superconductors. The highly celebrated high T c ceramic superconducting oxides are indeed mixed valency compounds.

Mixed-Valence Compounds

Theory and Applications in Chemistry, Physics, Geology,and Biology

Author: D.B. Brown

Publisher: Springer

ISBN:

Category: Science

Page: 520

View: 935

It has been a decade since two seminal reviews demonstrated that mixed-valence compounds share many unique and fascinating features. The insight pro vided by those early works has promoted a great deal of both experimental and theoretical study. As a result of extensive efforts, our understanding of the bonding and properties of mixed-valence compounds has advanced substantially. There has been no compre hensive treatment of mixed-valence compounds since 1967, and the meeting convened at Oxford in September, 1979, provided a unique opportunity to examine the subject and its many ramifications. Mixed-valence compounds play an important role in many fields. Although the major impact of the subject has been in chemistry, its importance has become increasingly clear in solid state physics, geology, and biology. Extensive interest and effort in the field of molecular metals has demonstrated that mixed-valency is a prerequisite for high elec trical conductivity. The intense colors of many minerals have been shown to be due to mixed-valency, and the electron-transfer properties of certain mixed-valence metalloproteins are important in biological processes. Experts from all of these areas participated in this meeting, and the truly interdisciplinary nature of the subject made it a unique learning experience for all in attendance.

The Effects of Electronic Delocalization in Highly Coupled Mixed Valence Systems

Author: Benjamin James Lear

Publisher:

ISBN:

Category:

Page: 146

View: 896

The trinuclear ruthenium cluster RuO(OAc)6L3 (where L is an ancillary ligand) is used to make a variety of mixed valence compounds in which two or more clusters are joined together by an organic bridging ligand. The magnitude of electronic coupling in the mixed valance state of these compounds is quite large and the complexes reside on the Robin-Day class II/class III borderline. The large degree of coupling in these complexes gives rise to ultrafast electron transfer whose effects are observable in the infrared (IR) spectra of these complexes. Utilizing the IR properties of the complexes we are able to arrive at thermodynamic estimates of the electronic coupling parameter (H AB) for asymmetric mixed valence compounds. These asymmetric compounds give rise to mixed valence isomers and the temperature dependence of the isomer populations is used to determine deltaH and deltaS for the electron transfer event in these complexes. The large coupling in these complexes reduces the barrier to electron transfer significantly (enabling ultrafast electron transfer). This places the rate of electron transfer under the control of the nuclear dynamics of the complex and the surrounding environment. The result is that the rate of electron transfer in these mixed valence complexes shows a strong dependence on kinetic parameters of the solvent (those that describe the movement of the nuclei of the system), but not on thermodynamic parameters of the solvent (that describe more static energetic contributions of the environment). This, in turn, leads to an unexpected temperature dependence of the electron transfer rate. It is found that the electron transfer rate dramatically increases when the solvent is frozen. This results form a decoupling of the relatively slow solvent motions from the electron transfer event allowing for the faster internal vibrational motions of the mixed valence complex to control the rate of electron transfer. The effects of the large electronic coupling in these complexes also gives rise to other surprising behaviors. The extent of the electronic coupling in the mixed valence systems is known to depend on the electron donor strength of the attached ancillary ligands. It is shown that, through supramoleuclar interactions at the ancillary ligands of these mixed valence systems, the electronic coupling may be modulated. There is a significant decrease in the resonance stabilization associated with breaking of symmetry in a mixed valence system and that this energy (together with the energy gained by restoration of symmetry) can provide substantial driving force for chemical interactions. This effect is explained in terms of both the direct stabilization of the compound through electronic coupling and in terms of resonance stabilization of the unpaired electron in the mixed valence compound. This result is then extended to molecular electronics where it is shown that changes in current effected by a chemical interaction can provide a driving force for said chemical interaction. The large magnitude of electronic coupling in these mixed valence systems is also shown to be sufficient to stabilize as the ground state what would be thought of as low-lying excited states. It is shown that an electron may transfer from a cluster to the bridging ligand and that this electron transfer gives rise to an increase in electronic coupling throughout the mixed valence state. This increase in electronic coupling is found to be sufficient to stabilize the radical state of the organic bridge. The large energy difference between uncoupled (diabatic) and coupled (adiabatic) mixed valence compounds is also exploited in order to determine whether an electron entering into the mixed valence molecule enters into a diabatic or adiabatic wavefunction. The electron transfer rate from photo-generated triplet zinc tetraphenylphorpyrin to the mixed valence compounds was observed. Comparisons of the observed electron transfer rate to the diabatic and adiabatic driving force for electron transfer are made. It is concluded that the electron enters into a diabatic wavefunciton of the mixed valence compound after which the compound evolves into the adiabatic wavefunciton. The major theme throughout this thesis is the exploitation of the huge value of electronic coupling (H AB) in order to give rise to and explain some very unique and unexpected behaviors of these mixed valence complexes.

Applications of the Moessbauer Effect to the Study of Mixed-Valence Compounds

Author: David B. Brown

Publisher:

ISBN:

Category:

Page: 31

View: 372

The principal utility of Moessbauer Effect spectroscopy in the study of mixed-valence materials is its ability (through measurement of the chemical shift) to distinguish between different oxidation states of the same element in similar chemical environments. By using Moessbauer Effect spectroscopy it is possible to distinguish between trapped and delocalized valence states in mixed-valence compounds. In addition, the rather long lifetime of the Moessbauer experiment, as compared to other forms of spectroscopy, permits evaluation of the rate of intervalence electron exchange in mixed-valence materials. (Author).

Molecules Into Materials

Case Studies in Materials Chemistry-mixed Valency, Magnetism and Super Conductivity

Author: Peter Day

Publisher: World Scientific

ISBN:

Category: Chemistry, Physical and theoretical

Page: 586

View: 489

The last decade has seen the emergence and explosive growth of a new field of condensed matter science: materials chemistry. Transcending the traditional boundaries of organic, inorganic and physical chemistry, this new approach aims to create new molecular and lattice ensembles with unusual physical properties. One of its pioneers, the author has worked on structure-property relations in the inorganic and metal-organic solid state for over 40 years. His seminal work on mixed-valency compounds and inorganic charge transfer spectra in the 1960s set the scene for this new type of chemistry, and his discovery of transparent metal-organic ferromagnets in the 1970s laid the ground rules for much current work on molecular magnets. He has also published extensively on molecular metals and superconductors, especially on charge transfer salts combining conductivity with magnetism. This indispensable volume brings together for the first time a selection of his articles on all these topics, grouped according to theme. Each group is prefaced by a brief introduction for the general reader, putting the articles into their context in the evolution of the subject and describing the intellectual circumstances in which each project was conceived and executed.

Magnetism of Mixed Valence Compounds

Author: William E. Hatfield

Publisher:

ISBN:

Category:

Page: 54

View: 705

Results of representative studies of magnetic properties of mixed valence compounds are described in terms of Heisenberg-Van Vleck-Direc exchange theory, which is developed. Systems of discrete clusters, chains, sheets, and three dimensinal polymeric materials are discussed. Several important problems requiring additional research are identified. (Author).