Showing papers on "Valence (chemistry) published in 1968"

Mixed Valence Chemistry-A Survey and Classification

Abstract: Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Piezo-Electroreflectance in Ge, GaAs, and Si

Abstract: Piezoelectroreflectance in Ge, Si and GaAs studied for uniaxial stress effects on electronic energy bands

Dielectric Definition of Electronegativity

Abstract: A simple model is proposed for the static electronic dielectric constants of zinc-blende and wurtzite crystals in terms of the dielectric constants of diamond-type crystals. The model suggests a natural definition of electronegativity in ($s{p}^{3}$)-hybridized valence states.

Covalent Bond in Crystals. I. Elements of a Structural Theory

Abstract: An a posteriori theory is developed for the structural energy of covalent crystals. The microscopic theory is based on ionic pseudopotentials and valence dielectric screening. The theory explains the difference between empirical pseudopotential form factors derived from the optical spectra of semiconductors and the metallic form factors calculated from free-ion term values by Animalu and Heine. A byproduct of the theory, which utilizes Penn's model isotropic semiconductor dielectric function, is a relation between the covalent bonding charge and the macroscopic dielectric constant. In self-consistent form the theory is an example of the "bootstrap" approach, applied here to treat the effect of covalent bonding on the ground-state energy of the valence electron gas. It is argued that the axiomatic character of the covalent theory is to be expected on symmetry grounds, and it is shown that the theory is superior to a nonlinear multiple-scattering theory based on the free-electron dielectric function. The extension of the theory to III---V and II---VI semiconductors is described briefly. The theory may be used to calculate elastic and macroscopic dielectric properties of covalent crystals starting only from ionic pseudopotential form factors.

Molecular geometry: Bonded versus nonbonded interactions

Abstract: Proposes simplified computational models to facilitate a comparison between the relative roles of bonded and nonbonded interactions in directed valence.

Matrix‐Isolation Study of the Reaction of Atomic and Molecular Fluorine with Carbon Atoms. The Infrared Spectra of Normal and 13C‐Substituted CF2 and CF3

Abstract: Carbon atoms, resulting from the photolysis of cyanogen azide, are found to react with molecular fluorine in an argon matrix at 14°K to produce CF2. The use of carbon‐13 has led to confirmation of the previous assignment of features at 1073 and 1191 cm−1 to 13CF2 present in natural abundance. Using recent structural data on CF2 and the 12CF2 and 13CF2 frequencies, it has been possible to calculate the complete valence force potential of CF2. Values of the potential constants are compared with those of the related species NF2 and OF2. When the sample is subjected to radiation of wavelengths effective in photolyzing F2, CF3 is also produced. Sufficient yields of both 12CF3 and 13CF3 have been obtained for observation of all four vibrational fundamentals. Data have been fitted to a four‐constant valence force potential. Agreement is most satisfactory for a C3v structure with a deviation of 13° from planarity. Using this structure, the thermodynamic properties of CF3 have been estimated.

Introduction to modern inorganic chemistry

Abstract: INTRODUCTION Inorganic Chemistry and the Discovery of the Elements Development Recent Advances Inorganic Nomenclature Approach to Inorganic Chemistry and Further Reading Problems THE ELECTRONIC STRUCTURE AND THE PROPERTIES OF ATOMS Introduction Theory of the Electronic Structure of Hydrogen Many-electron Atoms Shapes of Atomic Orbitals Further Properties of the Elements COVALENT MOLECULES: DIATOMICS General Background Diatomic Molecules POLYATOMIC COVALENT MOLECULES Introduction The Shapes of Molecules and Ions Containing p Bonds Only The Shapes of Species Containing p Bonds General Approaches to Bonding in Polyatomic Species Bonding in Polyatomics: The Two-centre Bond Approach Two-centred Orbitals: Hybridisation Delocalized, or Multi-centred, s Orbitals p Bonding in Polyatomic Molecules An Example of the Approach Using Delocalized Bonding Throughout Extension to Other Molecules Problems THE SOLID STATE Simple Ionic Crystals * The Formation of Ionic Compounds * The Born-Haber Cycle * The Lattice Energy * The Endothermic Terms in the Formation of an Ionic Solid * Bonding Which is not Purely Ionic * Metallic Bonding * Complex Ions * The Crystal Structures of Covalent Compounds * Defect Structures and Nonstoichiometric Solids Problems SOLUTION CHEMISTRY Aqueous Solutions * Solubility * Acids and Bases * Oxidation and Reduction Nonaqueous Solvents * Solubility and Solvent Interaction in Nonaqueous Solvents * Acid-base Behaviour in Nonaqueous Solvents * General Uses of Nonaqueous Solvents * Liquid Ammonia * Anhydrous Acetic Acid *'Superacid' Media * Bromine Trifluoride * Supercritical Fluids Problems EXPERIMENTAL METHODS Separation Methods * Ion Exchange * Chromatography * Solvent Extraction Structure Determination * Diffraction Methods * Spectroscopic Methods and the Electromagnetic Spectrum * Electronic Spectra * Vibrational Spectra * Nuclear Magnetic Resonance * Further Methods of Molecular Spectroscopy * Fourier Transform Methods * Other Methods Determination of Energy Levels: Photoelectron Spectroscopy Problems GENERAL PROPERTIES OF THE ELEMENTS IN RELATION TO THE PERIODIC TABLE Variation in Energies of Atomic Orbitals with Atomic Number Exchange Energy Stable Configurations Atomic and Ionic Sizes Chemical Behaviour and Periodic Position Methods of Showing the Stabilities of Oxidation States The Abundance and Occurrence of the Elements The Extraction of the Elements Problems HYDROGEN General and Physical Properties of Hydrogen Chemical Properties of Hydrogen Ionic Hydrides Metallic Hydrides Covalent Hydrides Electron Deficient Hydrides The Hydrogen Bond Problems THE 's' ELEMENTS General and Physical Properties, Occurrence and Uses Compounds with Oxygen and Ozone Carbon Compounds Complexes of the Heavier Elements Crown Ethers, Cryptates and Alkali Metal Anions Special Features in the Chemistry of Lithium and Magnesium Beryllium Chemistry Problems THE SCANDIUM GROUP AND THE LANTHANIDES General and Physical Properties Chemistry of the Trivalent State The Separation of the Elements Oxidation States Other Than III Properties associated with the Presence of f Electrons Problems THE ACTINIDE ELEMENTS Sources and Physical Properties General Chemical Behaviour of the Actinides Thorium Protactinium Uranium Neptunium, Plutonium and Americium The Heavier Actinide Elements Problems THE TRANSITION METALS: GENERAL PROPERTIES AND COMPLEXES Introduction to the Transition Elements The Transition Ion and its Environment:Ligand Field Theory Ligand Field Theory and Octahedral Complexes Coordination Number Four Stable Configurations Coordination Numbers Other Than Four or Six Effect of Ligand on Stability of Complexes Isomerism Mechanisms of Transition Metal Reactions Structural Aspects of Ligand Field Effects Spectra of Transition Element Complexes p Bonding Between Metal and Ligands Problems THE TRANSITION ELEMENTS OF THE FIRST SERIES General Properties Titanium, 3d24s2 Vanadium, 3d34s2 Chromium, 3d54s1 Manganese, 3d54s2 Iron, 3d64s2 Cobalt, 3d74s2 Nickel, 3d84s2 Copper, 3d104s1 The Relative Stabilities of the Dihalides and Trihalides of the Elements of the First Transition Series Problems THE ELEMENTS OF THE SECOND AND THIRD TRANSITION SERIES General Properties Zirconium, 4d25s2, and Hafnium, 5d26s2 Niobium, 4d45s1, and Tantalum, 5d36s2 Molybdenum, 4d55s1, and Tungsten, 5d46s2 Technetium, 4d65s1, and Rhenium, 5d56s2 Ruthenium, 4d75s1, and Osmium, 5d66s2 Rhodium 4d85s1, and Iridium, 5d96s0 Palladium, 4d105s0, and Platinum, 5d96s1 Silver, 4d105s1, and Gold, 5d106s1 The Zinc Group Problems TRANSITION METALS: SELECTED TOPICS Copper Oxide Ceramic Superconductors Carbonyl Compounds of the Transition Elements Metal-Organic Compounds p Bonded Cyclopentadienyls and Related Species The Organometallic Chemistry of the Lanthanides Actinide Organometallic Chemistry Multiple Metal-Metal Bonds Transition Metal Clusters Metal-Dioxygen Species Compounds containing M-N2 Units and their Relationship to Nitrogen Fixation Metal-Dihydrogen Complexes Post-Actinide 'Superheavy' Elements Relativistic Effects Problems THE ELEMENTS OF THE 'p' BLOCK Introduction and General Properties The First Element in a p Group The Remaining Elements of the p Group The Boron Group, ns2np1 The Carbon Group, ns2np2 The Nitrogen Group, ns2np3 The Oxygen Group, ns2np4 The Fluorine Group, ns2np5 (The Halogens) The Helium Group Bonding in Main Group Compounds: the Use of d Orbitals Problems SELECTED TOPICS IN MAIN GROUP CHEMISTRY AND BONDING The Formation of Bonds Between Like Main Group Atoms Polysulfur and Polyselenium Rings and Chains Nets and Linked Rings Cluster Compounds of the p Block Elements Polynuclear Ions and the Acid Strength of Preparation Media Silicates, Aluminosilicates and Related Materials Multiple Bonds Involving Heavier Main Group Elements Commentary on VSEPR Bonding in Compounds of the Heavier Main Group Elements GENERAL TOPICS Electron Density Determinations Metal-Polychalcogenide Compounds Fullerenes, Nanotubes and Carbon 'Onions' - New Forms of Elemental Carbon Dendrimeric Molecules BIOLOGICAL, MEDICINAL AND ENVIRONMENTAL INORGANIC CHEMISTRY Biological Inorganic Chemistry Medicinal Inorganic Chemistry Environmental Inorganic Chemistry APPENDICES INDEX RELATIVE ATOMIC MASSES PERIODIC TABLE OF THE ELEMENTS

Mass‐Spectrometric Studies of Bonding in the Group IIA Fluorides

Abstract: A mass spectrometer was used to study high‐temperature gaseous equilibria of the type M(g) + MF2(g) = 2MF(g), where M = Mg, Sr, and Ba. The heat of this reaction gives directly the difference between bond dissociation energies D (MF −F) − D (M −F). Results obtained for the above equilibrium were ΔH298 = 25.6, 2.1, and −6.7 kcal (all ±2 kcal) for Mg, Sr, and Ba, respectively. In addition, the dissociation energies, D°298, of MgF, SrF, and BaF were determined as 110.4 ± 1.2, 129.5 ± 1.6, and 140.3 ± 1.6. Although the bond dissociation energies of the lighter Group II fluorides are in qualitative accord with the simple s – p valence state scheme of covalent bonding, this model breaks down rather badly for the heavier metal fluorides. It appears, however, that the bond dissociation energies of these heavier fluorides can be accounted for quite well by an electrostatic model which considers each molecule as an assembly of polarizable ions. The measured dissociation energies of the metal monofluorides can be co...

Force Constants of Nickel Carbonyl from Vibrational Spectra of Isotopic Species

Abstract: The infrared spectra of gaseous Ni(13CO)4 and Ni(C18O)4 and the infrared and Raman spectra of CCl4 solutions of these isotopic molecules were recorded. General quadratic valence force constants have been calculated from the frequencies of the normal species and the two isotopic species. The resulting force constant solution is similar to that obtained previously assuming a pi electron interaction potential function. These results put the force constants of metal carbonyls on a firmer basis.

Studies of IBr, ICl, and I2Cl6 Crystal Properties by Means of the Mössbauer Effect in 129I. I. Chemical Bonds

Abstract: Using 129I as Mossbauer nucleus, the properties of ICl, IBr, and I2Cl6, as well as I2Cl4Br2, were investigated. From the well‐resolved spectra of the quadrupole splitting, the e2qQ, the η parameter, and the isomer shift with respect to the standard 66Zn129Te source are derived. These data are used to deduce the 5p electron population of I. On the basis of the sensitivity of the isomer shift to the s character of the bond, it is shown that no sp hybridization occurs. The linear dependence of the isomer shift on the p holes is found to extend to hp values higher than 1. A relationship is derived to include hs as well as hp when iodine is in a high valence state.

Localized Magnetic Moments and Pauling Valence in Manganese Metal, Some 3d-Transition Alloys and Intermetallic Compounds

Abstract: The dependence of magnetic moments localized at manganese atomic site on crystalline environment is discussed by using the concept of the Pauling valence which is defined by the relation between the atomic radius and the interatomic distance from neighbouring atoms. The linearlity of the magnitude of magnetic moments and the Pauling valence at individual manganese atomic site is found. It is concluded that the localized magnetic moment is mainly affected by kinds of nearest neighbour atoms and the coordination number. From this result, it is suggested that the covalency effect plays an important role in determining the magnitude of the magnetic moment assosiated with the manganese atomic site. Magnetic moments in σ-phase Fe-V alloys whose crystal structure is similar to that of α-Mn are also discussed.

Increased-valence theory of valence

Abstract: Describes several "increased valence" formulas for molecular systems with one or more sets of pour electrons distributed among three atomic orbitals of three atoms.

Solute Knight Shifts in Noble Metals

Abstract: Knight shifts are reported for Cu, Cd, Al, In, Sn, and Ga as dilute solutes in Au; Hg, In, and Al as dilute solutes in Ag; In as a dilute solute in Cu; and Sn as a dilute solute in Ag-Au alloys. These data are considered together with all other Knight-shift data on dilute solutes in the three noble metals. A valence effect is found which is opposite in sense for Cu and Ag on the one hand, and Au on the other. Cd resonance linewidths for Au-Cd alloys show appreciable broadening with increase in alloy content or test frequency. New values of hyperfine fields for the free atom are obtained, together with several choices of values for the paramagnetic spin susceptibility of the hosts. Implications of the rigid-band theory for lattice volume effects are examined. The problem of exchange enhancement of the spin susceptibility is reviewed. Values for Knight's parameter $\ensuremath{\xi}$, usually considered as a measure of the amount of $s$ character in the metal, are tabulated and discussed. The valence trends found in the Knight shifts are also evident in the $\ensuremath{\xi}$ values, and these cannot be attributed to a phase-shift analysis common to the three noble metals. Varying band character and local effects appear important to the results. The origin of the solvent Knight shifts in noble metal alloys is examined in terms of the rigid-band theory; the results emphasize several shortcomings in charge screening theory as currently applied to solvent shifts.

Valence Structure from Coherent X‐Ray Scattering: Fourier Difference Synthesis

Abstract: The one‐electron density function for a molecule of first‐row atoms is partitioned into core‐electron and valence‐electron density pieces. The SCF 1s AO's are used to define the core‐electron density pieces. The residual density, after removal of the core density, constitutes the valence density, which is assumed to be chemically interpretable. The partitioning is explored in reciprocal space to provide insight into x‐ray diffraction experiments. The contribution from valence scattering within the CuKᾱ sphere is 10%–30% of the total scattering. Valence‐electron density maps have been Fourier synthesized from x‐ray diffraction data of uracil. The density maps reveal trigonal bonding for the nitrogen atoms and bridge densities in the middle of the C(5)–C(6) and C(4)–C(5) bonds. The densities near the time‐average nuclear positions are unreliable.

Physical nature of the chemical bond II. Valence atomic orbital and energy partitioning studies of linear nitriles

Abstract: The valence atomic orbitals (VAO's) of several linear nitriles are determined using non-empirical SCF–LCAO–MO wave functions expanded in a minimal (CN−, HCN, FCN, C2N2), double-zeta (CN−, HCN), or double-zeta + polarization (HCN) basis of Slater atomic orbitals (AO's). The molecular energy of each system (except the double-zeta + polarization HCN system) is partitioned according to the procedure of Ruedenberg to obtain numerical values of nitrile C and N atomic and CN bond components of the energy. In addition, the nitrile results are compared with minimal AO basis results obtained previously by other authors for homonuclear diatomics, diatomic hydrides and H2O. The numerical data are used to test the internal self-consistency of the various definitions entering the partitioning method, i.e. whether or not analogous quantities assume similar values in chemically similar situations. The analysis of nitrile SCF–MO wave functions in terms of the set of VAO's characteristic of the system under consideration is shown to be a promising approach to the problem of extracting useful information from the wave functions. In general, numerical results for the nitrile systems studied are fairly consistent with the concepts on which the partitioning method is based: promotion, quasi-classical interaction, sharing penetration, sharing interference and charge transfer. However, the VAO expansions for several energy components need to be investigated further and possibly revised.

Synthesis, structure, and properties of π-cyclo-octenyl-π-cyclo-octa-1,5-dienecobalt

Abstract: Treatment of cobalt dichloride suspended in a mixture of cyclo-octa-1,5-diene, pyridine, and tetrahydrofuran with metal sodium affords a brown crystalline complex, Co(C8H13)(C8H12). Structural proof has been obtained which shows conclusively that the complex is π-cyclo-octenyl-π-cyclo-octa-1,5-dienecobalt. Thermally it is stable, and shows no tendency for valence tautomerism. Remarkable mobility of the hydrogen atoms of the ligand has been observed in some chemical reactions, and this is interpreted in terms of the molecular structure.