DOCSLIB.ORG

Valence-Bond Theory of Compounds of Transition Metals

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Valence-Bond Theory of Compounds of Transition Metals Proc. Nat. Acad. Sci. USA Vol. 72, No. 11, pp. 4200-4202, November 1975 Chemistry Valence-bond theory of compounds of transition metals (valence bonds/metal carbonyls/spd bond orbitals/bond angles) LINUS PAULING Linus Pauling Institute of Science and Medicine, 2700 Sand Hill Road, Menlo Park, California 94025 Contributed by Linus Pauling, August 14, 1975 ABSTRACT An equation relating the strength (bond- fying assumption that the bond orbitals have cylindrical forming power) of an spd hybrid bon orbital to the angles it symmetry about the bond axis. A treatment without this as- makes with other bond orbitals is formulated and applied in the discussion of the structures of transition-metal carbonyls sumption has been carried out by McClure (6). and other substances by the valence-bond method. The rath- The two equivalent and mutually orthogonal spd orbitals er simple theory gives results that agree well with those ob- with maximum strength in two directions making the angle tained by the complicated and laborious calculation of sets a with one another can be found by use of Lagrange's meth- of orthogonal hybrid bond orbitals with maximum strength. od of undetermined multipliers (4). Their strength is During the last two decades there has been intense activity S(a) = (3 - 6x + 7.5x2)1/2 in the field of the synthesis of compounds of the transition metals, such as those containing carbonyl and cyclopentadi- + (15 + 6x - 7..5x2)"/2 [1] enyl groups. Hundreds of these compounds have been made, and the structures of scores of them have been determined in which x = cos2(a/2). Values of S(a) are shown in Fig. 1. by the x-ray diffraction method. The theoretical treatment The best bond orbitals lie at a equal to 73.15' or 133.620. of the substances has been carried out by the molecular-or- The ranges 65°-85° and 120°-145° involve only a small de- bital method, and has yielded only qualitative results. For crease in S, and we may conclude that these ranges are fa- example, the short Re-Re distance 222 pm reported for the vored by atoms of the transition metals (7). ion Re2CI82- by Kuznetzov and Koz'min (1) was interpreted For an orbital i at angles aij with other orbitals j the value by Cotton as showing that the rhenium atoms are connected of Si is assumed to be by a quadruple bond, and Cotton formulated a simple mo- Si = 3 - Z[3 - S(a11)] [2] lecular-orbital theory that provided an explanation of the eclipsed configuration of the two ReCL4 groups, as follows J. (2): "The d.a2 y2orbital on each metal atom was assumed to This assumed additivity is seen to have moderate reliability be employed mainly in Re-Cl bonding and the remaining from the approximation of the points in Fig. 2 to the line four d orbitals on each metal atom were used to form the corresponding to Eq. 2. For example, each of the six octahe- Re-Re bond. Overlap of the d52 orbitals give rise to a a dral d2sp3 orbitals has four others at 900 and one at 180°, bond; overlap of corresponding pairs of the dX2 and dy4 or- giving 3 - S = 0.0852, which is 12% greater than the actual bitals leads to formation of a pair of r bonds. Finally, over- value (4), 0.0760. The point H4 is for Hultgren's best set of lap of the dxy orbitals gives rise to a a bond. The eclipsed four cylindrical orbitals with equal strength, with a = configuration is a consequence of the a component of the 73.820 and 136.10° and 3 - S = 0.008 by both direct calcu- bond since that is the only component that is angle-depen- lation and Eq. 2. The points 1 to 9 are for the set of nine best dent." Neither this nor any other molecular-orbital treat- spd orbitals reported by McClure, and R1, R2, R3 are for ment (3) provides any significant discussion of bond angles eight orbitals, as reported by Racah (see next section). and bond lengths or of the relative stability of alternative structures. Sets of eight or nine spd bond orbitals I have now found a simple relation between the strength Racah (8) pointed out that sets of eight equivalent spd orbit- (the bond-forming power) of a hybrid spd bond orbital and als could be formed. They are directed toward the corners of the angles that it makes with other similar orbitals that per- a square antiprism, point group 8 2m. The best set (R1 in mits the prediction to be made of bond angles, bond lengths, Fig. 2) has 3 - S = 0.0114, in good agreement with the the relative stability of alternative structures, and other value 0.0108 from Eq. 2. The angle with the 8 axis is 60.900 properties of compounds of the transition metals. and the bond angles are 72.340 (8), 76.32' (8), 121.80° (4), and 140.930 (8), all within the ranges corresponding to close Relation between bond strength and bond angles for to the maximum bond strength. Racah also determined the spd bond orbitals best orbitals for the tetragonal dodecahedron, which Hoard The bond strength S of an orbital has been defined (4) as the (9) had found in his x-ray study of K4Mo(CN)8-2H20. The value in the bond direction of the angular part of the wave best set of orbitals (with the sum of S a maximum) consists of function, with the angular wave functions normalized to 4wr. four with S = 2.9954, angle 34.550 with the 4 axis, and four It is a reasonably good measure of the energy of the bond with S = 2.9676, angle 72.78° with this axis. The bond an- that can be formed by the orbital, with corrections needed, gles are 69.100 (2), 72.670 (4), 75.80° (8), 95.030 (4), 132.72' of course; for promotion of electrons, partial ionic character, (2), 141.77° (4), and 145.560 (2). The values of 3 - S given deviation from electroneutrality, strain energy of bent by Eq. 2, 0.0048 and 0.0377, agree with those given by bonds, and other factors. The values of S for some spd bond Racah, 0.0046 and 0.0324 (R2 and R3 in Fig. 2). The angles orbitals were discussed long ago (4). Hultgren (5) then dis- of the Mo-CN bonds in the crystal are within one degree of cussed the problem thoroughly, with, however, the simpli- the values given by Racah's calculation. 4200 Downloaded by guest on October 1, 2021 Chemistry: Pauling Proc. Nat. Acad. Sci. USA 72 (1975) 4201 0.08 St 2.97 - TWO spd HYBR I D ORB ITA LS 2.96 p 2.95 - 400 600 800 1000 1200 1400 1600 1800 ct, ANGLE BETWEEN BOND DIRECTIONS FIG. 1. The strength S in two directions at angle a of two best 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 orthogonal spd orbitals as a function of a. FIG. 2. Comparison of values of 3 - S calculated by evaluation of the best sets of orthogonal orbitals (vertical) and by use of Eq. 2 The value of Eqs. 1 and 2 can be illustrated by their appli- (horizontal). Point 9 is between R2 and H4. cation to the square antiprism. The value of S is found by a few minutes of calculation to be a maximum (at 2.9789) for gives 2.9816 for McClure's set. This tetragonal structure is angle 61.40 with the 8 axis, only 0.50 from the value found accordingly only slightly less stable than McClure's and con- by Racah. Similarly, for the tetragonal dodecahedron this stitutes an alternative stable arrangement for nine spd simple calculation makes SI + S2 a maximum for angles bonds. 340 and 72.0°, 10 from Racah's values. SI and S2 have the values 2.9944 and 2.9644, respectively, close to those found Molecules containing quadruple bonds by Racah. The bond angles are 68.0° (2), 74.00 (4), 75.20 (8), I have pointed out (7) that the orientation of eight or nine 95.50 (4), 133.40 (4), 142.00 (4), and 144.00 (2). The angle bonds toward the corners of a square antiprism or a square 95.5° is the most seriously strained one. It is between pairs of antiprism with a pyramid permits the formation of a bonds in the oblate tetrahedron, and its contribution to S2, quadruple bond and the calculation of ratio of the -0.0276, explains its low value. quadruple-bond length to the single-bond length. These The nine best spd orbitals found by McClure are oriented structures-also account for the observed eclipsed configura- as shown in Fig. 3. They have values of S between 2.9539 tion of the other bonds and the values of the bond angles, and 2.9936. Of the 36 bond angles, 21 lie between 66.490 observed to be 1040 in Re2CJ82- and about the same in other and 92.570, average 74.040 (only 0.890 from the best bond complexes. The calculated value for this angle is 113'; a angle), and 15 lie between 112.920 and 140.490, average smaller value is expected, however, because it would relieve 133.350 (0.200 from the other best bond angle). The individ- the strain in the four bent bonds of the covalent bond and in ual values of 3 - S (Fig.
Load more

Recommended publications
  • Organometallic Chemistry from the Interacting Quantum Atoms Approach
    CORSO DI DOTTORATO DI RICERCA IN SCIENZE CHIMICHE CICLO XXIII TESI DI DOTTORATO DI RICERCA ORGANOMETALLIC CHEMISTRY FROM THE INTERACTING QUANTUM ATOMS APPROACH sigla del settore scientifico disciplinare CHIM03 NOME DEL TUTOR NOME DEL DOTTORANDO Prof: Angelo Sironi Davide Tiana NOME DEL COORDINATORE DEL DOTTORATO Prof. Silvia Ardizzone ANNO ACCADEMICO 2009/2010 1 2 Index Introduction ............................................................................................................................................................................ 5 The ligand field theory (LFT) .................................................................................................................................... 5 The chemistry from a real space point of view ................................................................................................. 9 Chapter 1: The quantum theory of atoms in molecules (QTAM) ............................................................... 12 Topological analysis of electron charge density ........................................................................................... 12 Analysis of the electronic charge density Laplacian ................................................................................... 16 Other properties ........................................................................................................................................................... 18 Chapter 2: The interacting quantum atoms theory (IQA) ............................................................................
    [Show full text]
  • Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory Review Questions
    Chemical Bonding II: Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory Review Questions 10.1 J The properties of molecules are directly related to their shape. The sensation of taste, immune response, the sense of smell, and many types of drug action all depend on shape-specific interactions between molecules and proteins. According to VSEPR theory, the repulsion between electron groups on interior atoms of a molecule determines the geometry of the molecule. The five basic electron geometries are (1) Linear, which has two electron groups. (2) Trigonal planar, which has three electron groups. (3) Tetrahedral, which has four electron groups. (4) Trigonal bipyramid, which has five electron groups. (5) Octahedral, which has six electron groups. An electron group is defined as a lone pair of electrons, a single bond, a multiple bond, or even a single electron. H—C—H 109.5= ijj^^jl (a) Linear geometry \ \ (b) Trigonal planar geometry I Tetrahedral geometry I Equatorial chlorine Axial chlorine "P—Cl: \ Trigonal bipyramidal geometry 1 I Octahedral geometry I 369 370 Chapter 10 Chemical Bonding II The electron geometry is the geometrical arrangement of the electron groups around the central atom. The molecular geometry is the geometrical arrangement of the atoms around the central atom. The electron geometry and the molecular geometry are the same when every electron group bonds two atoms together. The presence of unbonded lone-pair electrons gives a different molecular geometry and electron geometry. (a) Four electron groups give tetrahedral electron geometry, while three bonding groups and one lone pair give a trigonal pyramidal molecular geometry.
    [Show full text]
  • Pauling's Defence of Bent-Equivalent Bonds
    Pauling’s Defence of Bent-Equivalent Bonds: A View of Evolving Explanatory Demands in Modern Chemistry Summary Linus Pauling played a key role in creating valence-bond theory, one of two competing theories of the chemical bond that appeared in the first half of the 20th century. While the chemical community preferred his theory over molecular-orbital theory for a number of years, valence-bond theory began to fall into disuse during the 1950s. This shift in the chemical community’s perception of Pauling’s theory motivated Pauling to defend the theory, and he did so in a peculiar way. Rather than publishing a defence of the full theory in leading journals of the day, Pauling published a defence of a particular model of the double bond predicted by the theory in a revised edition of his famous textbook, The Nature of the Chemical Bond. This paper explores that peculiar choice by considering both the circumstances that brought about the defence and the mathematical apparatus Pauling employed, using new discoveries from the Ava Helen and Linus Pauling Papers archive. Keywords: Linus Pauling, Chemical Bond, Double Bond, Chemistry, Valence, Resonance, Explanation, Chemical Education Contents 1. Introduction………………………………………………………….. 2 2. Using Valence Bonds and Molecular Orbitals to Explain the Double Bond………………………………………………………................. 3 3. Textbook Explanations……………………………………………… 5 4. The Origins of Bent-Equivalent Bonds ……………………..……… 7 4.1 An Early Defence …………………………………………….. 8 5. Catalysing the Mature Defence……………………………………… 9 5.1 Soviet Suppression and Pedagogical Disfavour ………………. 11 5.2 Wilson…………………………………………………………. 12 5.3 Kekulé…………………………………………………………. 14 6. Concentrating Bond Orbitals into a Defence………………………… 15 7. Conclusions………………………………………………………….. 17 Word Count: 8658 (9936 with footnotes) Pauling’s Defence of Bent-Equivalent Bonds Page 2 of 19 1.
    [Show full text]
  • Chemical Applications of X-Ray Charge-Density Analysis
    Chem. Rev. 2001, 101, 1583−1627 1583 Chemical Applications of X-ray Charge-Density Analysis Tibor S. Koritsanszky† Department of Chemistry, University of the Witwatersrand, WITS 2050, Johannesburg, South Africa Philip Coppens* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York, 14260-3000 Received October 18, 2000 Contents VII. Physical Properties from the Experimental 1617 Density I. Charge Densities from X-ray Diffraction 1583 A. Net Atomic and Molecular Charges 1617 II. Elements of Electron Density Determination 1585 B. Solid-State Dipole and Higher Moments and 1618 A. Expressions 1585 Nonlinear Optical Properties 1. Electron Density 1585 C. Electric Field Gradient at the Nuclear 1621 2. Structure Factor and Difference Densities 1585 Positions 3. Aspherical-Atom Formalism 1586 D. Electrostatic Potential 1621 4. Electrostatic Properties from the Charge 1586 E. Quantitative Evaluation of Electrostatic 1623 Density Interactions from the X-ray Charge Density B. Experiment and Analysis 1587 VIII. Concluding Remarks 1624 1. Experimental Conditions 1587 IX. Glossary of Abbreviations 1624 2. Data Analysis 1588 X. Acknowledgments 1624 III. Chemistry from the Electron Density 1590 XI. References 1625 A. Use of the Deformation Density 1590 B. Theory of Atoms in Molecules 1591 I. Charge Densities from X-ray Diffraction 1. Topological Atom 1591 As X-ray scattering by electrons is much stronger 2. Topological Structure of Molecules 1591 than that of the nuclei, intensities of scattered X-rays 3. Topological Classification of Atomic 1591 are almost exclusively determined by the distribution Interactions of the electrons. Thus, X-ray diffraction is a priori a 4. Laplacian of the Electron Density 1592 tool for the determination of electron distribution in 5.
    [Show full text]
  • Theory of Three-Electron Bond in the Four Works with Brief Comments
    ry: C ist urr m en e t Dmytrovych, Organic Chem Curr Res 2017, 6:2 h R C e c s i DOI: 10.4172/2161-0401.1000182 e n a a r Organic Chemistry c g r h O ISSN: 2161-0401 Current Research ResearchResearch Article Article Open Accesss Theory of Three-Electron Bond in the Four Works with Brief Comments Bezverkhniy Volodymyr Dmytrovych* Street Chernivtsi 6, Village Nedoboevtsi, Chernivtsi Region, Khotyn District, Ukraine Abstract Using the concept of three-electron bond we can represent the actual electron structure of benzene and other molecules, explain specificity of the aromatic bond and calculate the delocalization energy. Gives theoretical justification and experimental confirmation of existence of the three-electron bond. It was shown, that functional relation y=a+b/x+c/ x2 fully describes dependence of energy and multiplicity of chemical bond from bond distance. Keywords: Benzene; Three-electron bond; Semi-virtual particle; Structure of the Benzene Molecule on the Basis of the Fermion; Entangled quantum state; Interfering universe Three-Electron Bond Introduction Results and Discussion Chemical bond has been always a basis of chemistry. Advancement Supposing that the chemical bond between two atoms can be of chemical science can be considered as evolution, development of established by means of three electrons with oppositely oriented spins concepts about chemical bond. Aromatic bond is fundamental basis ( ) the structure of the benzene molecule can be expressed as follows of organic chemistry. Concept of three-electron bond in benzene (Figures 2 and 3). molecule enables to explain specificity of aromatic bond. It also becomes ↑↓↑ It is interesting to point out that spins of central electrons on opposite apparent, why planar molecules with 6, 10 etc.
    [Show full text]
  • D. Michael P. Mingos Editor 100 Years Old and Getting Stronger
    Structure and Bonding 169 Series Editor: D.M.P. Mingos D. Michael P. Mingos Editor The Chemical Bond I 100 Years Old and Getting Stronger 169 Structure and Bonding Series Editor: D.M.P. Mingos, Oxford, United Kingdom Editorial Board: F.A. Armstrong, Oxford, United Kingdom X. Duan, Beijing, China L.H. Gade, Heidelberg, Germany K.R. Poeppelmeier, Evanston, IL, USA G. Parkin, NewYork, USA M. Takano, Kyoto, Japan Aims and Scope The series Structure and Bonding publishes critical reviews on topics of research concerned with chemical structure and bonding. The scope of the series spans the entire Periodic Table and addresses structure and bonding issues associated with all of the elements. It also focuses attention on new and developing areas of modern structural and theoretical chemistry such as nanostructures, molecular electronics, designed molecular solids, surfaces, metal clusters and supramolecular structures. Physical and spectroscopic techniques used to determine, examine and model structures fall within the purview of Structure and Bonding to the extent that the focus is on the scientific results obtained and not on specialist information concerning the techniques themselves. Issues associated with the development of bonding models and generalizations that illuminate the reactivity pathways and rates of chemical processes are also relevant. The individual volumes in the series are thematic. The goal of each volume is to give the reader, whether at a university or in industry, a comprehensive overview of an area where new insights are emerging that are of interest to a larger scientific audience. Thus each review within the volume critically surveys one aspect of that topic and places it within the context of the volume as a whole.
    [Show full text]
  • Description of Conjugation and Hyperconjugation in Terms of Electron Distributions
    J. Am. Chem. SOC.1983, 105, 5061-5068 5061 crownlike geometry with pseudoaxial methyl is observed as the can be rationalized by least hindered attack on a local conformer preferred local conformation of the minor epoxides (19, for ex- similar to 1. Similar results have been observed for systems having ample) obtained from E-disubstituted 3-methylcycloalkenes. either allylic hydroxyl or alkyl substitution, so the effect must be Transition-state geometries resembling any of these (E)-olefin relatively large. conformers will encounter considerable transannular interactions We believe that consistently high selectivity results from a good as rehybridization proceeds. Thus, several factors complicate the match between preferred local ground-state and transition-state correlation of favored ground-state and transition-state local geometry. Both CH3and R occupy innocuous locations near the conformation in reactions involving E-disubstituted cycloalkenes olefin plane in conformers such as 1, and there is sufficient and bulky reagents. As noted earlier, our generalization is intended flexibility to minimize transannular effects as rehybridization for use only with (E)-olefins having allylic alkyl substitution. occurs. The selectivity is often large enough (>20:1 product ratios) Although there are examples in the literature where allylic OH to justify stereochemical predications based on the dominant local directs epoxidations in the same stereochemical sense as in our conformer even in more complex systems where other important system with allylic CH3,11b9cthe effect is smaller and not as reliable. conformational factors may not be known. This is evident from the nonselective epoxidation of 20. Predictions in the (E)-olefin series must be made with ca~ti0n.l~ Acknowledgment.
    [Show full text]
  • Bent Bonds and the Antiperiplanar Hypothesis As a Simple Model to Predict Dielsealder Reactivity: Retrospective Or Perspective?
    Tetrahedron 69 (2013) 6022e6033 Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet Perspectives Bent bonds and the antiperiplanar hypothesis as a simple model to predict DielseAlder reactivity: retrospective or perspective? Ghislain Deslongchamps a, Pierre Deslongchamps b,c,* a Department of Chemistry, University of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada b Departement de Chimie, Universite Laval, Quebec, QC G1V 0A6, Canada c OmegaChem Inc., 480 rue Perreault, St-Romuald, QC G6W 7V6, Canada article info Article history: Received 24 April 2013 Accepted 3 May 2013 Available online 10 May 2013 Keywords: Bent bonds Antiperiplanar hypothesis DielseAlder Diradicaloid Contents 1. Introduction . ...................... 6022 2. The DielseAlder reaction revisited . ................................................ 6023 2.1. 1,3-Butadiene . .................................................6023 2.2. Prototypical DielseAlder reaction . .........................................6024 2.3. Dimerization of cyclopentadiene . .........................................6025 2.4. Thiele’sacid ........................................................... .. .................................................6026 2.5. Dimerization of 1,3-butadiene . .........................................6026 2.6. Furanone dienophiles . .........................................6026 2.7. Substituent effects on facial selectivity . .........................................6027 2.8. Retro-DielseAlder reaction
    [Show full text]
  • Organometallic Chemistry and Catalysis Grenoble Sciences
    ATOMIC WEIGHTS OF THE ELEMENTS a, b (adapted from T.B. Coplen et al., Inorg. Chim. Acta 217, 217, 1994) Element Symbol Atomic Atomic Element Symbol Atomic Atomic Number Weight Number Weight Actiniumc 227Ac 89 227.03 Mercury Hg 80 200.59(2) Aluminum Al 13 26.982 Molybdenum Mo 42 95.94 Americiumc Am 95 241.06 Neodynium Nd 60 144.24(3) Antimony Sb 51 121.76 Neon Ne 10 20.180 Argon Ar 18 39.948 Neptuniumc 237Np 93 237.05 Arsenic As 33 74.922 Nickel Ni 28 58.693 Astatinec 210At 85 209.99 Niobium Nb 41 92.906 Barium Ba 56 137.33 Nitrogen N 7 14.007 Berkeliumc 249Bk 97 249.08 Nobeliumc 259No 102 259.10 Beryllium Be 4 9.0122 Osmium Os 76 190.23(3) Bismuth Bi 83 208.98 Oxygen O 8 15.999 Boron B 5 10.811(5) Palladium Pd 46 106.42 Bromine Br 35 79.904 Phosphorus P 15 30.974 Cadmium Cd 48 112.41 Platinum Pt 78 195.08(3) Calcium Ca 20 40.078(4) Plutoniumc 239Pu 94 239.05 Californiumc 232Cf 98 252.08 Poloniumc 210Po 84 209.98 Carbon C 6 12.011 Potassium K 19 39.098 Cerium Ce 58 140.12 Praseodynium Pr 59 140.91 Cesium Cs 55 132.91 Promethiumc 147Pm 61 146.92 Chlorine Cl 17 35.453 Proactiniumc Pa 91 231.04 Chromium Cr 24 51.996 Radiumc 226Ra 88 226.03 Cobalt Co 27 58.933 Radonc 222Rn 86 222.02 Copper Cu 29 63.546(3) Rhenium Re 75 186.21 Curiumc 244Cm 96 244.06 Rhodium Rh 45 102.91 Dysprosiumc Dy 66 162.50(3) Rubidium Rb 37 85.468 Einsteiniumc 252Es 99 252.08 Ruthenium Ru 44 101.07(2) Erbium Er 68 167.26(3) Samarium Sm 62 150.36(3) Europium Eu 63 151.96 Scandium Sc 21 44.956 Fermiumc 257Fm 100 257.1 Selenium Se 34 78.96(3) Fluorine F 9 18.998 Silicon
    [Show full text]
  • Molecular Modeling Experiment
    (004:141) Experiment #11 MOLECULAR MODELING: AROMATIC RESONANCE EFFECTS IN CYCLIC KETONES Goal: In this experim ent, you will us e computational chemistry to probe the effects of aromaticity upon bond strengths and lengths. The vibrational spectral frequencies for a variety of cyclic ketones will b e predicted using m olecular orbital methods, and the trends in bond length and C=O stretching frequency will be com pared to expectations based on resonance theory and aromaticity models. If you wish you may work in groups of two or three, and share the results of your computations. Discuss th e results am ongst yourselves, but each person must write up and submit his or her own report. For the calculations, use the PC SPART AN ‘02 com puter program (Wavefunction Inc.) on PCs in the Chemistry Department (locations will be announced). You will draw and calculate properties of each of the molecules shown below: O O O O O O 1 (004:141) Experiment #11 Background Principles of Resonance Kekulé structural formulas are essential tools for understanding organic chem istry. However, the structures of som e compounds and ions cannot be represented by a single formula. For example, sulfur dioxide (SO2) and nitric acid (HNO3) may each be described by two equivalent formulas (equations 1 & 2). For clarity the two am biguous bonds to oxygen are given different colors in these formulas. If only one formula for sulfur dioxide were correct and accurate, then the double bond to oxygen would be shorter and stronger than the single bond. Since experim ental evidence indicates that this molecule is bent (bond angle 120°) and has equal lengths of sulfur-oxygen bonds (1.432 Å), a single formula is inadequate, and the actual structure resembles an average of the two form ulas.
    [Show full text]
  • Molecular Mechanics Calculations of Dimetal Bonds
    MOLECULAR MECHANICS CALCULATIONS OF DIMETAL BONDS Frangoise Marcelle Mary O'Neill A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. (1 Auqu~~t 1992 ----. ,_' ..Iium:_~ • __ , .;;.... ...... ....__ Abstract Two aspects of the beautiful conceptual str'lcture of M-M multiple bonding remain puzzling; the large variability of the Cr(II)-Cr(II) quadruple bonds within the Cr2 (X"Y)4Lz compounds, and their response to axial ligation. Most theoretical workers emphasize the inductive effect of the bridging ligand X"Y, while most experimental workers view the influence of the axial ligands L, as most important in determining the Cr-Cr distance. The controversy concerning the relative effects has been diffi':::ultto resolve. Previous efforts to understand the bond-length variations in the system have been unfruitful because of added steric distortions on the M-M distance. A method capable of probing the electronic nature of the M-M bond, in the absence of steric complications, is required. The electronic and steric effects can effectively be separated in molecules with dimetal centres using the methods of molecular mechanics. using the M02 system as our model of M-M bonding in these binuclear comp xes belonging to group VIA, unique or characteristic (~, ro) coup Les , where k, is the force (; constant and ro the electronic bond distance, for the Mo-Mo bond of orders 4.0, 3.5, 3.0, 2.0 and 1.0 were generated. Relationships bebveen bond order and either k, or ro are then formulated.
    [Show full text]
  • The C–F Bond As a Conformational Tool in Organic and Biological Chemistry
    The C–F bond as a conformational tool in organic and biological chemistry Luke Hunter Review Open Access Address: Beilstein Journal of Organic Chemistry 2010, 6, No. 38. School of Chemistry, The University of Sydney, NSW 2006, Australia doi:10.3762/bjoc.6.38 Email: Received: 12 January 2010 Luke Hunter - [email protected] Accepted: 15 March 2010 Published: 20 April 2010 Keywords: Guest Editor: D. O’Hagan conformation; functional molecules; organofluorine chemistry; stereochemistry; stereoelectronic effects © 2010 Hunter; licensee Beilstein-Institut. License and terms: see end of document. Abstract Organofluorine compounds are widely used in many different applications, ranging from pharmaceuticals and agrochemicals to advanced materials and polymers. It has been recognised for many years that fluorine substitution can confer useful molecular prop- erties such as enhanced stability and hydrophobicity. Another impact of fluorine substitution is to influence the conformations of organic molecules. The stereoselective introduction of fluorine atoms can therefore be exploited as a conformational tool for the synthesis of shape-controlled functional molecules. This review will begin by describing some general aspects of the C–F bond and the various conformational effects associated with C–F bonds (i.e. dipole–dipole interactions, charge–dipole interactions and hyper- conjugation). Examples of functional molecules that exploit these conformational effects will then be presented, drawing from a diverse range of molecules including pharmaceuticals, organocatalysts, liquid crystals and peptides. Review General aspects of the C–F bond Fluorine is a small atom, with an atomic radius intermediate charges attract each other. Hence, the C–F bond has significant between that of hydrogen and oxygen (Table 1).
    [Show full text]