Showing papers on "Chemical bond published in 1998"

Characterization of a Dihydrogen Bond on the Basis of the Electron Density

Abstract: A new type of hydrogen bond, called a dihydrogen bond, has recently been introduced. In this bond a hydrogen is donated to another (hydridic) hydrogen. We apply a set of criteria developed in the context of the theory of “atoms in molecules” that were previously successfully used to study conventional hydrogen bonds. This method enables one to characterize the dihydrogen bond on the basis of the electron density only. We investigated a dimer structure of BH3NH3 at the ab initio level which contains two dihydrogen bonds that differ in strength. The combination of a theoretical density with our hydrogen-bonding criteria turns out to be a valuable new and independent source of information complementary to techniques such as NMR, IR, and structural crystallography.

Microwave spectroscopy of a quantum-dot molecule

Abstract: Quantum dots are small conductive regions in a semiconductor, containing a variable number of electrons (from one to a thousand) that occupy well-defined, discrete quantum states—for which reason they are often referred to as artificial atoms1. Connecting them to current and voltage contacts allows the discrete energy spectra to be probed by charge-transport measurements. Two quantum dots can be connected to form an ‘artificial molecule’. Depending on the strength of the inter-dot coupling (which supports quantum-mechanical tunnelling of electrons between the dots), the two dots can form ‘ionic’ (26) or ‘covalent’ bonds. In the former case, the electrons are localized on individual dots, while in the latter, the electrons are delocalized over both dots. The covalent binding leads to bonding and antibonding states, whose energy difference is proportional to the degree of tunnelling. Here we report a transition from ionic bonding to covalent bonding in a quantum-dot ‘artificial molecule’ that is probed by microwave excitations5,6,7,8. Our results demonstrate controllable quantum coherence in single-electron devices, an essential requirement for practical applications of quantum-dot circuitry.

Natural resonance theory: II. Natural bond order and valency

Abstract: Resonance weights derived from the Natural Resonance Theory .NRT , introduced in the preceding paper are used to calculate ''natural bond order,'' ''natural atomic valency,'' and other atomic and bond indices reflecting the resonance composition of the wave function. These indices are found to give significantly better agreement with observed properties empirical valency, . bond lengths than do corresponding MO-based indices. A characteristic feature of the NRT treatment is the description of bond polarity by a ''bond ionicity'' . index resonance-averaged NBO polarization ratio , which replaces the ''covalent-ionic resonance'' of Pauling-Wheland theory and explicity exhibits the complementary relationship of covalency and electrovalency that underlies empirical assignments of atomic valency. We present ab initio NRT applications . to prototype saturated and unsaturated molecules methylamine, butadiene , . polar compounds fluoromethanes , and open-shell species: hydroxymethyl . radical to demonstrate the numerical stability, convergence, and chemical reasonableness of the NRT bond indices in comparison to other measures of valency and bond order in current usage. Q 1998 John Wiley & Sons, Inc. J Comput Chem 19: 610)627, 1998

Interface Property and Apparent Strength of High-Strength Hydrophilic Fiber in Cement Matrix

Abstract: This study addresses the characterization of fiber-matrix interfacial properties and the apparent strength of high-strength hydrophilic fibers. Single-fiber pullout bond tests and single-fiber pull-to-rupture strength tests were conducted by employing polyvinyl alcohol (PVA) fibers. The pullout bond tests showed that these fibers have surprisingly high chemical and frictional bond strengths. The chemical bond strength was relatively stable independent of a water-to-cement ratio of matrix and the fiber type tested, contrary to the friction bond strength. The pull-to-rupture strength tests revealed that the apparent strength of the PVA fibers in cementitious composites is considerably lower than that in standard fiber strength tests. The apparent strength was further reduced with inclining angle of fiber alignment. This effect was captured by a simple phenomenological model in this study, which introduces the apparent strength reduction factor. The combined effects of high bond strength and degraded fiber strength will likely contribute to composite performance less than would be expected from a high-performance fiber.

Experimental Electron Density in a Transition Metal Dimer: Metal−Metal and Metal−Ligand Bonds

Abstract: The accurate experimental electron density distribution of Co2(CO)6(AsPh3)2 has been determined through X-ray diffraction at T = 123 K. Metal−metal and metal−ligand bonds have been investigated by means of deformation densities and the quantum theory of atoms in molecules. The “expected” lack of charge accumulation in the deformation density map is “contradicted” by the presence of a bond critical point and a bond path line linking the two Cobalt atoms, in agreement with theoretical predictions on similar compounds. A careful analysis of the properties of ρ(r) at the bond critical points and of the Laplacian distribution along the bond paths has allowed the full characterization of all bonds in the title compound and, in particular, to discard the apparently straightforward classification of Co−Co as a closed-shell interaction. The radial shape of the atomic Laplacian makes (covalent or polar) shared interactions similar to donor−acceptor ones when at least one “heavy atom” is concerned. Thus, even if it ...

SERS spectra of poly(3‐hexylthiophene) in oxidized and unoxidized states

Abstract: Surface enhanced Raman scattering (SERS) is being increasingly used for the study of the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin, arising from the excitation of surface plasmons in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, for which available experimental data are scarce. The chemical process originates from the increase in the polarizability of the molecules at the metal surface under the action of the incident radiation, which leads to the formation of new chemical bonds with the atoms of the metal support. The present work was devoted to the study of the SERS spectra of poly(3-hexylthiophene) deposited on rough Ag and Au supports by evaporating the solvent from a solution of known concentration. The experiments revealed the existence of a chemical surface effect. The results obtained show that the SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds of the type MeX (Me=Ag or Au, X=Cl or O). The SERS measurements reported here reveal an increase in the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. It was observed for the first time by SERS spectroscopy that the doping of 3-PHT with FeCl3 leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. © 1998 John Wiley & Sons, Ltd.

Temperature and pressure dependence of hydrogen bond strength: A perturbation molecular orbital approach

Abstract: Lengths and strengths of hydrogen bonds are exquisitely sensitive to temperature and pressure. Temperature and pressure sensitivity is the result of the fact that hydrogen bonds are so weak that the internal energy of the bond is important to bond strength, and the equilibrium bond distance is controlled by a combination of thermodynamics and quantum mechanics, rather than quantum mechanics alone. The importance of thermodynamics in the bond length, and strength, of hydrogen bonds is the result of a breakdown in the Born-Oppenheimer approximation that occurs when the energy of the first vibrational excitation of a bond is of the order of kT. Variation of water–water hydrogen bond length and strength with temperature and pressure is discussed in light of the data for the specific volume of ice Ih, the enthalpy of vaporization of liquid water, and the internal energy of the liquid. In most chemical contexts, correction of covalent bond strength for internal energy is not necessary. For hydrogen bonds this i...

Lengthening of the covalent x-h bond in heteronuclear hydrogen bonds quantified from organic and organometallic neutron crystal structures

Abstract: The lengthening of the covalent X−H bond in heteronuclear hydrogen bonds is quantified from neutron crystal structures for combinations of the donors N−H and O−H with the acceptors O, N, S, and Cl-. For N−H···O hydrogen bonds, only high-precision low-temperature crystal structures are used. For the other hydrogen bond types, there are too few neutron crystal structures available to allow restriction to low-temperature data. For all donor/acceptor combinations studied, systematic lengthening of the X−H bond due to the hydrogen bond interaction is definitely confirmed, and this includes the relatively weak sulfur acceptors. The experimental data are discussed in the frame of the valence model of the hydrogen bond, and on this basis, the geometries of a number of hydrogen bonds are predicted for which only X-ray crystal structures have been published.

Chemical Bonding in Hypervalent Molecules Revised. Application of the Atoms in Molecules Theory to Y3 X and Y3 XZ (Y = H or CH3; X = N, P or As; Z = O or S) Compounds

Abstract: Atoms in Molecules Theory has been applied to analyze bonding properties, in potentially hypervalent pnicogen (N, P or As)−chalcogen(O or S) bonds within the framework of three plausible models: (i) one σ bond and two π back-bonds (negative hyperconjugation), (ii) one σ bond and three π back-bonds, and (iii) three Ω (banana) bonds The topological analyses (based upon the electron charge density (ρ(r)), its Laplacian (∇2ρ(r)), bond ellipticity, etc) and the charges were consistent with a highly polarized σ bond, with bond strength dependent on the electrostatic interactions The equilibrium geometries were optimized by both density functional theory with a hybrid functional (B3LYP) and by ab initio methods at the MP2(full) level, using the 6-311G basis set augmented by polarization and/or diffuse functions

Surface-tip interactions in noncontact atomic-force microscopy on reactive surfaces: Si(111)

Abstract: Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic-force microscopy on Si(111) surfaces At the distance of closest approach between the tip and the surface, there is an onset of covalent chemical bonding between the dangling bonds of the tip and the surface Displacement curves and lateral scans on the surface show that this interaction energy and force are comparable to the macroscopic Van der Waals interaction However, the covalent interaction completely dominates the force gradients probed in the experiments Hence, this covalent interaction is responsible for the atomic resolution obtained on reactive surfaces and it should play a role in improving the resolution in other systems Our results provide a clear understanding of a number of issues such as (i) the experimental difficulty in achieving stable operation, (ii) the quality of the images obtained in different experiments and the role of tip preparations and (iii) recently observed discontinuities in the force gradient curves

Bond Length–Electron Density Relationships: From Covalent Bonds to Hydrogen Bond Interactions

Abstract: It is possible to treat bond distances of covalent C-H bonds and C⋯H hydrogen bonds simultaneously assuming a logarithmic relationship with the electron density at the bond critical point. Similar relationships have been found for other X-H/X⋯H bonds. The data used for obtaining these equations have been determined theoretically. All the systems have been fully optimized and their electron densities calculated at the B3LYP/6-311 + + G(d,p) level.

Thermal and corrosion behavior of P2O5-Na2O-CuO glasses

Abstract: Lead free sodium-copper-phosphate glasses in the range (mol%) 30-80P2O5, 0-70Na2O and 0-70CuO were investigated. The glass formation region is P2O5≥40, CuO≤50 and Na2O≤60 mol%. Glasses with a glass transition temperature (Tg) below 420°C, a thermal expansion coefficient from 258 × 10−7 to 99 × 10−7/°C and a dissolution rate in 30°C deionized water at the order of 10−7 g/cm2 min can be prepared. Substitution of CuO for Na2O increases Tg, the softening temperature (Td), hardness and chemical durability and decreases the thermal expansion coefficient. The structure of 50P2O5-(50 - x)Na2O-xCuO glass was examined by XPS and FTIR spectroscopies, which reveal the formation of POCu bonds in these glasses. The formation of POCu bonds, which replace PωO− ⋯ Na+ bonds while keeping the same fraction of POP bonds, increases the crosslink density in the glass network and, therefore, enhances the chemical durability and increases Tg, Td and the hardness of the glasses.

Theoretical Study of Stable Trans and Cis Isomers in [UO2(OH)4]2- Using Relativistic Density Functional Theory

Abstract: The title compound, uranyl(VI) tetrahydroxide [UO{sub 2}(OH){sub 4}]{sup 2{minus}}, has been studied in detail using density functional theory (DFT) in the first systematic theoretical study of the compound Scalar relativistic effects are included approximately by replacing the uranium core with a relativistic effective core potential A total of nine stable structures have been characterized Four of them (I-IV) possess the usual linear uranyl bond, and rapid exchange between these conformations is expected at finite temperatures The uranyl and U-OH bond lengths of the minimum energy structure, I, are calculated as 1842 and 2334 {angstrom}, respectively This compares well with the experimental crystal structure values of 1824(3) {angstrom} and 2258(3) {angstrom}, respectively The existence of stable structures with a bent uranyl bond (cis-uranyl) is predicted for the first time (structures V-IX) These conformers are only 18--19 kcal/mol higher in energy than the global energy minimum, and their uranyl bond angles cover a range of 113--132{degree} Harmonic vibrational frequencies for all stable conformers, I-IX, were calculated They are compared to experiment where possible A mechanism is suggested for the nonaqueous intramolecular oxygen ligand exchange in [UO{sub 2}(OH){sub 4}]{sup 2{minus}} between uranyl and hydroxide involving a cis-uranyl structure as a stable intermediate inmore » a two-step process with a calculated activation energy of 38 kcal/mol« less

HXeSH, the First Example of a Xenon-Sulfur Bond

Abstract: Quite recently, we have reported a number of novel rare-gas compounds of type HXY (X ) Xe or Kr, and Y is a fragment with a large electron affinity). The compounds obtained so far include HXeH, HXeCl, HXeBr, HXeI, HXeCN, HXeNC, HKrCl, and HKrCN.1-4 They are prepared in rare-gas matrices at low temperatures (<50 K) by photolyzing a suitable precursor HY, which yields isolated hydrogen atoms and Y-fragments. After photolysis, thermal mobilization of hydrogen atoms leads to their reactions with the rare-gas atoms surrounding the Y-fragments. The resulting novel HXY rare-gas compounds are detected by IR spectroscopy mainly via extremely intense X-H stretching absorptions. In this report we introduce a new member of this family, HXeSH. This molecule expands the number of elements capable of making chemical bonds with xenon and represents the first compound with a xenon-sulfur bond. The electronic structure calculations for HXeSH were carried out with the Gaussian 94 package of computer codes.5 The relativistic effective core potential (ECP) by LaJohn et al. was used on Xe.6 This ECP includes the d-subshell in the valence space resulting in 18 valence electrons and was used in a decontracted form. The standard split-valence 6-311++G(2d,2p) basis set was used for hydrogen and sulfur. The calculated structural parameters of HXeSH at different computational levels are given in Table 1. Like H2S, HXeSH is also a bent molecule with an H-S-Xe angle near 90 degrees. The calculated S-H bond distance of 1.34 A in HXeSH is essentially the same as the experimental value in H2S. The Xe-H distance (1.84 A at the CCSD(T)-level) is somewhat larger than the value of free XeH+ (1.60 A)8 but much smaller than the van der Waals distance of neutral Xe-H (3.95 A).9 The calculated length of the Xe-H bond compares well with XeH2 and HXeI molecules, which both have experimental Xe-H stretching absorptions in the 1200 cm-1 region of the spectrum.1,2,10 The calculated Xe-S bond distance of 2.77 A can be compared with the recently calculated values for the excited states of Xe-S. For the 11Σ+ state (Xe+ S(1D)) and the lowest chargetransfer (Xe+S-) state 23Π, 2.62 and 3.32 A were obtained for the Re-values, respectively.11 Also, our Xe-S bond distance is near the I-S bond distance of 2.67 A for the linear S-I-S unit in [(S7I)2I)]. The calculated charge distribution indicates significant ion-pair character as in all the other HXY molecules observed thus far.1-4 Xenon carries a relatively large positive charge, and the SH group is negatively charged.

π‐Electron “Donor–Acceptor” Complexes B⋅⋅⋅ClF and the Existence of the “Chlorine Bond”

Abstract: Does the “chlorine” bond exist? Comparison of the properties of several π-electron-donor–acceptor complexes B⋅⋅⋅ClF and B⋅⋅⋅HCl (right and far right) reveals a parallelism that suggests that a “chlorine” bond, the analogue of a hydrogen bond, exists in B⋅⋅⋅ClF complexes.

Electronic structure of FeS 2 : The crucial role of electron-lattice interaction

Abstract: Using the results of fully self-consistent all-electron first-principles calculations for semiconducting iron pyrite we discuss the major factors governing the semiconducting properties as well as the chemical bonding of this material. The calculations are based on density functional theory within the local density approximation and employ the augmented spherical wave method in its scalar-relativistic implementation. The electronic properties are dominated by strongly hybridized Fe $3d$ and S $3p$ states. The chemical bonding is analyzed using an ab initio implementation of the crystal orbital overlap population. Chemical stability is shown to result mainly from the Fe-S bonding. While the upper part of the valence band is formed mainly from Fe ${3dt}_{2g}$-derived states the conduction band comprises the ${e}_{g}$-derived levels. The conduction band minimum, in contrast, is exclusively due to S $3p$ states, this fact explaining the observed high optical absorption. For the same reason the optical properties are strongly influenced by the short sulfur-sulfur bonds. We demonstrate that only small deviations in the sulfur pair bond lengths involve rather drastic changes of the near-gap electronic states which might even turn the indirect band gap into a direct one. These findings allow us to understand the rather high sensitivity of the optical band gap to the incorporation of defects. Finally, our results open perspectives for photovoltaic applications of ${\mathrm{FeS}}_{2}$.

On the electronic nature of low-barrier hydrogen bonds in enzymatic reactions

Abstract: The electronic nature of low-barrier hydrogen bonds (LBHBs) in enzymatic reactions is discussed based on combined low temperature neutron and x-ray diffraction experiments and on high level ab initio calculations by using the model substrate benzoylacetone. This molecule has a LBHB, as the intramolecular hydrogen bond is described by a double-well potential with a small barrier for hydrogen transfer. From an “atoms in molecules” analysis of the electron density, it is found that the hydrogen atom is stabilized by covalent bonds to both oxygens. Large atomic partial charges on the hydrogen-bonded atoms are found experimentally and theoretically. Therefore, the hydrogen bond gains stabilization from both covalency and from the normal electrostatic interactions found for long, weak hydrogen bonds. Based on comparisons with other systems having short-strong hydrogen bonds or LBHBs, it is proposed that all short-strong and LBHB systems possess similar electronic features of the hydrogen-bonded region, namely polar covalent bonds between the hydrogen atom and both heteroatoms in question.

Valence bond concepts applied to the molecular mechanics description of molecular shapes. 3. applications to transition metal alkyls and hydrides

Abstract: Recently we reported a qualitative, valence bond derived model for describing the shapes of transition metal complexes, with a focus on metal hydrides and alkyls. This model, based on the concepts of hybridization and resonance, rationalizes the unusual and varied shapes of hydride and alkyl complexes with transition metals. This paper demonstrates the quantitative incorporation of these valence bond concepts into molecular mechanics algorithms. The resulting force field method (HV-VB) accurately describes the structures of alkyls and hydride complexes of the transition metals. For a wide variety of crystallographically characterized molecules, the HV-VB computations faithfully reproduce the observed structures.

Isolation of an intrinsic precursor to molecular chemisorption

Abstract: Over the past 70 years, numerous gas-surface adsorption studies have indicated the existence of a weakly bound, mobile intermediate that is a precursor to chemical bond formation. The direct observation and characterization of such a species are presented. Precursor and chemisorbed benzene on a silicon surface were clearly distinguished with the use of a tunable-temperature scanning tunneling microscope. Precursor decay to chemisorption was observed, allowing the salient features of the potential energy surface to be determined.

Ultra precision and reliable bonding method

Abstract: The bonding of two materials through hydroxide-catalyzed hydration/dehydration is achieved at room temperature by applying hydroxide ions to at least one of the two bonding surfaces and by placing the surfaces sufficiently close to each other to form a chemical bond between them. The surfaces may be placed sufficiently close to each other by simply placing one surface on top of the other. A silicate material may also be used as a filling material to help fill gaps between the surfaces caused by surface figure mismatches. A powder of a silica-based or silica-containing material may also be used as an additional filling material. The hydroxide-catalyzed bonding method forms bonds which are not only as precise and transparent as optical contact bonds, but also as strong and reliable as high-temperature frit bonds. The hydroxide-catalyzed bonding method is also simple and inexpensive.

Nature of the three-electron bond in H2S∴SH2+

Abstract: We have investigated the model system H2S∴SH2+, i.e., the sulfur−sulfur bound dimer radical cation of H2S, using both density functional theory (LDA, BP86, PW91) and traditional ab initio theory (up to CCSD(T)). Our purpose is to better understand the nature of the three-electron bond. The S−S bond length is 2.886 A and the bond enthalpy (for 298.15 K) amounts to −40.7 kcal/mol at the BP86/TZ2P level. The best ab initio estimates for the S−S bond strength (our CCSD(T)/6-311++G(2df,2pd)//MP2(full) and literature values) are some 10 kcal/mol weaker than those from nonlocal DFT. It is shown, using an energy decomposition scheme for open-shell systems, that the sulfur−sulfur bond (ΔE = ΔE2c-3e + ΔEelst) is nearly 60% provided by the three-electron bond (ΔE2c-3e) between the unpaired sulfur 3px electron on H2S+• and the sulfur 3px lone pair on H2S; electrostatic attraction (ΔEelst) is important, too, with a contribution of somewhat more than 40%. We show furthermore that the three-electron bond (ΔE2c-3e = ΔE2c...

Semiempirical study on the valences of Cu and bond covalency in Y1-xCaxBa2Cu3O6+y

Abstract: The valences of Cu and bond covalencies in Y1-xCaxBa2Cu3O6+y, have been investigated using complex chemical bond theory, This theory is the generalization of Phillips, Van Vechten, Levine, and Tanaka's scheme. The results indicate that the valences of Cu(1) and Cu(2) in our calculation agree well with those obtained by the bond valence sum method. The valences of Cu(1) and Cu(2) in our calculation also suggest that the holes introduced by Ca substitution only reside in CuO2 planes and there is a competing mechanism for the hole density in CuO2 planes between,Ca doping and oxygen depletion. These conclusions are in satisfactory agreement with experiments. The calculated ordering of covalencies is Cu(1)-O(4)>Cu(1)-O(1)>Cu(2)-O(2,3)>Cu(2)-O(1)>Ca-O>Y-O similar to Ba-O, regardless of the Ca doping level and oxygen content. [S0163-1829(98)03325-6].

Power law relationships between bond length, bond strength and electron density distributions

Abstract: The strength of a bond, defined as p=s/r, where s is the Pauling bond strength and r is the row number of an M cation bonded to an oxide anion, is related to a build-up of electron density along the MO bonds in a relatively large number of oxide and hydroxyacid molecules, three oxide minerals and three molecular crystals. As p increases, the value of the electron density is observed to increase at the bond critical points with the lengths of the bonds shortening and the electronegativities of the M cations bonded to the oxide anion increasing. The assertion that the covalency of a bond is intrinsically connected to its bond strength is supported by the electron density distribution and its bond critical point properties. A connection also exists between the properties of the electron density distributions and the connectivity of the bond strength network formed by the bonded atoms of a structure.

Theoretical Investigation of the Relationship between Proton NMR Chemical Shift and Hydrogen Bond Strength.

Abstract: Hartree−Fock, Moller−Plesset, and DFT (BLYP, B3LYP) calculations have been carried out using the 6-31+G(d,p) basis set to study the relationship between calculated 1H NMR chemical shifts and calculated hydrogen bond strengths in several model low-barrier hydrogen bond complexes. For both the formic acid-substituted formate anion and enol-substituted enolate anion model systems, we find an excellent linear correlation between calculated hydrogen bond strength and predicted 1H NMR chemical shift, with an average slope of 1.5 kcal/mol per ppm chemical shift.

Electron pairing and chemical bonds. Molecular structure from the analysis of pair densities and related quantities

Abstract: The Fermi holes are presented as a new means of analysis and visualisation of molecular structure. Based on these quantities it is possible to get clear and highly visual insight into the structure of molecular fragments (functional groups) even in molecules with complex bonding patterns like multicenter bonding, hypervalence, etc. In addition to allowing the detection and localization of multicenter bonding, the new approach also brings some new interesting possibilities for the quantitative evaluation of molecular similarity.

Electronic structure of BN, BP and BAs

Abstract: The empirical tight-binding method is used to investigate the electronic structure of the zinc-blende boron compounds BN, BP and BAs. Results are given for band structures, ionicity factors and elastic constants. The electronic structure of the boron compounds exhibits features that differ from those of the other III–V materials. In particular we found that these compounds are characterized by a strong cation-anion s–s repulsion effect. The calculated ionicity shows a weak charge transfer effect for BP and BAs which makes these compounds as the prototype covalent materials of the III–V family, while BN is found more ionic.

A valence bond perspective on the molecular shapes of simple metal alkyls and hydrides

Abstract: Many chemists use qualitative valence bond concepts to rationalize molecular structures and properties, particularly for main group elements. Extension of Pauling's valence bond concepts to transition metal compounds dominated by covalent bonding leads to simple prescriptions for determining bond hybridizations and molecular shapes. As a result, transition metal structures can be discussed in the familiar terminology of Lewis structures, lone pairs, hybrid orbitals, hypervalence, and resonance. A primary feature of these prescriptions is the relative impotence of valence p-orbitals in the formation of covalent bonds at transition metals: sdn hybridization dominates. This feature is consistent with detailed analyses of high level quantum mechanical computations. Unlike Pauling's original treatments of hypervalency, rationalization of empirical structures and high level electronic structure computational results requires consideration of multiple resonance structures. Valence bond theory constitutes a comp...