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Richard A. Andersen - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and Reactions of [Cp*Yb-2](2)(mu-Me) and [Cp*Yb-2](2)(mu-Me)(Me) and Related Yb-2(II, III) and Yb-2(III, III) Compounds
Organometallics, 2017Co-Authors: Marc D. Walter, Laurent Maron, Philip T. Matsunaga, Carol J. Burns, Richard A. AndersenAbstract:A new type of synthesis, referred to as oxidative methylation, is developed for [Cp*Yb-2](2)(mu-X) and [Cp*Yb-2](2)(mu-X)(X), where X = Me, using MeCu or Cp*2VMe as the methyl transfer reagent and Cp*Yb-2. The synthetic methodology is extended to other X derivatives such as the halides and BH4. Reaction of [Cp*Yb-2](2)(mu-Me)(Me) and H-2 yields the mixed-valent hydride [Cp*Yb-2](2)(mu-H), which eliminates H-2 on gentle heating, forming Cp*Yb-2. When Cp*2VX is replaced by Cp*2TiX, 1:1 adducts based upon Ti(III,d(1)) are isolated. The X-ray crystal Structure of [Cp*Yb-2](mu-Me)[TiCp*(2)] shows that the methyl group bridges the two different decamethylmetallocene fragments in a near-linear fashion, a geometry that is likely to resemble the transition state of the single-electron-transfer precursor complex. A CASSCF computational study on the mixed-valent hydride [Cp*Yb-2](2)(mu-H) shows that the ground state is a spin doublet in which the hydride forms a symmetric bridge to both Yb atoms. The three spins forming the ground-state doublet are aligned as Yb(f(13)(alpha),(d(z2))(0))center dot center dot center dot H center dot center dot center dot Yb(f(13)(alpha),(d(z2))(1)(beta)), and the unpaired d electron is delocalized between the d(z2) orbitals of the two Yb centers via the hydride bridge using the sigma* orbital of the Yb(d(z2))-H bond. The first excited state lies 0.09 eV (725 cm(-1)) higher in energy and is a spin quartet in which the three spins are aligned as Yb(f(13)(alpha),(d(z2))(0))center dot center dot center dot H center dot center dot center dot Yb(f(13)(alpha),(d(z2))(1)(alpha)), also giving rise to delocalization of the d electron between the d(z2) orbitals of the two Yb centers. The second spin doublet resembles the Lewis Structure with an asymmetric mu-H bridge in which the Yb(II) metallocene has a closed-shell electronic configuration and is approximately 0.15 eV (1210 cm(-1)) higher in energy than the ground-state delocalized open-shell doublet. The electronic Structure of the mixed-valent methyl is closely related to that of the hydride, but the methyl group is localized on the Cp*Yb-2(III) fragment. Electronic energies (Delta E) computed at the DFT (B3PW91) level of theory provide insights into the thermochemistry of the formation and decomposition of [Cp*Yb-2](2)(mu-H). The BDE for Yb-H is ca. 15 kcal/mol stronger than that for the corresponding Yb-Me in the monomeric metallocenes. In contrast, formation of [Cp*Yb-2](2)(mu-CH3) is ca. 60 kcal/mol more exothermic than the formation of [Cp*Yb-2](2)(mu-H). This difference is ascribed to enhanced intramolecular steric repulsion between the Cp*Yb-2 moieties in the linear Yb-H-Yb unit.
Laurent Maron - One of the best experts on this subject based on the ideXlab platform.
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Understanding the Multiconfigurational Ground and Excited States in Lanthanide Tetrakis Bipyridine Complexes from Experimental and CASSCF Computational Studies
Inorganic Chemistry, 2019Co-Authors: Robert Halbach, Grégory Nocton, Jorge Amaro-estrada, Laurent Maron, Corwin Booth, Richard AndersenAbstract:An alternative synthesis for M(κ2-bipy)4 (M = La, Ce) and [Li(thf)4][M(κ2-bipy)4] (M = Tb, Dy) and the crystal Structures for M = La, Ce, and Tb are described. The isomorphous and isostructural neutral molecules, M = La and Ce, are polymeric in the solid-state, as are those of M = Sm and Eu, which were reported in earlier work. The polymeric network is built from eight coordinate units whose geometry in all four cases is that of a square prism. The known molecules, M = Yb and Lu, are also polymeric, but the eight coordinate units have dodecahedral geometries. The Structure of the anions in the separated ion pair, [Li(thf)4][M(κ2-bipy)4], in which Tb is reported in this work and Lu is known, are monomeric with geometries that are between that of a square antiprism and a dodecahdron. The electronic Structure, from CASSCF multireference quantum mechanical calculations, shows that the electronic ground states for M = La and Lu are multiconfigurational spin doublets and those for the M = Ce and Yb are multiconfigurational spin triplets. This is confirmed by magnetic susceptibility studies as a function of temperature that are consistent with the metals (La, Ce, Sm, Tb, Dy, Yb, and Lu) being trivalent, as are the LIII-edge XANES spectra (Ce, Yb), and divalent for Eu. The multiconfigurational nature of the ground states, developed from CASSCF molecular orbital calculations, renders a single Lewis Structure and a single reference molecular orbital representation misleading. The results from the multireference calculations are extended to the other lanthanide molecules and are the genesis of a new model for understanding the magnetic properties of these molecules.
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Synthesis and Reactions of [Cp*Yb-2](2)(mu-Me) and [Cp*Yb-2](2)(mu-Me)(Me) and Related Yb-2(II, III) and Yb-2(III, III) Compounds
Organometallics, 2017Co-Authors: Marc D. Walter, Laurent Maron, Philip T. Matsunaga, Carol J. Burns, Richard A. AndersenAbstract:A new type of synthesis, referred to as oxidative methylation, is developed for [Cp*Yb-2](2)(mu-X) and [Cp*Yb-2](2)(mu-X)(X), where X = Me, using MeCu or Cp*2VMe as the methyl transfer reagent and Cp*Yb-2. The synthetic methodology is extended to other X derivatives such as the halides and BH4. Reaction of [Cp*Yb-2](2)(mu-Me)(Me) and H-2 yields the mixed-valent hydride [Cp*Yb-2](2)(mu-H), which eliminates H-2 on gentle heating, forming Cp*Yb-2. When Cp*2VX is replaced by Cp*2TiX, 1:1 adducts based upon Ti(III,d(1)) are isolated. The X-ray crystal Structure of [Cp*Yb-2](mu-Me)[TiCp*(2)] shows that the methyl group bridges the two different decamethylmetallocene fragments in a near-linear fashion, a geometry that is likely to resemble the transition state of the single-electron-transfer precursor complex. A CASSCF computational study on the mixed-valent hydride [Cp*Yb-2](2)(mu-H) shows that the ground state is a spin doublet in which the hydride forms a symmetric bridge to both Yb atoms. The three spins forming the ground-state doublet are aligned as Yb(f(13)(alpha),(d(z2))(0))center dot center dot center dot H center dot center dot center dot Yb(f(13)(alpha),(d(z2))(1)(beta)), and the unpaired d electron is delocalized between the d(z2) orbitals of the two Yb centers via the hydride bridge using the sigma* orbital of the Yb(d(z2))-H bond. The first excited state lies 0.09 eV (725 cm(-1)) higher in energy and is a spin quartet in which the three spins are aligned as Yb(f(13)(alpha),(d(z2))(0))center dot center dot center dot H center dot center dot center dot Yb(f(13)(alpha),(d(z2))(1)(alpha)), also giving rise to delocalization of the d electron between the d(z2) orbitals of the two Yb centers. The second spin doublet resembles the Lewis Structure with an asymmetric mu-H bridge in which the Yb(II) metallocene has a closed-shell electronic configuration and is approximately 0.15 eV (1210 cm(-1)) higher in energy than the ground-state delocalized open-shell doublet. The electronic Structure of the mixed-valent methyl is closely related to that of the hydride, but the methyl group is localized on the Cp*Yb-2(III) fragment. Electronic energies (Delta E) computed at the DFT (B3PW91) level of theory provide insights into the thermochemistry of the formation and decomposition of [Cp*Yb-2](2)(mu-H). The BDE for Yb-H is ca. 15 kcal/mol stronger than that for the corresponding Yb-Me in the monomeric metallocenes. In contrast, formation of [Cp*Yb-2](2)(mu-CH3) is ca. 60 kcal/mol more exothermic than the formation of [Cp*Yb-2](2)(mu-H). This difference is ascribed to enhanced intramolecular steric repulsion between the Cp*Yb-2 moieties in the linear Yb-H-Yb unit.
Marc D. Walter - One of the best experts on this subject based on the ideXlab platform.
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Synthesis and Reactions of [Cp*Yb-2](2)(mu-Me) and [Cp*Yb-2](2)(mu-Me)(Me) and Related Yb-2(II, III) and Yb-2(III, III) Compounds
Organometallics, 2017Co-Authors: Marc D. Walter, Laurent Maron, Philip T. Matsunaga, Carol J. Burns, Richard A. AndersenAbstract:A new type of synthesis, referred to as oxidative methylation, is developed for [Cp*Yb-2](2)(mu-X) and [Cp*Yb-2](2)(mu-X)(X), where X = Me, using MeCu or Cp*2VMe as the methyl transfer reagent and Cp*Yb-2. The synthetic methodology is extended to other X derivatives such as the halides and BH4. Reaction of [Cp*Yb-2](2)(mu-Me)(Me) and H-2 yields the mixed-valent hydride [Cp*Yb-2](2)(mu-H), which eliminates H-2 on gentle heating, forming Cp*Yb-2. When Cp*2VX is replaced by Cp*2TiX, 1:1 adducts based upon Ti(III,d(1)) are isolated. The X-ray crystal Structure of [Cp*Yb-2](mu-Me)[TiCp*(2)] shows that the methyl group bridges the two different decamethylmetallocene fragments in a near-linear fashion, a geometry that is likely to resemble the transition state of the single-electron-transfer precursor complex. A CASSCF computational study on the mixed-valent hydride [Cp*Yb-2](2)(mu-H) shows that the ground state is a spin doublet in which the hydride forms a symmetric bridge to both Yb atoms. The three spins forming the ground-state doublet are aligned as Yb(f(13)(alpha),(d(z2))(0))center dot center dot center dot H center dot center dot center dot Yb(f(13)(alpha),(d(z2))(1)(beta)), and the unpaired d electron is delocalized between the d(z2) orbitals of the two Yb centers via the hydride bridge using the sigma* orbital of the Yb(d(z2))-H bond. The first excited state lies 0.09 eV (725 cm(-1)) higher in energy and is a spin quartet in which the three spins are aligned as Yb(f(13)(alpha),(d(z2))(0))center dot center dot center dot H center dot center dot center dot Yb(f(13)(alpha),(d(z2))(1)(alpha)), also giving rise to delocalization of the d electron between the d(z2) orbitals of the two Yb centers. The second spin doublet resembles the Lewis Structure with an asymmetric mu-H bridge in which the Yb(II) metallocene has a closed-shell electronic configuration and is approximately 0.15 eV (1210 cm(-1)) higher in energy than the ground-state delocalized open-shell doublet. The electronic Structure of the mixed-valent methyl is closely related to that of the hydride, but the methyl group is localized on the Cp*Yb-2(III) fragment. Electronic energies (Delta E) computed at the DFT (B3PW91) level of theory provide insights into the thermochemistry of the formation and decomposition of [Cp*Yb-2](2)(mu-H). The BDE for Yb-H is ca. 15 kcal/mol stronger than that for the corresponding Yb-Me in the monomeric metallocenes. In contrast, formation of [Cp*Yb-2](2)(mu-CH3) is ca. 60 kcal/mol more exothermic than the formation of [Cp*Yb-2](2)(mu-H). This difference is ascribed to enhanced intramolecular steric repulsion between the Cp*Yb-2 moieties in the linear Yb-H-Yb unit.
Tse-chiang Chang - One of the best experts on this subject based on the ideXlab platform.
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Photoelectron spectra, penning ionization electron spectra, and character of canonical molecular orbitals
Journal of Computational Chemistry, 1998Co-Authors: Erh-hao Chen, Tse-chiang ChangAbstract:When canonical molecular orbitals are expanded in terms of a set of localized molecular orbital building blocks, called bond orbitals, the character of the canonical molecular orbitals can be characterized according to the component bond orbitals resembling the core, lone pair, and localized bond building blocks in an intuitive Lewis Structure. Weinhold's natural bond orbital method can produce a unique Lewis Structure with total occupancy of its occupied bond orbitals exceeding 99.9% of the total electron density for simple molecules. Two useful indices, Lewis bond order and weight of lone pair orbitals, can be defined according to the weights of the bonding and lone pair components of this unique Lewis Structure. Calculation results for molecules N2, CO, CS, NO, HCN, C2H2, H2O, and H2S show that the former index can account for the vibrational Structures of photoelectron spectroscopy, whereas the latter index can account for the band intensity enhancement of Penning ionization electron spectroscopy. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 882–892, 1998
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ORBITAL INTERACTION AND THE PHOTOELECTRON SPECTROSCOPY OF N2, CO, HCN AND C2H2
Journal of Molecular Structure: THEOCHEM, 1998Co-Authors: Erh-hao Chen, Tse-chiang ChangAbstract:Abstract The bond orbitals, associated with the Lewis Structure, can be used as the basis set of canonical molecular orbitals. The concept of linear combination of bond orbitals for canonical molecular orbitals provides a link between two seemingly rival theories: valence bond theory and molecular orbital theory. The bond orbital basis set can reveal the character of the canonical molecular orbitals and the orbital delocalization due to intramolecular interaction. An index, called Lewis bond order, defined as the weight of bonding type bond orbitals, can be used to account for the bonding nature of the canonical molecular orbitals and thus also account for the vibrational Structures of photoelectron spectroscopy. Each canonical molecular orbital of the molecules HCN and C 2 H 2 may have several Lewis bond orders corresponding to several bonds in the Lewis Structure.
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Lewis bond order for some controversial canonical molecular orbitals
Journal of Molecular Structure-theochem, 1997Co-Authors: Tse-chiang ChangAbstract:Abstract The Lewis Structure, consisting of core, lone-pair and bonding-type localized molecular orbitals, can give an unambiguous bonding picture and an intuitive bond order. Therefore a new scale of bond order, the Lewis bond order, can be assigned to localized molecular orbitals in terms of intuitive Lewis Structures. The Lewis bond orders of canonical molecular orbitals can be obtained by expanding them into localized molecular orbitals with the assigned Lewis bond orders. Test calculations have been performed for the molecules N 2 , O 2 , F 2 , P 2 , S 2 , CO, CS, NO and SO, whose Hartree-Fock wavefunctions can be well localized. The total Lewis bond orders of these molecules turn out to be exactly the bond orders revealed by the Lewis Structures. For canonical molecular orbitals with an ambiguous bonding nature, e.g. 2 σ g , 2 σ u and 3 σ g of the molecule N 2 , their Lewis bond orders match quite well with the photoelectron spectroscopy results.
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The canonical molecular orbital bond order and the photoelectron spectroscopy
Journal of Molecular Structure: THEOCHEM, 1997Co-Authors: Tse-chiang ChangAbstract:Abstract Recently, a new method to define the bond order for the canonical molecular orbitals according to the Lewis Structure has been proposed. The new method provides a link between the MO and VB theories. Not only the ionization potentials, but also the vibrational progressions of the photoelectron spectroscopy can be accounted for by the new bond order proposed. The predicted spacing of the vibrational progressions are in good agreement with the photoelectron spectroscopy results for several simple diatomic molecules: N2, O2, F2, CS, NO and CO.
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Pauli repulsion in the open shell species BeH and Co
Journal of Computational Chemistry, 1992Co-Authors: Tse-chiang ChangAbstract:Using the natural bond orbital method, one may associate the valence bond configuration and Lewis Structure concepts to wave functions consisting of molecular orbitals and thus gain intuitive insight into the molecular potential energy curves. Natural bond orbital analysis of the restricted open shell Hartree–Fock and unrestricted Hartree–Fock wave functions for the BeH ground state provides an intuitive model to help understand the nature of the bonding in this open shell species. The contrasting behavior of the bonding orbitals for different spins can be attributed to differences in the Pauli repulsive interactions with the lonepair orbitals. Such behavior occurs in BeH(2Σ) but does not in CO+(2Π) because the Pauli repulsion depends on the orbital overlap.
Richard D. Harcourt - One of the best experts on this subject based on the ideXlab platform.
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Some Electron-Excess σ Bonded Systems
Lecture Notes in Chemistry, 2015Co-Authors: Richard D. HarcourtAbstract:Most of the “increased-valence” Structures that we have discussed so far may be derived from Lewis Structures by delocalizing lone-pair \( \uppi \) and/or \( {\overline{\uppi }} \) electrons into vacant bonding or antibonding orbitals. The atomic orbital overlaps that are appropriate for some of these delocalizations are shown in Figs. 1-5 and 2-4. We shall now consider a few systems whose “increased-valence” Structures can be constructed by delocalizing one or more lone-pair σ electrons of a Lewis Structure into bonding or antibonding σ orbitals. Some other examples will also be discussed in Chapter 20, where the theory will be presented in a slightly different form. However, the principles for both chapters are the same.
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Ab initio double-ζ (D95) valence bond calculations for the ground states of S2N2 and S42+
The Journal of Physical Chemistry A, 2004Co-Authors: Thomas M. Klapötke, Richard D. HarcourtAbstract:The results of double-ζ D95 valence-bond (VB) calculations are reported for the ground states of S 2 N 2 and S 4 2+. The Mulliken, Lowdin, and Hiberty structural weights are reported for ten Lewis Structures that differ in the locations of six n electrons. The most important Lewis Structure for S 2 N 2 is the nitrogen singlet diradical Structure (I) with a structural weight of 0.47. For S 4 2+ the two singlet diradical Structures (XI and XII) with structural weights of 0.23 each are by far the most important individual Lewis Structures.
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A note on Wiberg-type atomic valencies for a 3-electron 3-centre valence bond Structure
Journal of Molecular Structure-theochem, 2002Co-Authors: Richard D. HarcourtAbstract:An expression is deduced for a Wiberg-type atomic valence for the A-atom of the 3-electron 3-centre valence bond Structure with the localised molecular orbital configuration ðy þ laÞ 1 ða þ kbÞ 2 : It is demonstrated that for l . 0; the A-atom valence is less than unity. However for k – 1 and small kl; the value of this valence is larger than it is for the Lewis Structure with the same value of k in the ðyÞ 1 ða þ kbÞ 2 configuration for the latter Structure. Elaboration of similar theory is also provided for the 4-electron 3-centre valence bond Structure
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Increased‐Valence Structures for Qualitative Valence‐Bond Representations of Electronic Structure for Electron‐Rich Molecules
European Journal of Inorganic Chemistry, 2000Co-Authors: Richard D. HarcourtAbstract:Whereas familiar Lewis-type valence-bond Structures for singlet-spin systems involve electron-pair bonds and lone-pairs of electrons, increased-valence Structures involve one-electron bonds and fractional electron-pair bonds. An increased-valence Structure may be easily derived from a familiar Lewis Structure via one-electron delocalizations of lone-pair electrons into two-centre bonding molecular orbitals. The increased-valence Structure is equivalent to resonance between several Lewis Structures, including the familiar Lewis Structure from which it was derived, and therefore it is more stable than the latter Structure. Some of the properties of increased-valence Structures, and their associated Pauling three-electron bonds as diatomic components of these Structures, are discussed, with specific reference to N2O and the FeIIO2 linkages of oxyhaemoglobin. Applications to numerous other systems that involve 4-electron 3-centre, 6-electron 4-centre, and 6-electron 5-centre bonding units are presented. Brief consideration is also given to a speculative Bohr approach to electronic Structure.
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On the “pentavalent” nitrogen atom and nitrogen pentacoordination
Journal of Molecular Structure, 1993Co-Authors: Richard D. HarcourtAbstract:Abstract Hydrazoic acid (HN3) is an example of a molecule whose bond lengths suggest hat the central nitrogen atom is apparently pentavalent, as indicated in the classical valence bond Structure (I). However, unless the nitrogen atom expands its valence shell, the π bonds of this Structure are fractional electron-pair bonds. The increased-valence Structure (II) with fractional electron-pair π and π′ bonds, and 1-electron π and π′ bonds, also involves an apparent pentavalence. Some of the properties of these two VB Structures are used to restate the nature of the origin of the apparent electronic pentavalence for nitrogen, namely appreciable contributions of Dewar-type Structures such as (III) to the component Lewis Structure resonance scheme. It is shown that although the valence of the central nitrogen atom of Structure (II) is able to exceed a value of 4, it can never attain a value of 5. Increased-valence Structures for the C2 isomer of N6 are also presented, and the bond lengths that are associated with the most stable of these Structures are in accord with those calculated using ab initio techniques. The results of some ab initio VB calculations, with minimal basis sets, are reported for: (a) N2, to demonstrate the effect of variation in σ bond atomic orbital hybridization on the lengths of the N″N‴ bonds of HN3 (as HN′N″N‴) and the NN bond of N2; (b) trigonal bipyramidal NH3F2 and PH3F2, to suggest that the unwillingness of nitrogen to form stable pentacoordinate compounds is associated with some reluctance by nitrogen to participate in the formation of axial 4-electron 3-centre σ bonding units.