Nonbonding Electron Pairs Research Articles

Sur les chalcogeno-iodures d'antimoine Sb XI( X =S, Se, Te): Structures et spectroscopie Mo¨ssbauer de 121Sb

The crystal structure of Sb XI( X =Se, Te) compounds has been determined by means of three-dimensional intensity data. The crystal structure of SbSeI, orthorhombic, space group Pnma with a = 8.698(2), b = 4.127(1), c = 10.412(2) Å, was refined at several temperatures (180 K, R = 0.021; 293 K, R = 0.020; 320 K, R = 0.023) in correlation with the paraelectric structure or SbSI stable above 293 K. The crystal structure of SbTeI, triclinic, space group P1¯, with a = 7.570(3), b = 7.159(3), c = 4.228(3) Å, α = 107.22(5), β = 106.18(4), γ = 77.19(3)° has been determined by symbolic addition method and refined to a final R value of 0.035. These structures are built up from infinite weakly linked ribbons (Sb X 2) n of trigonal Sb X 3 with Sb X bonds of 2.605(1), 2.795(1)Å( X =Se), and 2.829(1), 2.953(1), 2.955(1)Å( X =Te). The nature of Sb X and Sb I bonds is discussed in terms of the S, Se, Te substitution. Antimony-121 Mo¨ssbauer spectra have been recorded at liquid helium temperature. The data are discussed with regard to the stereochimical activity of the antimony (III) lone pair of electrons. For SbTeI the Mo¨ssbauer parameters are interpreted in terms of direct population of conductance bands by nonbonding electron pairs.

Iodine-127 Mössbauer study of iodine(III) interhalogen cations and related compounds

The 127I Mössbauer spectra of a number of iodine(III) cations and related molecules having four ligands and two non-bonding electron pairs about the central iodine have been recorded at 4.2 K. The Mössbauer parameters are dominated by the primary bonds to the iodine which are largely of p character.

Solid-state properties of materials of the type Cs4MX6(where M = Sn or Pb and X = Cl or Br)

Two new compounds Cs4SnBr6 and Cs4SnCl6 have been prepared and the CsBr–SnBr2 phase diagram is described. Phases of the type Cs4Sn1 –nPbnBr6 –xClx(where n= 0–1, x= 0–6) have been examined by X-ray diffraction, electrical conductivity, Mossbauer spectral, and optical reflectance techniques. The electrical conductivity and optical properties of the phases are explained in terms of the population of solid-state bands by the non-bonding electron pairs of the Group 4 atoms. The structures of all the phases are related to that of Cs4PbCl6 which has been redetermined.

The structure of thianthrenium biscarbethoxymethylide and the corresponding sulfoxide

AbstractThianthrene reacts with one equivalent of diethyl diazomalonate to form the monosulfonium ylide thianthrenium biscarbethoxymethylide. Single crystal X‐ray and 13C‐nmr establish the conformation as pseudoequatorial (e') in both the solid state and solution. The angle of fold between aryl rings is 135.7°, the nearlyplanar malonylide fragment bisecting this angle. The carbonyl oxygen of the endo carbethoxy group is anti to the nonbonding electron pair on sulfur. The methylide carbon resonates at 50 ppm. The aromatic solvent induced shift (ASIS) technique indicates that thianthrene Soxide also is e'.

Effet Mössbauer de 119Sn dans les systèmes SnSBaS et SnSTl 2S

Tin-119 Mössbauer spectra have been recorded at liquid nitrogen and room temperatures for ternary sulfides isolated from the systems SnSBaS and SnSTl 2S. The chemical isomer shifts observed (2.8 – 3.4 mm/sec relative to BaSnO 3 at 293 K) are characteristic of divalent tin compounds. The data are consistent with the known structures of these compounds and are discussed with regard to the stereochemical activity of the tin(II) lone pair of electrons. For Tl 2Sn 2S 3 and Tl 4SnS 3 the Mössbauer parameters are interpreted in terms of direct population of conductance bands by nonbonding electron pairs.

Complex formation between antimony trifluoride and alkali-metal sulphates: the X-ray crystal structure of K2SO4·SbF3and antimony-121 Mössbauer studies of some related compounds

The X-ray crystal structure of K2SO4·SbF3 is reported. It crystallizes in the orthorhombic space group P212121, with a= 5.601(2), b= 9.072(4), c= 14.180(6)A, and Z= 4 and a final R= 0.035. The antimony environment is that of a distorted octahedron, SbF3O2E, where E represents the non-bonding electron pair of SbIII The 121Sb Mossbauer data of this and other M2SO4·SbF3 complexes are interpreted in terms of SbX5E and SbX6E environments. In all cases the non-bonding electron pair is stereochemically active.

Fluorination with positive fluorine generated from isoelectronically related reagents

Compounds such as PhIF 2, Ph 3PF 2 and XeF 2, which have been used previously as unrelated fluorinating agents, are shown to be periodically related as isoelectronic molecules E 3AF 2 of trigonal-bipyramidal shape, where E represents a bonded or non-bonded electron pair and A a main Group V – VIII element. These compounds are arranged in order of halogenating ability by estimating the magnitude of reduction couples, approximated by δH o f(E 3AF 2-E 3A), or by noting the direction of redox reactions involving the couples. The A sequence deduced Kr>Xe⋍Cl>Br>I>S>Se>Te⋍As⋍Sb>P agrees with the limited experimental data available. Evidence for an ionic mechanism involving ‘onium’ monohalide ions is given for halogenations with these reagents when carried out under “Friedel-Craft” conditions although no stable salts containing these ions have as yet been isolated because of intramolecular halogenation. These ions act as sources of positive fluorine. The use of ring deactivated reagents to achieve halogenation is discussed.

Die kristallstruktur des tris(monohapto)cyclopentadienylantimon(III), ( 1h-C 5H 5) 3Sb

(C 5H 5) 3Sb was synthesized by treatment of Sb(NMe 2) 3 with excess monomeric cyclopentadiene in quantitative yield. The compound crystallizes in the space group P2 1/ c with Z = 4; d c 1.606 g cm −3; a 1036.5(15) pm; b 872.1(10 pm; c 1563.6(21) pm; β 109.47(12)°. At 223 K a set of 1849 unique reflections was obtained, 1574 with F > 2σ( F) have been used for refining the structure; full-matrix treatment with individual anisotropic temperature factors led to a reliability index R w = 0.025. The molecular structure unequivocally shows the existence of σ-bonded ( 1 h-monohapto)cyclopentadienyl rings with approximate tetrahedral angles between the “best planes” of the five-membered rings and the SbC bond directions. The space-demanding non-bonding electron pair at the Sb atom influences the molecular packing: channels are formed along the b-axis of the elementary cell.

Crystal structure of di-µ3-oxo-octakis-µ-(trifluoroacetato)-ditin(II)ditin(IV)–benzene (1/1)

The crystal and molecular structure of [SnIISnIVO(O2CCF3)4]2·C6H6 has been determined by single-crystal X-ray crystallography to R′= 0.058. Crystals are monoclinic, space group C2/c, with a= 24.182(10), b= 9.828(3), c= 17.882(6)A, β= 104.77(3)°, and Z= 4. The molecular structure consists of independent centrosymmetric [SnIISnIVO(O2CCF3)4]2 units. The two symmetry-related tin(IV) atoms are bridged by two µ3-oxygen atoms to form a SnIV2O2 ring. Octahedral co-ordination at SnIV is completed by trifluoroacetates which bridge the SnIV and SnII. The tin(II) atoms have a distorted square-based-pyramidal geometry with the apical site occupied by a µ3-oxygen, and the basal positions by oxygens from bridging trifluoroacetates. The O(basal)–SnII–O(basal) angles range from 74.40(26) to 118.40(27)°. The remaining apical site is presumably occupied by a non-bonding electron pair. One lattice benzene molecule is present per unit.

Topological methods of quantum chemistry for a study of chemical reactivity

In this paper all approximations are discussed which lead to a quantum chemical model which is adequate for structural formulas known in classical organic chemistry. Neglect of the energy differences caused by changes of valence and dihedral angles, i.e. neglect of all nonbonding interactions, leads to the separation of the Hartree-Fock matrix in blocks of core electrons, nonbonding electron pairs, two center blocks of σ-bonds and delocalized π-electronic structures. Such a procedure can be formulated at all levels of sophistication - from the one electron approximation to the MC-SCF method. Even on the one electron level, the average error in heats of atomization (35.9 kJ/mol) is lower than that of the more complicated geometrical methods MINDO/3 (52.1 kJ/mol) and MNDO (39.6 kJ/mol). The procedure suggested is about two orders of magnitude more efficient than geometrical ones of the same level and can be, therefore, used for a study of reaction mechanisms of medium size systems (30-50 atoms) without large expense.

Remote substituent effects in the Baeyer-Villiger oxidation. II. regioselection based on the hydroxyl group orientation in the tetrahedral intermediate

A remote directing effect in the Baeyer-Villiger oxidation of 8-oxabicyclo[3.2.1]-octan-3-one derivatives has been interpreted in terms of chiral orientation of the hydroxyl nonbonding electron pairs in the tetrahedral intermediates.

Xe(OTeF5)6, A Deep‐Colored Noble Gas Compound, and OXe(OTeF5)4—The Existence of Kr(OTeF5)2

The red‐violet color of Xe(OTeF5)6 (1) must be attributed to the bonding state of hexavalent xenon which possesses a nonbonding electron pair: Te(OTeF5)6 has no such pair and is colorless. The extremely light‐sensitive compound (1) probably exists as a monomeric molecule with covalent bonds. equation image

The Mössbauer effect in tin(II) compounds. Part 14. Data for some chalcogenide halides

The 119Sn Mossbauer data for Sn7Br10S2, Sn4Br6Se, and Sn4Br10Te and for compositions from the SnCl2–SnS system are compared with those of the parent halides and chalcogenides. The data for Sn7Br10S2 show the presence of two tin(II) sites but these are not associated with distinct Sn–Br and Sn–S environments. The data for all the phases studied are consistent with random distribution of halide and chalcogenide anions. The variation in Mossbauer parameters of Sn7Br10S2 with temperature and the development of colour in the chalcogenide halides is discussed in terms of direct population of solid-state bands by non-bonding electron pairs.

Tellurium-125 Mössbauer spectra of some tellurium subhalides

The 125Te Mossbauer spectra at 4.2 K for the crystalline tellurium subhalides Te3Cl2, Te2X (X = Br or I), Te2Br0.75- I0.25, β-Tel, and α-Tel show the presence of at least two different tellurium sites in each subhalide in accordance with their known crystal structures. The chemical isomer shifts δ(relative to Zn125mTe at 4.2 K) in four different types of co-ordination have been found to be 0.6 mm s–1 for trigonal bipyramidal tellurium atoms [Te(Te)2(Cl)2E], 0.8–1.0 mm s–1 for square-planar (pseudo-octahedral) tellurium atoms [Te(Te)2(X)2E2](X = Br or I), and 0.8 mm s–1 for two-co-ordinate (pseudo-tetrahedral) tellurium atoms [Te(Te)2E2]. The symbols in square brackets indicate the nearest-neighbour environment of the relevant tellurium atom, the symbol E being used to represent an essentially non-bonding pair of electrons. The corresponding quadrupole splittings for the four environments were 11.0, 12.1–13.7, 3.4–6.1, and 8.1 mm s–1 respectively. The bonding scheme for square-planar tellurium atoms in the tellurium subhalides is considered to involve two three-centre four-electron bonds and is thus similar to that in the isostructural and isoelectronic species [ICl4]– and XeCl4. The Mossbauer spectra at 4.2 K for the glassy phases Te2Br0.75I0.25 and TeBr2 were different from those of the corresponding crystalline compounds but again showed the existence of at least two different tellurium sites in each compound.

The role of non-bonding electron pairs in the asymmetric coordination environments of 1,1-dithiolate complexes

Various forms of asymmetry in the lengths of the bond between the central atom and sulphur, found in differing coordination environments of 1,1-dithiolate compounds involving main group atoms, have been successfully rationalized by considering both the valence shell electron pair repulsion theory and the effect of the restricted ligand bite distance.

Effects of the presence of valence-shell non-bonding electron pairs on the properties and structures of caesium tin(II) bromides and of related antimony and tellurium compounds

The ideal perovskite structure of CsSnIIBr3 at room temperature has been confirmed by a single-crystal X-ray diffraction study. It is proposed that the high-symmetry environment for the SnII in this compound arises because the distorting effect of the non-bonding electrons is reduced by their populating an empty low-energy band in the solid, thus giving rise to the black colour and metallic-conducting properties. The high-temperature phases of CsSn2IIBr5, Cs4SnIIBr6, and of compositions from the CsSnII2Br5–CsSnII2Cl5 system show similar properties. Colour can be introduced into the Cs2SnIVBr6 system by formation of mixed phases with CsSnIIBr3 which has a closely related structure. The colour and electrical properties of the mixed SnII–SnIV material can be explained by population of the low-energy delocalised solid-state bands by the SnII non-bonding electrons without recourse to intervalence-transfer-absorption ideas. Comparison of the complex bromides and chlorides of SnII and TeIV with the mixed-valence SbIII–SbV compounds suggests that direct population of bands rather than intervalence charge transfer may also be the dominant process in determining the properties of the antimony derivatives, l.r. data for the mixed-valence antimony compounds are consistent with the direct-population explanation.

On the Origin of the Bohlmann Bands

A perturbational molecular orbital analysis has been performed of the strengths of the CH bonds of methylamine and methanol in their staggered conformations. This analysis leads to the prediction that a CH bond anti-coplanar to a directed lone pair is stronger than a gauche CH bond, and is in disagreement with experimental observation. The origin of the disagreement is to be found in the underestimation of the role of the nuclear–nuclear contribution to the bond strengths. Abinitio computation of the gauche and anti stretching force constants of methylamine provides quantitative theoretical support for the view that these differ because of a nuclear-dominated effect. It is suggested that effects, analogous to those observed in the Bohlmann bands, may be seen even in the absence of nonbonded electron pairs.

The Electron Pair in Chemistry

The reality of the electron pair as a fundamental unit in the electronic structure of molecular systems is evidenced by calculations which show that the most probable partitioning of a system is the one which localizes pairs of electrons in well-defined spatial regions or loges. The loges in turn, correspond to those regions of space generally associated with core, bonded, and non-bonded electrons. In terms of information theory, they yield the maximum amount of information concerning the localizability of the electrons. The most probable three-loge partitioning of the six-electron BH(X1∑+) system, for example, is dominated by the event which places two electrons in each of three loges, the location and shape of the loges being such as to justify the labelling of the electron pairs they localize as core, bonded and nonbonded. Since the loges are defined in real space and are totally nonoverlapping, one may define the volume of space occupied by pairs of electrons. In BH, for example, the volume of space required to contain 95% of the nonbonded pair of electrons is over two times larger than that required to contain 95% of the bonded pair. It is possible to define core loges which exhibit pair occupation probabilities ranging in value from 95% in LiH+ to 85% in BH. Corresponding probabilities ranging in value from 75% to 90% are obtained for bonded and nonbonded loges. In the set of molecules studied here, the occurrence of events with such high probabilities is found only for loges which maximize the probability of a pair occupation.

On the Concept of Orbital Steering in Catalytic Reactions

Angular displacement from linear overlap of but a few degrees in the transition state of the enzyme-substrate complex has been postulated to be of great kinetic significance ("orbital steering"). The concept of orbital steering is shown to have evolved from the orientation parameters of an equation previously proposed to evaluate the kinetic importance of propinquity. This equation is shown to be naive. Arguments provided against the concept of orbital steering include: (a) force constants predicted from orbital steering are about 100 times those experimentally determined from displacement of nuclei in a direction normal to the axis of a covalent bond (for example, at room temperature vibrational bending amplitudes of +5 degrees or more are common); (b) because of the lessened directionality of orbitals containing nonbonded electron pairs, the force constants in transition states should be even smaller than in the case of a covalent bond; and (c) molecular orbital calculations predict shallow total energy minima for orbital alignment. The experimental rate data offered as a basis for the concept of orbital steering are shown to find rationalization in the previously observed dependence of DeltaSdouble dagger on kinetic order and the energy requirements for the freezing-out of single bonds in the transition state leading to the formation of medium-size ring compounds from extended ground states. It is concluded that if orbital steering does exist, experimental and theoretical evidence to support this concept have yet to be presented.

Über den lösungsmitteleinfluss auf die konformative beweglichkeit des 1,2,4,5-tetroxan-systems

Activation parameters Δ H ‡, Δ G ‡, and Δ S ‡ have been measured for the conformational isomerization of 3,3,6,6-tetramethyl-1,2,4,5-tetroxan in solvents with acceptor properties using the PMR method of Gutowsky and Holm. The results show that repulsive forces between the non-bonding electron-pairs of the O atoms apparently contribute essentially to the enthalpy of activation of the conformational isomerization and thus determine to a large extent the residence time of the conformations.