Three-center Two-electron Research Articles

Three-center bond index profiles

Three-center two-electron bond index profiles of some simple molecules containing H, Li and B atoms have been calculated using suitable basis sets at the HF level. These profiles correspond to the formation of H 3 + from H 2 and H +, rotation of the BH 2 units of B 2H 6 and variation of the HLiH angle of LiH 3. The three-center bond index is found to be maximum near to the geometry of energy minimum. Higher three-center bond indices are observed for LiH dimer and B 4 + at their equilibrium geometry than at an alternative structure.

Theoretical studies of (d-p)? bonding, electronic spectra, and reactivities in homo- and heterometallic clusters: [Mo3- n W n X4(H2O)9]4+ ( X = O, S, Se, Te;n= 0-3)

Ab initio calculations with relativistic effective potentials have been carried out on 12 trinuclear molybdenum/tungsten cluster aqua ions [M3X4(H2O)9]4− (M3= Mo3, W3 for X = O, S, Se, Te; M3=Mo2W, MoW2 for X = O, S). The electronic structures and bonding pictures of l-12 are discussed in terms of the delocalized and localized molecular orbitals as well as the Mulliken populations, natural populations, and Mayer bond orders. It is shown that the (d-p)ρ bonding in the puckered six-membered ring of the [M3(µ-X)3] core arises from a closed continuous ring of three mutually adjacent localized (d-p-d)ρ bonds with strong interactions. It is these three-centered two-electron (d-p-d)ρ bonds that account for the unusual physicochemical properties and reactivities of these cluster compounds. The wavelengths and the assignment of electronic spectra have been given, and the relation between the wavelength shili and the (d-p)ρ bonding is discussed, The reactivities of the ligand substitution reactions and two kinds of addition reactions as weil as some kinetic and redox properties of these compounds are briefly discussed by taking advantage of this Iocalization (d-p-d)ρ a bonding picture.

Transition Metal Silyl Complexes. 53.1 Magnitude of the Chelate Effect in the Oxidative Addition of Si−H Bonds

The chelate complexes (CO)4W(Ph2PCH2CH2SiHR2) (R = Me, Ph), containing W,H,Si three-center two-electron bonds, were photochemically prepared from W(CO)6 and Ph2PCH2CH2SiR2H. The phosphine complexes (CO)5W(Ph2PCH2CH2SiR2H) are intermediates in this reaction. The high NMR coupling constants JSiWH in the chelate complexes (98.1 and 95.2 Hz) indicate that due to the chelating situation an earlier stage of the oxidative addition of the Si−H bond is frozen than in the corresponding nonchelated complexes. The stabilization brought about by the chelating (phosphinoethyl)silyl ligand is slightly larger than the electronic effect caused by substituting another CO ligand by PR3 or by replacing SiR3 (R = alkyl, aryl) for SiCl3 in nonchelated complexes.

Comparison and Search for CH53+ and CH64+ and Their Isoelectronic Boron Analogues BH52+ and BH63+ 1

The structures and stabilities of CH53+ and CH64+ and their isoelectronic boron analogues BH52+ and BH63+, respectively, were calculated at the MP2/6-31G** and QCISD (T)/6-311G** levels. The planar D5h symmetric structure 3 is a minimum for the CH53+ trication. The five hydrogens in structure 3 are bonded to the carbon atom by sharing the six valence electrons. Calculations showed that the CH64+ tetracation does not correspond to a minimum at the levels studied. The planar C2v symmetric structure 6, with two 3c−2e (three-center two-electron) bonds, on the other hand, was found to be a minimum for the BH52+ dication. The D3 symmetric structure 8 also corresponds to a stable minimum for the BH63+ trication.

Gas-Phase Bond Strength and Atomic Connectivity Studies of the Unsymmetrical Two-Center Three-Electron Ion, [Et2S∴SMe2]+

A computational and experimental study of the unsymmetrical 2c−3e bond in [Et2S∴SMe2]+ is presented. For the first time, MS/MS collision-induced dissociation experiments provide strong experimental support of the atomic connectivity in a gas-phase S∴S 2c−3e association adduct. The strongest peak in the collision-induced dissociation spectrum corresponds to Et2S+, consistent with the lower ionization potential of Et2S compared to Me2S and the proposed structure. High-pressure equilibrium mass spectrometry experiments yield a reaction enthalpy of −104 kJ/mol for the equilibrium reaction Et2S+ + Me2S ⇄ [Et2S∴SMe2]+ at 506 K. Correcting this value to 0 K using ab initio molecular parameters results in a bond energy of 107 kJ/mol which can be compared to a calculated value of 107.9 kJ/mol at the B3LYP/6-31G(d)//B3LYP/6-31G(d)+ZPC level. Studies on a competing reaction, Et2S+ + Et2S ⇄ [Et2S∴SEt2]+, yield an experimental bond enthalpy of 119 kJ/mol at 506 K and a calculated bond energy of 121.3 kJ/mol, in excellent agreement with previously reported values.

Aromaticity of Substituted Cyclopropenes: A Theoretical Study

The aromaticity of several heterosubstituted cyclopropenes and corresponding protonated forms was investigated using ab initio calculations at the G2-MP2 level of theory, topological charge density, and NBO analysis. It was shown that according to geometric, energetic, charge density, and magnetic criteria these systems are remarkably aromatic, especially the protonated forms. It was shown that the aromatic delocalization of double bond in the cyclopropene ring can be well modeled using a three-center two-electron bond in the framework of NBO analysis proposed by Reed and Weinhold.

Evaluation of thermodynamic parameters for highly correlated chemical systems: a spectrophotometric study of the 1 : 1 and 2 : 1 equilibria between I2and 1,1′-methylenebis(3-methyl-4-imidazoline-2-thione)(mbit) and 1,1′-ethylenebis(3-methyl-4-imidazoline-2-thione)(ebit). Crystal and molecular structures of mbit·2I2and ebit·2I2

The reactions of I2 with 1,1′-methylene- and 1,1′-ethylene-bis(3-methyl-4-imidazoline-2-thione)(mbit and ebit) have been investigated in CHCl3 solution at different temperatures by spectrophotometry. The experimental data have been processed using two different procedures of computer analysis: SUPERSPEC based on the conventional Gauss–Newton–Marquardt least-squares method; and POWELSPEC, based on Powell's direct search method for the refinement of ΔH° and ΔS°. The ability of the two methods to identify the involved equilibria correctly was compared. Evidence for the stepwise formation of the 1 : 1 and 1 : 2 adducts has been obtained by the two methods for the mbit case, while POWELSPEC only was able to solve the ebit case satisfactorily. The crystal structures of the adducts mbit·2I2 and ebit·2I2 have been determined. They show that the thionic sulfur atoms co-ordinate two diiodine molecules, with S–I 2.683(2)(mbit·2I2) and 2.642(3)(ebit·2I2)A and with I–I 2.897(1) and 2.903(2)A respectively, the two S–I–I groups beings related by a two-fold axis. The structural features of the S–I–I linear group are in accordance with Fourier-transform IR and Raman spectral data and can be taken into account with a three-centre two-electron axial orbitally deficient bonding scheme.

Hydrogen bonds involving transition metal centres – a brief review

Hydrogen bonding is a topic which has received much attention over the years and continues to do so as the importance of such interactions is established in all areas of chemistry. The class of hydrogen bonds that directly involves electron-rich transition metal centres in the three-centre interaction has received little attention until quite recently. Such interactions are of importance in understanding intermolecular interactions between organometallic molecules and are particularly relevant to understanding proton transfer reactions that directly involve transition metal centres. Hydrogen bonds in which transition metal centres serve as the hydrogen-bond acceptor, i.e. X―H...M (X = C, N, O, S), can be identified on the basis of geometric and spectroscopic criteria which confirm: (i) that such interactions should be classified alongside conventional hydrogen bonds (X―H...X'), and (ii) that these three-centre four-electron interactions can be distinguished from the better known three-centre two-electron X―H...M (X = B, C, Si, P, S) interactions. In salts of the type R 3 NH + Co(CO) 3 L - , where L = CO, P(OR) 3 or PR 3 , it is shown that increasing the basicity of the hydrogen-bond acceptor [Co(CO) 3 L - ] by changing L leads to strengthening of the N―H...Co hydrogen bond. Furthermore, in reactions where the products were dictated by a competition between N―H...N and N―H...Co hydrogen-bond formation, results suggested that N―H...N were formed preferentially, inferring that those hydrogen bonds involving metal centres are the weaker hydrogen bonds. Some initial results that point towards the construction of larger hydrogen-bonded assemblies involving X―H...M hydrogen bonds are also discussed.

Infrared spectroscopy of the siliconium ion, SiH+5

The infrared spectrum for the H–H stretching mode of the siliconium ion SiH+5 in the frequency range of 3650–3740 cm−1 is presented. The observed vibration–rotation transitions were fitted with the A-type rotational transitions of an asymmetric top using the Watson S-type asymmetric top rotational Hamiltonian. The results suggested that the siliconium ion SiH+5 can be described as a complex between SiH+3 and a freely internally rotating H2 groups, with a highly localized three-center two-electron bond.

Structural quantum effects and three-centre two-electron bonding in CH+5

HYPERCOORDINATE carbonium ions can be formed by protonating saturated hydrocarbons with superacids1–3. As this leaves a deficiency of bonding electrons, the resulting non-classical carbocations contain bonds in which two electrons are shared between three nuclei. Protonated methane, CH+5, might be seen as the prototype of such species1–3. But recent calculations4,5 have suggested that all five C–H bonds are effectively equivalent and exchange dynamically very rapidly. It was therefore concluded4 that CH+5 is a highly fluxional molecule without a definite structure, in which the representation in terms of three-centre two-electron bonding is misleading. Here we use a recently developed technique6 to perform ab initio electronic structure calculations that include quantum effects of the nuclei. We find that, although there are prominent quantum-mechanical effects on the structure, including fluxional-ity, pseudo-rotations and hydrogen scrambling, the quantum ground state is nevertheless dominated on average by configurations in which an H2 moiety is attached to a CH3 group forming a three-centre two-electron bond. To this extent, CH+5 should therefore resemble other carbonium ions.

Synthesis and structure of tris[dihydrobis(pyrazolyl)borate](tetrahydrofuran)uranium(III), U[H(μ-H)Bpz 2] 3 (THF): three-center BH§d U(III) interactions in the presence of coordinated THF ligand

Reaction of UI 3(THF) 4 with three equivalents of KH 2Bpz 2 gives the complex U[H(μ-H)Bpz 2] 3(THF) ( 1). The molecular structure has been determined by single-crystal X-ray diffraction: monoclinic, space group P2 1/ c, a = 10.662(2), b = 13.586(3), c = 20.868(6) A ̊ , β = 103.32(2)°, V = 2941(1) A ̊ 3 and Z = 4 . The six nitrogen donor atoms of the chelating H 2Bpz 2 − ligands are arranged in a slightly distorted trigonal prismatic geometry. Two of the H 2Bpz 2 − ligands span triangular edges on a single square face of the trigonal prism, while the third spans the edge common to the remaining two square faces. The oxygen atom of the coordinated THF ligand caps one of the square faces. The remaining two rectangular faces and one of the two triangular faces of the trigonal prism are capped by three-center two-electron BH⋯U bridge bonds, providing an effective ten-coordinate U(III) center. The observation of low frequency BH stretching bands is in accord with the presence of these interactions. In solution the molecule is fluxional. Although the rearrangement is nearly stopped at −100 °C, its nature could not be elucidated. Repeated cycles of dissolution of 1 in toluene and solvent removal yield the THF-free complex U[H(μ-H)Bpz 2] 3 ( 2). The molecule is also fluxional. The activation energy for equilibration of the BH 2 hydrogens is 51 kJ mol −1.

A Definitive Investigation of the Gas-Phase Two-Center Three-Electron Bond in [H2S-SH2]+, [Me2S-SMe2]+, and [Et2S-SEt2]+: Theory and Experiment

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA Definitive Investigation of the Gas-Phase Two-Center Three-Electron Bond in [H2S-SH2]+, [Me2S-SMe2]+, and [Et2S-SEt2]+: Theory and ExperimentYing Deng, Andreas J. Illies, Mary A. James, Michael L. McKee, and Michael PeschkeCite this: J. Am. Chem. Soc. 1995, 117, 1, 420–428Publication Date (Print):January 1, 1995Publication History Published online1 May 2002Published inissue 1 January 1995https://doi.org/10.1021/ja00106a048RIGHTS & PERMISSIONSArticle Views165Altmetric-Citations65LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Theoretical study on tetranuclear boron clusters: B4X4 (X = H, F, Cl, Br, I)

AbstractThe molecular orbitals for B4H4, B4F4, B4Cl4, B4Br4 and B4I4 have been calculated by using all‐electron or effective core potential ab initio method at the self‐consistent field level using basis sets with diffuse and polarization functions. The boron—boron and boron—halide (—hydrogen) distances of these cage compounds are optimized with three kinds of basis sets constrained to a tetrahedral symmetry. According to the localization scheme of Boys, four three‐centered two‐electron (3c2e) B—B—B bonds localized on each of the faces of the B4 tetrahedron are derived for B4X4 clusters. The HOMO‐LUMO energy gaps, atomization energies and Mulliken overlap populations of these compounds indicate that the stabilities of the clusters decrease in the sequence of B4F4 > B4Cl4, B4H4 > B4Br4 > B4I4.

Localized Molecular Orbital Studies of Fullerenes: C60and C70

Abstract Localized molecular orbitals (LMOs) have been calculated for C60 and C70 by using the CNDO/2 approximation and the energy-localization scheme of Edmiston-Ruedenberg. It is found that the π-bonding on the surfaces of the spherical carbon cages is localized into two-centered two-electron (2c2e) and three-centered two-electron (3c2e) bonds with additional charge distributions delocalized in the neighboring atoms. The localized electronic structures, the hybridized states of carbon atoms, the spherical aromaticity, and the reactivity for forming metal complexes of C60 and C70 are discussed.

Synthesis and crystal structures of the gold–carbaborane complexes [9-exo-{Au(PPh3)}-9-(µ-H)-10-endo-{Au(PPh3)}7,8-Me2-nido-7,8-C2B9H8] and [10-exo-{Au2(PPh3)2}-10-endo-{Au(PPh3)}-7,8-Me2-nido-7,8-C2B9H8

Treatment of the salt [NEt4][10-endo-{Au(PPh3)}-7,8-Me2-nido-7,8-C2B9H9] with [AuCl(PPh3)] in CH2Cl2 afforded the digold complex [9-exo-{Au(PPh3)}-9-(µ– H)-10-endo-{Au(PPh3)}-7,8-Me2-nido-7,8-C2B9H8]. Deprotonation of the latter with NaH in tetrahydrofuran followed by addition of [AuCl(PPh3)] gave the trigold compound [10-exo-{Au2(PPh3)2}-10-endo-{Au(PPh3)}-7.8-Me2-nido-7,8-C2B9H9]. The crystal structures of the new compounds have been determined by X-ray diffraction. In the digold species one Au(PPh3) group is endo-bound to the nido-C2B9 cage, primarily through a connectivity [Au–B 2.28(1)A] with the boron atom which is in the β-site with respect to the two carbons in the open [graphic omitted] face. There is a weaker attachment [Au–B 2.37(1)A] to one of the α-boron atoms, while the other α-BH unit forms a three-centre two-electron B–H⇀Au bond with the second Au(PPh3) group [Au–B 2.28(1), Au–µ– H 1.9(1)A]. The latter gold fragment lies exo with respect to the open face of the carbaborane cage, with an Au–Au separation of 2.875(1)A. The structure of the trigold complex is formally related to that of its digold precursor by replacement of the µ–H atom in the latter by an Au(PPh3) moiety. This results in the molecule having an essentially isosceles triangle of gold atoms [Au–Au 2.691(1), 2.922(1), 3.010(1)A], asymmetrically bridged by the β-boron [Au–B 2.192(9), 2.227(7), 2.307(8)A] of the open [graphic omitted] face of the C2B9 cage. The NMR data (1H, 13C –{1H}, 31P –{1H} and 11B –{1H}) of the compounds are reported and discussed.

A novel mechanism for reactions of thiirane with the thiirane radical cation. An experimental and ab initio study

An experimental and computational study of the ion-molecule reactions initiated by the reaction of SC 2 H 4 .+ with SC 2 H 4 has been carried out. The dimer ion is observed in a hybrid drift tube/ion source under chemical ionization conditions at low temperatures and high pressures. However, the ion intensity is much lower than expected for a product containing a stable two-center three-electron (2c-3e) S. . .S bond. Above room temperature, ion intensities for m/z 92 S 2 C 2 H 4 .+ , m/z 124 S 3 C 2 H 4 .+ , m/z 156 S 4 C 2 H 4 .+ , and m/z 184 S 4 C 4 H 8 .+ dominate

Ab initio study on the structure and H2 dissociation reaction of a tetrahydride-bridged dinuclear Ru complex, (C5H5)Ru(μ-H)4Ru(C5H5)

Abstract The structure and bonding nature, and the H 2 dissociation reaction of (C 5 H 5 )Ru(μ-H) 4 Ru(C 5 H 5 ) have been studied using an ab initio molecular orbital (MO) method at the RHF and the MP2 level. Between the two Ru atoms there are four hydride-bridged, three-center two-electron bonds (RuHRu) with a bond energy of 74 kcal mol −1 , but no direct RuRu bonds. The four hydrides are almost equivalent and there is no HH bond. The H 2 dissociation reaction which gives coordinatively highly unsaturated CpRu(μ-H) 2 RuCp (Cp  C 5 H 5 ) is uphill and endothermic by 57 kcal mol −1 as a result of loss of the strong hydride-bridged bonds.

Mechanisms of electrophilic substitutions of aliphatic hydrocarbons: methane + nitrosonium cation

The substitution reaction of methane with the nitrosonium cation, a model electrophile, was investigated computationally at the Hartree-Fock and correlated MP2, MP4SDTQ, and CISD levels of theory, using standard basis sets (6-31G(d), 6-31G(dp), and 6-31+G(dp) for geometry optimizations and TZ2P for energy single points on the most critical structures). The energetically favored reaction course leads to N-protonated nitrosomethane. H[sub 3]/CHNO[sup 4] (6). The initial complex of CH[sub 4] and NO[sup +] in C[sub s] symmetry is bound by -3.7 kcal mol[sup 1] MP4SDTQ/6-31+G(dp)/MP2/6-31+G(dp) + ZPVE/MP2/6-31+G(dp). In the critical step, electrophile NO[sup +] attacks carbon directly, rather than a C-H bond, to yield a pentacoordinate intermediate (3) with a hydrogen unit attached to a H[sub 2]CNO[sup +] cation moiety [[Delta]H[sub O](CISD+Q/TZ2P/MP2/6-31G(dp)+ZPVE//MP 2/6-31G(d p)) = 57.3 kcal mol[sup [minus]1]]. This unusual mode of attack, proceeding through a transition structure which also has three-center two-electron (3c-2e) CHH bonding, can be visualized in two ways. During the reaction, tetrachedral methane distorts to lower symmetry (C[sub s]) and binding between the electrophile and the developing lone pair occurs. The energy required for the methane distortion is partly recovered from the new bonding interaction to the electrophile. An alternative pathway involving the insertion of NO[sup +]more » into a CH bond is less favorable by 14.4 kcal mol[sup [minus]1]. 27 refs., 9 figs., 3 tabs.« less

Role of the central atom in three-centre bonding

Sannigrahi and Kar [l] have recently extended the definition of bond index to multi-centre cases. They and Sannigrahi et al. [2] have observed that a genuinely three-centre two-electron bond is always associated with a positive and significantly high value (greater than 0.1) of the bond index IABC. Using the symmetric unpolarised three-centre bond model of Mayer [3], Kar and Marcos [4] showed that the more electronegative atom prefers the bridging or central position of the three-centre bond. Examples are the BHB bond in B2Hs and the LiHLi bonds in Li2H+ and in (LiHh, where the more electronegative hydrogen atom is the central one. However, recent investigation [5] on LiBH2 and HBeBH2 dimers has indicated that the bond index of the three-centre BXB bonds with electropositive X (X = Li or Be) in the central position is higher than those of the three-centre XBX bonds. In the present work the three-centre model calculation of Mayer [3] was extended in order to examine the role of the central atom in three-centre bonding. The pertinent expressions for two-centre bond indices (IAB and I,& for cases where the symmetric (A = C) three-centre bonding orbital is more or less polarised, are [3]: r,, = IBC = i(l -p2) (1)

Electronic structure of the Fe3S4 cluster and its quasi-aromaticity

The localized molecular orbitals and their energy levels for the clusters [Fe3S4(SH)3]2−, [(HS)3Fe3S4·Ni(PH3)]2−, [Mo3S4(OH2)9]4+, and [Mo3S4·Ni]4+ have been calculated by mean of the Edmiston-Ruedenberg energy localization technique under the CNDO/2 approximation in order to reveal the resemblance between [Fe3S4]+ and [Mo3S4]4+ in the geometrical configurations and the addition reactivities with heterometal atoms. It is shown that in these two cluster species with core {M 3(μ3-S)(α-S)3} of similar structure (M = Mo, Fe) there exist three synergically connected three-centered two-electron (M-S-M) π-bonds around the puckered six-membered {M3S3} rings on account of delocalization of a lone electron pair on each bridging S atom; these (M-S-M) π-bonds are thus capable of forming cubane-like heterometal clusters with “intruder” metal atoms through the (π → M) bonding. It is therefore seen that unlike the [Mo3S4]4+ with appreciable bonding between the Mo atoms, the extra d-electrons on the metal atoms in the [Fe3S4]+ cluster are localized on the Fe atoms, exhibiting an electronic structure significantly different from that of the [Mo3S4]4+ cluster.