Theoretical study of M(+)-RG complexes (M = Ga, In; RG = He-Rn)

We present potential energy curves calculated at the CCSD(T) level of theory for Ga(+)-RG and In(+)-RG complexes (RG = He-Rn). Spectroscopic parameters have been derived from these potentials and compared to previously calculated parameters for the Al(+)-RG and Tl(+)-RG complexes. Additionally, for some cases, we compare these parameters with those obtained from electronic spectroscopic studies on excited states of the neutral species, arising from atomic-based d ← p excitations. The Ga(+)-RG and In(+)-RG potentials have also been used to calculate the transport coefficients for M(+) traveling through a bath of RG atoms.     

Theoretical study of Al+-RG (RG = He-Rn)

We present the results of CCSD(T) calculations on the full set of Al(+)-RG complexes (RG = He-Rn). Potential energy curves are calculated pointwise, employing the full counterpoise correction and basis sets of quadruple-ζ and quintuple-ζ quality, and then extrapolated to the complete basis set limit. Each curve has been employed to calculate rovibrational energy levels, from which spectroscopic parameters have been derived. These are compared to the available experimental data, and it is seen that there is excellent agreement with the values obtained from both Rydberg state extrapolations and high-resolution laser-induced fluorescence studies. Finally, we have also used our potentials to calculate transport coefficients for Al(+) moving through a bath of RG.     

Interaction potentials for Br(-)-Rg (Rg=He-Rn): spectroscopy and transport coefficients

High-level ab initio CCSD(T) calculations are performed in order to obtain accurate interaction potentials for the Br(-) anion interacting with each rare gas (Rg) atom. For the Rg atoms from He to Ar, two approaches are taken. The first one implements a relativistic core potential and an aug-cc-pVQZ basis set for bromine, an aug-cc-pV5Z basis set for Rg, and a set of bond functions placed at the midpoint of the Rg-Br distance. The second one uses the all-electron approximation with aug-cc-pV5Z bases further augmented by an extra diffuse function in each shell. Comparison reveals close similarity between both sets of results, so for Rg atoms from Kr to Rn only the second approach is exploited. Calculated potentials are assessed against the previous empirical, semiempirical, and ab initio potentials, and against available beam scattering data, zero electron kinetic energy spectroscopic data, and various sets of the measured ion mobilities and diffusion coefficients. This multiproperty analysis leads to the conclusion that the present potentials are consistently good for the whole series of Br(-)-Rg pairs over the whole range of internuclear distances covered.     

Accurate potential energy curves for F(-)-Rg (Rg = He-Rn): spectroscopy and transport coefficients

High-quality ab initio potential energy curves are presented for the F(-)-Rg series (Rg = He-Rn). Calculations are performed at the CCSD(T) level of theory, employing d-aug-cc-pV5Z quality basis sets, with "small core" relativistic effective core potentials being used for Kr-Rn. The quality of the curves is judged by agreement with recent high-level calculations in the case of F(-)-He and F(-)-Ne and by excellent agreement with mobility data for the systems F(-)-Rg (Rg = He-Xe). Except for these recent high-level calculations on the two lightest systems, we are able to deduce that all other previous potentials for the whole set of these systems are inadequate. We also present spectroscopic information for the titular species, derived from our potential energy curves.     

Evidence for emergent chemical bonding in Au+-Rg complexes (Rg = Ne, Ar, Kr, and Xe)

Evidence is presented that there is a clear covalent component in the bonding of Au+ to Kr and Au+ to Xe, with some evidence that there may be such bonding between Au+ and Ar; for Au+ and Ne, there is no such evidence, and the bonding seems to be entirely physical. A model potential analysis shows that when all attractive inductive and dispersive terms out to R-8 are properly included in the Au+-Ne case, with an Ae(-bR) Born-Mayer repulsive term, essentially all the bonding in Au+-Ne can be rationalized by physical attraction alone. This is consistent with a natural bond order (NBO) analysis of the Au+-Ne ab initio wavefunctions, which shows the charge on Au+ to be very close to 1.0. In contrast, similar model potential and NBO analyses show quite clearly that physical interactions alone cannot account for the large bond energy values for the Au+-Kr and Au+-Xe complexes and are consistent with covalent contributions to the Au+-Kr and Au+-Xe interactions. Au+-Ar is seen to lie on the borderline between these two limits. In performing the model potential analyses, high-level ab initio calculations are employed [CCSD(T) energies, extrapolated to the complete basis set limit], to obtain reliable values of Re, De and omegae as input. A comparison of the gold-Xe bond distances in several solid-state Au(I, II and III) oxidation-state complex ions, containing "ligand" Xe atoms, prepared by Seppelt and co-workers, with that of the "free" Au+-Xe gas-phase ion is made, and a discussion of the trends is presented.     

Nature of the chemical bonding in D3h [MH3M]+ cations (M = Be,Mg)

Motivated by the particularly short metal-metal distance that has been predicted for the D_3h [BeH₃Be]⁺ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D_3h [MH₃M]⁺ cations (M = Be, Mg). CCSD(T)/cc-pVQZ calculations are used to determine optimized geometries for all of the various species, including those “capped” by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-center two-electron M─H─M bonding character instead of any significant direct metal-metal bonding.     

Triimidosulfonic acid and organometallic triimidosulfonates: S(+)-N(-) versus S=N bonding

Sulfonic acids RSO(2)OH and their metal salts MO(3)SR are versatile catalysts in large-scale industrial cyclization and polymerization processes. Isoelectronic replacement of the oxygen atoms by NR imido groups gives triimidosulfonic acid and triimidosulfonates. The salts form nonaggregated soluble molecules rather than infinite solid-state lattices such as their oxo analogues. In this paper, we present the synthesis and structure of the basic starting material MeS(N(t)Bu)(3)H (1), the metal complexes [Me(2)Al(N(t)Bu)(3)SMe] (2) and [Zn[(N(t)Bu)(3)SMe](2)] (3), and the mixed metal adduct [(thf)Li[(N(t)Bu)(3)SMe].ZnMe(2)] (4). The chelating coordination, rather than the tripodal coordination, cannot be attributed to steric effects of the S-bonded methyl group, as the less demanding Ph-C triple bond C-alkynyl substituent at sulfur in [(thf)(2)Li[(N(t)Bu)(3)SCCPh]] (5) causes the same conformation. S-N bond shortening to the pendant imido group has to be attributed to closed-shell electrostatic attraction rather than to S-N double bonding by valence expansion at the central sulfur atom. Coordination to an additional N-->Zn dative bond in 4 widens the bond length to values normally interpreted as S-N single bonds. We take this fact as experimental evidence that S-N bonding is predominantly governed by electrostatic interaction rather than by valence expansion employing d-orbitals. This was predicted by theoreticians more than a decade ago.     

RgBF2(+) complexes (Rg = Ar, Kr, and Xe): the cations with large stabilities

Rare gas containing cations with general formula [Rg, B, 2F](+) have been investigated theoretically by second-order Mo?ller-Plesset perturbation, coupled cluster, and complete active space self-consistent field levels of theory with correlation-consistent basis sets. Totally two types of minima, i.e., boron centered C(2) (v) symmetried RgBF(2) (+) (Rg = Ar, Kr, and Xe) which can be viewed as loss of F(-) from FRgBF(2) and linear FRgBF(+) (Rg = Kr and Xe) are obtained at the CCSD(T)∕aug-cc-pVTZ∕SDD and CASSCF(10,8)∕aug-cc-pVTZ∕SDD levels, respectively. It is shown that the RgBF(2) (+) are global minima followed by FRgBF(+) at 170.9 and 142.2 kcal∕mol on the singlet potential-energy surfaces of [Rg, B, 2F](+) (Rg = Kr and Xe) at the CASPT2(10,8) ∕aug-cc-pVTZ∕SDD∕∕CASSCF(10,8)∕aug-cc-pVTZ∕SDD, respectively. The interconversion barrier heights between RgBF(2) (+) and FRgBF(+) (Rg = Kr and Xe) are at least 39 kcal∕mol. In addition, no dissociation transition state associated with RgBF(2) (+) and FRgBF(+) can be found. This suggests that RgBF(2) (+) (Rg = Ar, Kr, and Xe) can exist as both thermodynamically and kinetically stable species, while linear FRgBF(+) (Rg = Kr and Xe) can exist as metastable species compared with the lowest dissociation limit energies just like isoelectronic linear FRgBO and FRgBN(-). From natural bond orbital and atoms-in-molecules calculations, it is found that the positive charge is mainly located on Rg and boron atoms for both types of minima, the Rg-B bonds of ArBF(2) (+), KrBF(2) (+), and XeBF(2) (+) are mostly electrostatic, thus can be viewed as ion-induced dipole interaction; while that of linear FKrBF(+) and FXeBF(+) are covalent in nature. The previous experimental observation of ArBF(2) (+) by Pepi et al. [J. Phys. Chem. B. 110, 4492 (2006)] should correspond to C(2) (v) minimum. The presently predicted spectroscopies of KrBF(2) (+), XeBF(2) (+), FKrBF(+), and FXeBF(+) should be helpful for their experimental identification in the future.     

Geometry, chemical bonding, and electronic spectra of Si(n) and Si(n)-glycine (n = 3-5) complexes

Structures and spectra are calculated for Si(n) and Si(n)-Gly (n = 3-5) complexes. Relative stability differences of Gly conformers are magnified by interactions with the Si(n) cluster, so that one conformer of Si(n)-Gly is stabilized. Significant charge transfer occurs from the amino group in Gly to a Si atom in the cluster. Interactions with Gly are predicted to shift the excitation energies of Si(n) significantly to the blue to 2.1-2.7 eV, although they are still lower than in a Si cluster passivated by hydrogen.     

IR spectra of C2H5(+)-N2 isomers: evidence for dative chemical bonding in the isolated ethanediazonium ion

The potential energy surface (PES) of C(2)H(5)(+)-N(2) is characterized in detail by infrared photodissociation (IRPD) spectroscopy of mass-selected ions in a quadrupole tandem mass spectrometer and ab initio calculations at the MP2/6-311G(2df,2pd) level. The PES features three nonequivalent minima. Two local minima, 1-N(2)(H) and 1-N(2)(C), are adduct complexes with binding energies of D(0) = 18 and 12 kJ/mol, in which the N(2) ligand is weakly bonded by electrostatic forces to either the acidic proton or the electrophilic carbon atom of the nonclassical C(2)H(5)(+) ion (1), respectively. The global minimum 3 is the ethanediazonium ion, featuring a weak dative bond of D(0) = 38 kJ/mol. This interaction strength is sufficient to switch the C(2)H(5)(+) structure from nonclassical to classical. The 1-N(2)(C) isomer corresponds to the entrance channel complex for addition of N(2) to 1 yielding the product 3. This reaction involves a small barrier of 7 kJ/mol as a result of the rearrangement of the C(2)H(5)(+) ion. The partly rotationally resolved IRPD spectrum of C(2)H(5)(+)-N(2) recorded in the C-H stretch range is dominated by four bands assigned to 3 and one weak transition attributed to 1-N(2)(H). The abundance ratio of 1-N(2)(H) and 3 estimated from the IRPD spectrum as ～1% is consistent with the calculated free energy difference of 12 kJ/mol. As the ethanediazonium ion escaped previous mass spectrometric detection, the currently accepted value for the ethyl cation affinity of N(2) is revised from -ΔH(0) = 15.5 ± 1.5 to ～42 kJ/mol. The first experimental identification and characterization of 3 provides a sensitive probe of the electrophilic character and fluxionality of the ethyl cation. Comparison of 3 with related alkanediazonium ions reveals the drastic effect of the size of the alkyl chain on their chemical reactivity, which is relevant in the context of hydrocarbon plasma chemistry of planetary atmospheres and the interstellar medium, as well as alkylation reactions of (bio)organic molecules (e.g., carcinogenesis and mutagenesis of DNA material).     

Formation and photodissociation of M(+)-C(6)H(6) (M(+) = V(+) and Ta(+)) and Ta(+)-C(6)H(4) complexes in a time-of-flight mass spectrometer

A series of cyclic hydrocarbons were introduced to react with V(+) and Ta(+) using a pulsed beam expansion source in a time-of-flight mass spectrometer. The third-row metal Ta(+) displayed high reactivity in dehydrogenation to form benzyne complexes, whereas benzene complexes were the terminal products for V(+). M(+)-C(6)H(6) (M(+) = V(+) and Ta(+)) and Ta(+)-C(6)H(4) were selected to perform the photodissociation experiments. In contrast to the V(+) fragment formation via simple cleavage of the V(+)-C(6)H(6) bond, a photoinduced loss of C(2)H(2) occurred in both the Ta(+)-C(6)H(6) and Ta(+)-C(6)H(4) complexes. Plausible explanations involved in the formation of Ta(+)-C(6)H(6) and Ta(+)-C(6)H(4) complexes are given for observing such photo-induced dissociation. The observed photodissociation in Ta(+)-C(6)H(6) is analogous to the dissociative process previously investigated in metal ion-molecule reactions. The photodissociation spectrum of Ta(+)-C(6)H(4) was obtained by recording the appearance of Ta(+)-C(4)H(2) as a function of wavelength and yielded a dissociation energy of 91 +/- 1 kcal mol(-1).     

Univalent gallium and indium phosphane complexes: from pyramidal M(PPh3)3(+) to carbene-analogous bent M(PtBu3)2(+) (M=Ga, In) complexes

In a new oxidative route, Ag(+)[Al(OR(F))(4)](-) (R(F)=C(CF(3))(3)) and metallic indium were sonicated in aromatic solvents, such as fluorobenzene (PhF), to give a precipitate of silver metal and highly soluble [In(PhF)(n)](+) salts (n=2, 3) with the weakly coordinating [Al(OR(F))(4)](-) anion in quantitative yield. The In(+) salt and the known analogous Ga(+)[Al(OR(F))(4)](-) were used to synthesize a series of homoleptic PR(3) phosphane complexes [M(PR(3))(n)](+), that is, the weakly PPh(3)-bridged [(Ph(3)P)(3)In-(PPh(3))-In(PPh(3))(3)](2+) that essentially contains two independent [In(PPh(3))(3)](+) cations or, with increasing bulk of the phosphane, the carbene-analogous [M(PtBu(3))(2)](+) (M=Ga, In) cations. The M(I)-P distances are 27 to 29 pm longer for indium, and thus considerably longer than the difference between their tabulated radii (18 pm). The structure, formation, and frontier orbitals of these complexes were investigated by calculations at the BP86/SV(P), B3LYP/def2-TZVPP, MP2/def2-TZVPP, and SCS-MP2/def2-TZVPP levels.     

H(3) (+) as a trap for noble gases--2: structure and energetics of XH(3) (+) complexes from X=neon to xenon

The affinity of H(3) (+) to combine with noble gases X has been investigated from neon to xenon using ab initio coupled cluster [CCSD and CCSD(T)] and density functional BH&HLYP levels of theory. For all noble gases, the stable structures belong to a C(2v) symmetry with an apex of the H(3) (+) triangle pointing to the noble gas. The structure of the complexes changes gradually from a practically pure Ne-H(3) (+) arrangement to a situation close to XeH(+)-H(2). A topological analysis of the electron localization function is used to illustrate the changes in the bonding along the series. The lowest dissociation energies of NeH(3) (+) and ArH(3) (+) ( approximately 1 and approximately 7 kcalmol) correspond to the breaking of the complexes according to X+H(3) (+), while the lowest dissociation energies of KrH(3) (+) and XeH(3) (+) ( approximately 8 and approximately 3 kcalmol) correspond to the breaking according to XH(+)+H(2). Rotational constants and harmonic frequencies are reported. Apart from XeH(3) (+) whose dipole moment (mu=2.6 D) may not be large enough, all the other complexes with dipole moments in the range of 6-8 D should be reasonable targets for detection by microwave spectroscopy. The present calculations are intended to stimulate both laboratory experiments and spatial observations since the possible sequestration of noble gases by H(3) (+) may have strong implications on the composition of astrophysical objects.     

Spectroscopic and quantum chemical studies on low-spin FeIV=O complexes: Fe-O bonding and its contributions to reactivity

High-valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes and have been structurally defined in model systems. Variable-temperature magnetic circular dichroism (VT-MCD) spectroscopy has been used to evaluate the electronic structures and in particular the Fe-O bonds of three FeIV=O (S = 1) model complexes, [FeIV(O)(TMC)(NCMe)]2+, [FeIV(O)(TMC)(OC(O)CF3)]+, and [FeIV(O)(N4Py)]2+. These complexes are characterized by their strong and covalent Fe-O pi-bonds. The MCD spectra show a vibronic progression in the nonbonding --> pi* excited state, providing the Fe-O stretching frequency and the Fe-O bond length in this excited state and quantifying the pi-contribution to the total Fe-O bond. Correlation of these experimental data to reactivity shows that the [FeIV(O)(N4Py)]2+ complex, with the highest reactivity toward hydrogen-atom abstraction among the three, has the strongest Fe-O pi-bond. Density functional calculations were correlated to the data and support the experimental analysis. The strength and covalency of the Fe-O pi-bond result in high oxygen character in the important frontier molecular orbitals (FMOs) for this reaction, the unoccupied beta-spin d(xz/yz) orbitals, that activates these for electrophilic attack. An extension to biologically relevant FeIV=O (S = 2) enzyme intermediates shows that these can perform electrophilic attack reactions along the same mechanistic pathway (pi-FMO pathway) with similar reactivity but also have an additional reaction channel involving the unoccupied alpha-spin d(z2) orbital (sigma-FMO pathway). These studies experimentally probe the FMOs involved in the reactivity of FeIV=O (S = 1) model complexes resulting in a detailed understanding of the Fe-O bond and its contributions to reactivity.     

Linear uranium complexes X2UL5 with L=cyanide, isocyanate: DFT evidence for similarities between uranyl (X=O) and uranocene (X=Cp) derivatives

A DFT study of the isostructural compounds [UO2L5](n-) with n=3-5 and linear [Cp2UL5](m-) with m=1-3 has been carried out for two different anionic ligands. Structurally stable structures are obtained for all systems. The coordination competition between cyanide (CN(-)) and isocyanide (NC(-)) as well as between cyanate (OCN(-)) and isocyanate (NCO(-)) has been studied in the uranyl case. A clear preference for cyanide and isocyanate complexes is reported. The coordination of five ligands in the equatorial plane is rationalised by the analysis of the MO diagram of both systems. Moreover, the qualitative comparison of the two MO diagrams shows a high similarity in agreement with the isolobality concept. The existence of linear [Cp2UL5](-) organometallic U(VI) complexes is thus proposed, as well as the possibility of obtaining complexes of both types for U(VI) and U(V) with OCN(-) ligands. In addition, the U(IV) linear metallocene is calculated to be stable for the latter ligand.     

Unsupported d8-d8) interactions in cationic Pd(II) and Pt(II) complexes: evidence for a significant metal-metal bonding character

Isostructural dinuclear Pd and Pt complexes that exhibit unique d(8)-d(8) interactions between dicationic metal centers are reported. These metal-metal interactions are not supported by any bridging ligands and suggest a significant metal-metal bonding character for both Pd and Pt systems.     

Probing the structures and chemical bonding of boron-boronyl clusters using photoelectron spectroscopy and computational chemistry: B(4)(BO)(n) (-) (n = 1-3)

The electronic and structural properties of a series of boron oxide clusters, B(5)O(-), B(6)O(2) (-), and B(7)O(3) (-), are studied using photoelectron spectroscopy and density functional calculations. Vibrationally resolved photoelectron spectra are obtained, yielding electron affinities of 3.45, 3.54, and 4.94 eV for the corresponding neutrals, B(5)O, B(6)O(2), and B(7)O(3), respectively. Structural optimizations show that these oxide clusters can be formulated as B(4)(BO)(n) (-) (n = 1-3), which involve boronyls coordinated to a planar rhombic B(4) cluster. Chemical bonding analyses indicate that the B(4)(BO)(n) (-) clusters are all aromatic species with two π electrons.     

Thione complexes of Rh(I): a first comparison with the bonding and catalytic activity of related carbene and imine compounds

Heterocyclic mono(thione), trans-bis(thione), cis-bis(thione), trans-(carbene-thione), cis-(carbene-thione), trans-(phosphine-thione) and mono(imine) complexes of rhodium(I) have been prepared and fully characterised. Chloro(eta(4)-1,5-cyclooctadiene)(L)rhodium(I)(1a, L = 1,3-diisopropyl-4,5-dimethyl-2,3-dihydro-1H-imidazol-2-thione; 1b L = 1,3,4,5-tetramethyl-2,3-dihydro-1H-imidazol-2-thione) appear as isomers at room temperature due to slow coordination exchange on the S-donor atom. In the three structures determined, the substituent on the sulfur appears syn to Cl. Hindered rotation about the Rh-carbene bond is revealed in the NMR spectra of seven new complexes with isopropyl substituents on the heterocyclic carbene ligands. The trans influence of the thione ligands is smaller than that of carbenes but larger than that shown by imines and chloride. Thione complexes are better catalyst precursors than the carbene complexes for the hydroformylation of 1-hexene under the chosen reaction conditions: 80 degrees C, 8 MPa CO-H2(1:1), 16 h, 1:1000 catalyst to 1-hexene ratio.     

On the chemical bonding of gold in auro-boron oxide clusters AunBO- (n = 1-3)

During experiment on Au-B alloy clusters, an auro-boron oxide cluster Au2BO- was observed to be an intense peak dominating the Au-B mass spectra, along with weaker signals for AuBO- and Au3BO-. Well-resolved photoelectron spectra have been obtained for the three new oxide clusters, which exhibit an odd-even effect in electron affinities. Au2BO- is shown to be a closed shell molecule with a very high electron detachment energy, whereas AuBO and Au3BO neutrals are shown to be closed shell species with large HOMO-LUMO gaps, resulting in relatively low electron affinities. Density functional calculations were performed for both AunBO- (n = 1-3) and the corresponding HnBO- species to evaluate the analogy between bonding of gold and hydrogen in these clusters. The combination of experiment and theory allowed us to establish the structures and chemical bonding of these tertiary clusters. We find that the first gold atom does mimic hydrogen and interacts with the BO unit to produce a linear AuBO structure. This unit preserves its identity when interacting with additional gold atoms: a linear Au-[AuBO] complex is formed when adding one extra Au atom and two isomeric Au2-[AuBO] complexes are formed when adding two extra Au atoms. Since BO- is isoelectronic to CO, the AunBO- species can be alternatively viewed as Aun interacting with a BO- unit. The structures and chemical bonding in AunBO- are compared to those in the corresponding AunCO complexes.