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The synthesis of the characterization of a series of dirhenium metallaboranes containing 5-10m boron atoms is described. Competition between main group and transition metal electronic properties is also presented.

The density functional theory is utilized to study the bonding and electronic features of the homo- and heterobimetallic sp carbon chain complexes. The diiron and rhenium or iron dications are identified to contain three positions of approximately similar power and clearly specifying the magnetic information for experiments.

Aromaticity of cyclopentafused pyrene congeners has been investigated at various levels of theory. Aromaticity of these compounds decreased with increasing number of externally fused five-membered rings.

The first observation of vibrational structure in the ionization of metal ligand sigma bonding valence orbital, specifically the Re- H sigma-bond ionizations of CpRe(NO)(CO)H and CpRe(NO)(CO)H [Cp = eta(super 5)-C5H5, Cp = eta(super 5)-C5(CH3)5] is reported. The observations suggest that even in low symmetry molecules the orbital corresponding to the Re-H sigma bond and the Re-H vibrational mode is much localized.

A computational study of MO3S9 as a model of amorphous molybdenum trisulfide is presented.

The experimental enthalpy of the formation of triquinacene (1) was increased by 4 kcal/mol after determining it through the measurement of its energy of combustion in a microcalorimeter. The modified enthalpy value contradicts stabilization by homoaromatic interaction. The homoaromaticity of 1 is also indicated by the negligible computed stabilization energy, diamagnetic susceptibility exaltation and nucleus-independent chemical shift values.

The aromaticity and antiaromaticity of charged fluorenyl, indenyl, and cyclopentadienyl systems were evaluated using ab initio computations at high level and density theory. Results showed that fluorenyl, indenyl, and cyclopentadienyl anions were highly aromatic when compared with anthracene, benzene, and naphthalene. Singlet cyclopentadienyl and indenyl cations were found as antiaromatics while fluorenyl cation as non-aromatic. Results also revealed that delocalization energies of aromatic cations were greater than those of the antiaromatic cations.

The homoaromatic characteristics of strained semibullvalenes were established through analyses of high level ab initio and density functional theory aided by UV and 13C NMR spectral studies. Results have shown that the C2 v symmetric transition structure for the Cope rearrangement in the parent semibullvalene was bishomoaromatic. However, the 4,6-ethanosemibullvalene was localized and not homoaromatic.

The cyclization of 1,3-cis-5-hexatriene to 1,3-cyclohexadiene is accelerated by the complexation of Lithium cation. These electrostatic accelerations are due to higher electrostatic stabilization of the transition structure than the ground state. The electrostatic acceleration effect of the 1,5-hydrogen shift in cyclopentadiene is also generated by the complexation of Na+, Mg2+, HBe+ and Be2+. The acceleration is maximum with Be2+ and minimum with Na+.

A (10)annulene obeys Huckels rule but prefers nonaromatic, nonplanar structure. Second order perturbation and density functional theories help characterize a new aromatic, planar mono-trans isomer of (10)annulene. The geometry of the isomer shows Cs point group with the inward pointing hydrogen bent about 20 degrees from the carbon plane and the carbon-carbon bond lengths are very close to those of benzene. Energetic and magnetic properties support the aromatic character of the isomer, while UV and NMR structural studies reveal ambiguous behavior in the isomer.

The aromatic stability of 3,5-dehydrophenyl cation is due to the presence of two (4n+2)-electron conjugated systems. The cation exhibits higher stability than its cyclic isomers and its open-chain isomers. 2,4,6-trisilyl substitution increases the stability of the cation through hyperconjugation. Magnetic susceptibility exaltation demonstrates the double aromaticity of the cation and of the D6h cyclo(6)carbon. Both compounds have the necessary geometric, energetic and magnetic requirements for aromaticity.

The strategy and design in catalysts and reagents for fluorous and supercritical CO2 chemistry is advanced by clearly identifying the methylene spacer lengths needed for the effective insulation of a typical active site from a perfluoroalkyl chain. Computations show that the first carbon of a perfluoroalkyl segment exhibits a much greater inductive effect than the second, and that ionization potentials of nonfluorinated phosphines P((CH2)mCH3)3 reach a limit at approximately nine carbons (m=8).

A study was conducted to examine the affinity of the all-trans-cyclodecapentaene to the central unit of dodecahedrapentaene in the [n]-trannulenes, a member of the all-trans-[n]annulene family which exhibited in-plane conjugation. The proton nuclear magnetic resonance was performed and the aromaticity of the compound was tested. The findings suggest an inverse relationship between the heat formation and ring size for each CH group for both the aromatic and antiaromatic trannulenes.

It is possible to use magnetic criteria to assess the degree of aromaticity of a series of inorganic ring systems. The homodesmotic reaction can be used to evaluate both aromatic stabilization energy values and exalted magnetic susceptibilities. The nucleus-independent chemical shift (NICS)(delta) and NICS(pi) tend to cancel when delocalization is not present. Cyclic delocalized systems have enhanced NICS(pi) contributions. Both NICS(pi) and NICS(delta) decrease with increase of the ring size (bond length).

The first stable silanethione, Tbt(Tip)Si=S (Tip = 2,4,6-triisopropylphenyl) has been synthesized, using 2,4,6-tris(bis(trimethylsilyl)methyl)-phenyl (Tbt). Tbt was applied to the kinetic stabilization of silaaromatic species, leading to the isolation of 2-silanaphthalene. The structure was confirmed by NMR spectral data.

A novel aromacity/antiaromacity criterion uses absolute magnetic shieldings at ring centers and quantum mechanics programs to determine the aromacity/antiaromacity of a range of five-membered rings. The method corresponds to the popular NMR chemical shift convention and is superior to aromaticity determination through diamagnetic susceptibility exaltation. The negative nucleus-independent chemical shifts (NICS) denote aromaticity whereas positive NICS represent antiaromacity. The antiaromatic 4n pi electron compounds exhibit highly positive NICS.

Ab initio self-consistent reaction field study of the donor-acceptor complexes HCN-BF3 and H3CCN-BF3 reveals that medium effects are responsible for the experimentally observed geometry differences of the complexes in solid and gas states. A crystal dipolar field interaction induces medium effects and only complexes having donor molecules with permanent dipole moment exhibit this effect.

Ab initio calculations predict observed 3He NMR chemical shifts in He at C60 and He at C70. Host fullerene structures influence the calculated 3He chemical shifts. This calculation also predicts 7Li chemical shifts in isostructural Li(+) at C60 and Li(+) at C70. These calculations also prove the aromatic nature of C60.

A study on the 1,3-dipolar cycloaddition chemical reaction and its mechanistic aspects was conducted to analyze in-plane aromaticity pertaining to significant variables in chemical reactions such as substituent and solvent effects. The aim of the study was to add to the knowledge of the factors which determine the energetic parameters of these reactions and their regiochemistry and stereochemistry. Computational techniques and results are discussed.

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R 1 R 2 PH, R 2 = H or R 1 ) with hydrosilanes (R 3 R 4 R 5 SiH, R 4 , R 5 = H or R 3 ) to produce synthetically useful silylphosphines (R 1 R 2 P-SiR 3 R 4 R 5 ) has been achieved using B(C 6 F 5 ) 3 as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C 6 F 5 ) 3 but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R 1 R 2 HP·B(C 6 F 5 ) 3 adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C 6 F 5 ) 3 catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R 1 R 2 P-SiR 3 R 4 R 5 ). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C 6 F 5 ) 3 at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H 2 -acceptor.

The reactivity of variously substituted alkynylsilanes toward selected group 4 metallocene complexes was investigated. Reactions of the alkynylsilanes R(1)C(2)SiR(2)(2)H (R(1) = SiMe(3), R(2) = Me, 1; R(1) = SiMe(3), R(2) = Ph, 2; R(1) = SiMe(2)H, R(2) = Me, 5) with Cp(2)TiMe(2) (Cp = eta(5)-cyclopentadienyl) resulted, upon methyl group transfer to the silyl group, in the previously described titanocene alkyne complexes Cp(2)Ti(R(1)C(2)SiR(2)(2)R(3)) (R(1) = Me(3)Si, R(2) = R(3) = Me, 3; R(1) = HMe(2)Si, R(2) = R(3) = Me, 6) or the unreported complex 4 (R(1) = Me(3)Si, R(2) = Ph, R(3) = Me). The Cp(2)TiCl(2)/n-BuLi system yielded alkyne complexes 6 and 7 (R(1) = HMe(2)Si, R(2) = Me, R(3) = H); no alkyl group transfer was detected. On the other hand, reactions utilizing the Cp(2)ZrCl(2)/n-BuLi system afforded inseparable mixtures; however, complexes of the type Cp(2)Zr[R(1)C(2)SiMe(2)(n-Bu)] (R(1) = Me(3)Si, 8; R(1) = HMe(2)Si, 9) were detected. Cp(2)Hf(n-Bu)(2) reacted with the alkynylsilanes in a diverse way, depending on the substituents of the alkyne substrate. The reaction with an excess of alkyne 1 (R(1) = Me(3)Si, R(2) = Me) afforded only an intractable mixture, which contained Me(3)SiC(2)SiMe(2)(n-Bu) (10). Hafnacyclopentadienes 13-15 as precedented product types were obtained when alkyne 12 (R(1) = Ph, R(2) = Me) was used. In sharp contrast, the symmetrically substituted alkynes 5 (R(1) = HMe(2)Si, R(2) = Me) and H(2)PhSiC(2)SiPhH(2) (18) yielded the hitherto unknown Si-containing metallacycles 16 and 19. A reaction mechanism leading to these products was proposed and subsequently supported by DFT calculations. In addition, the reduction of Cp(2)HfCl(2) with magnesium in THF in the presence of alkynylsilanes was shown to be an alternative route to compounds 14-16 and 19. Presumably due to steric reasons, alkyne 1 could not form any of the product types described above. Nevertheless, it was utilized for the preparation of the PMe(3)-stabilized hafnocene alkyne complex 11.

Spin-polarized density functional theory calculations have been performed to investigate the carbon pathways and hydrogenation mechanism for CH(4) formation on Fe(2)C(011), Fe(5)C(2)(010), Fe(3)C(001), and Fe(4)C(100). We find that the surface C atom occupied sites are more active toward CH(4) formation. In Fischer-Tropsch synthesis (FTS), CO direct dissociation is very difficult on perfect Fe(x)C(y) surfaces, while surface C atom hydrogenation could occur easily. With the formation of vacancy sites by C atoms escaping from the Fe(x)C(y) surface, the CO dissociation barrier decreases largely. As a consequence, the active carburized surface is maintained. Based on the calculated reaction energies and effective barriers, CH(4) formation is more favorable on Fe(5)C(2)(010) and Fe(2)C(011), while Fe(4)C(100) and Fe(3)C(001) are inactive toward CH(4) formation. More importantly, it is revealed that the reaction energy and effective barrier of CH(4) formation have a linear relationship with the charge of the surface C atom and the d-band center of the surface, respectively. On the basis of these correlations, one can predict the reactivity of all active surfaces by analyzing their surface properties and further give guides for catalyst design in FTS.

The first examples of vibrational structure in metal-ligand sigma-bond ionizations are observed in the gas-phase photoelectron spectra of CpRe(NO)(CO)H and CpRe(NO)(CO)H [Cp = eta(5)-C(5)H(5), Cp = eta(5)-C(5)(CH(3))(5)]. The vibrational progressions are due to the Re-H stretch in the ion states formed by removal of an electron from the predominantly Re-H sigma-bonding orbitals. A vibrational progression is also observed in the corresponding ionization of the deuterium analogue, CpRe(NO)(CO)D, but with lower vibrational energy spacing as expected from the reduced mass effect. The vibrational progressions in these valence ionizations are directly informative about the nature of the metal-hydride bonding and electronic structure in these molecules. Franck-Condon analysis shows that for these molecules the Re-H or Re-D bond lengthens by 0.25(1) A when an electron is removed from the Re-H or Re-D sigma-bond orbital. This bond lengthening is comparable to that of H(2) upon ionization. Removal of an electron from the Re-H or Re-D bonds leads to a quantum-mechanical inner sphere reorganization energy (lambda(QM)) of 0.34(1) eV. These observations suggest that even in these low symmetry molecules the orbital corresponding to the Re-H sigma bond and the Re-H vibrational mode is very localized. Theoretical calculations of the electronic structure and normal vibrational modes of CpRe(NO)(CO)H support a localized two-electron valence bond description of the Re-H interaction.