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Facile metal-free splitting of molecular hydrogen (H 2 ) is crucial for the utilization of H 2 without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H 2 in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons R to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H 2 provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.

The synthesis and characterization of a stable, acyclic two-coordinate silylene, Si(SArMe6 )2 [ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2], by reduction of Br2Si(SArMe6 )2 with a magnesium(I) reductant is described. It features a V-shaped silicon coordination with a S–Si–S angle of 90.52(2)° and an average Si–S distance of 2.158(3) Å. Although it reacts readily with an alkyl halide, it does not react with hydrogen under ambient conditions, probably as a result of the ca. 4.3 eV energy difference between the frontier silicon lone pair and 3p orbitals.

The electronic structures of fifteen Group 13-16 carbene analogues are analyzed using various quantum chemical methods and compared to the data obtained for the parent N-heterocylic carbene (NHC), imidazol-2-ylidene. The results of this study present a uniform analysis of the similarities and differences in the electronic structures of p-block main group carbene analogues. Though all systems are formally isovalent, the theoretical analyses unambiguously indicate that their electronic structures run the gamut from C=C localized (Group 13) to C=N localized (Group 16) via intermediate, more delocalized, systems. In particular, neither the stibenium ion nor any of the chalcogenium dications is a direct analogue of imidazol-2-ylidene as they all contain two lone pairs of electrons around the divalent main group center, instead of the expected one. The reason behind the gradual change in the electronic structure of main group analogues of imidazol-2-ylidene was traced to the total charge of the systems, which changes from anionic to dicationic when moving from left to right in the periodic table. Results from theoretical analyses of aromaticity show that all Group 13-16 analogues of imidazol-2-ylidene display some degree of aromatic character. The heavier Group 13 anions benefit the least from π-electron delocalization, whereas the cationic Group 15 systems are on par with the parent carbon system and display only slightly less aromatic character than cyclopentadienide, a true 6π-electron aromatic species. The σ-donor and π-acceptor ability of the different main group carbene analogues is also evaluated.

The reaction of Cpx 2ZrCl2 (Cpx = Cp, Cp*) with ammonia borane in presence of n-butyllithium yielded Cp2Zr(Cl)NH2BH3 and Cpx 2Zr(H)NH2BH3. These derivatives are isoelectronic with the ethyl zirconocene chloride and hydride, respectively, and feature a chelating amidoborane ligand coordinating through a Zr−N bond and a Zr−H−B bridge. In solution, each of the complexes consists of an equilibrium mixture of two isomers differing in the orientation of the amidoborane ligand with respect to the Zr−X bond (X = H, Cl), while in the solid state, only one isomer was observed. Such isomers have not been characterized for any metal complexes containing the isoelectronic β-agostic ethyl ligand or any other agostic alkyl group.

The electronic structures and molecular properties of square-planar 6-electron ring molecules and ions E2N2 and E4 2+ (E = S, Se, Te) were studied using various ab initio methods and density functionals. All species were found to contain singlet diradical character in their electronic structures. Detailed analysis of the CAS wave function of S2N2 in terms of different valence bond structures gives largest weight for a Lewis-type singlet diradical VB structure in which the two unpaired electrons reside on nitrogen atoms, though the relative importance of the different VB structures is highly dependent on the level of theory. The diradical character in both E2N2 and E4 2+ was found to increase in the series S < Se < Te. The diradical nature of the chemical species is manifested in the prediction of molecular properties, in which the coupled cluster and multiconfigurational approaches, as well as the BPW91 functional show consistent performance. 77 Se NMR chemical shifts of chalcogen cations SxSe4-x 2+ (x = 0-3) were calculated with CAS, BPW91 and B3PW91 methods using the GIAO formalism. The hybrid functional B3PW91 shows inferior performance, but both CAS and BPW91 unquestionably confirm the experimental assignment and are able to predict the NMR chemical shifts of these computationally difficult cases with excellent accuracy.

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A wide range of structurally characterized adducts of CO2are discussed in this review, from the strongly bound, charge assisted carbamate complexes through the weaker halide and pseudo-halide complexes to the weakest possible inclusion complexes.