Inert Pair Research Articles

Why the Mechanisms of Digermyne and Distannyne Reactions with H2 Differ So Greatly

Despite their formal relationship to alkynes, Ar'GeGeAr', Ar'SnSnAr', and Ar*SnSnAr* [Ar' = 2,6-(2,6-iPr(2)C(6)H(3))(2)C(6)H(3); Ar* = 2,6-(2,4,6-iPr(3)C(6)H(2))(2)-3,5-iPr(2)C(6)H] exhibit high reactivity toward H(2), quite unlike acetylenes. Remarkably, the products are totally different. Ar'GeGeAr' can react with 1-3 equiv of H(2) to give mixtures of Ar'HGeGeHAr', Ar'H(2)GeGeH(2)Ar', and Ar'GeH(3). In contrast, Ar'SnSnAr' and Ar*SnSnAr* react with only 1 equiv of H(2) but give different types of products, Ar'Sn(μ-H)(2)SnAr' and Ar*SnSnH(2)Ar*, respectively. In this work, this disparate behavior toward H(2) has been elucidated by TPSSTPSS DFT computations of the detailed reaction mechanisms, which provide insight into the different pathways involved. Ar'GeGeAr' reacts with H(2) via three sequential steps: H(2) addition to Ar'GeGeAr' to give singly H-bridged Ar'Ge(μ-H)GeHAr'; isomerization of the latter to the more reactive Ge(II) hydride Ar'GeGeH(2)Ar'; and finally, addition of another H(2) to the hydride, either at a single Ge site, giving Ar'H(2)GeGeH(2)Ar', or at a Ge-Ge joint site, affording Ar'GeH(3) + Ar'HGe:. Alternatively, Ar'Ge(μ-H)GeHAr' also can isomerize into the kinetically stable Ar'HGeGeHAr', which cannot react with H(2) directly but can be transformed to the reactive Ar'GeGeH(2)Ar'. The activation of H(2) by Ar'SnSnAr' is similar to that by Ar'GeGeAr'. The resulting singly H-bridged Ar'Sn(μ-H)SnHAr' then isomerizes into Ar'HSnSnHAr'. The subsequent facile dissociation of the latter gives two Ar'HSn: species, which then reassemble into the experimental product Ar'Sn(μ-H)(2)SnAr'. The reaction of Ar*SnSnAr* with H(2) forms in the kinetically and thermodynamically more stable Ar*SnSnH(2)Ar* product rather than Ar*Sn(μ-H)(2)SnAr*. The computed mechanisms successfully rationalize all of the known experimental differences among these reactions and yield the following insights into the behavior of the Ge and Sn species: (I) The active sites of Ar'EEAr' (E = Ge, Sn) involve both E atoms, and the products with H(2) are the singly H-bridged Ar'E(μ-H)EHAr' species rather than Ar'HEEHAr' or Ar'EEH(2)Ar'. (II) The heavier alkene congeners Ar'HEEHAr' (E = Ge, Sn) cannot activate H(2) directly. Instead, Ar'HGeGeHAr' must first isomerize into the more reactive Ar'GeGeH(2)Ar'. Interestingly, the subsequent H(2) activation by Ar'GeGeH(2)Ar' can take place on either a single Ge site or a joint Ge-Ge site, but Ar'SnSnH(2)Ar' is not reactive toward H(2). The higher reactivity of Ar'GeGeH(2)Ar' in comparison with Ar'SnSnH(2)Ar' is due to the tendency of group 14 elements lower in the periodic table to have more stable lone pairs (i.e., the inert pair effect) and is responsible for the differences between the reactions of Ar'EEAr' (E = Ge, Sn) with H(2). Similarly, the carbene-like Ar'HGe: is more reactive toward H(2) than is Ar'HSn:. (III) The doubly H-bridged Ar'E(μ-H)(2)EAr' (E = Ge, Sn) species are not reactive toward H(2).

Redetermination of (D-penicillaminato)lead(II)

In the title coordination polymer, [Pb(C5H9NO2S)]n {systematic name: catena-poly[(μ-2-amino-3-methyl-3-sulfido­butano­ato)lead(II)]}, the d-penicillaminate ligand coordin­ates to the metal ion in an N,S,O-tridentate mode. The S atom acts as a bridge to two neighbouring PbII ions, thereby forming a double thiol­ate chain. Moreover, the coordinating carboxyl­ate O atom forms bridges to the PbII ions in the adjacent chain. The overall coordination sphere of the PbII ion can be described as a highly distorted penta­gonal bipyramid with a void in the equatorial plane between the long Pb—S bonds probably occupied by the stereochemically active inert electron pair. The amino H atoms form N—H⋯S and N—H⋯O hydrogen bonds, resulting in a cluster of four complex units, giving rise to an R 4 4(16) ring lying in the ab plane. The crystal structure of the title compound has been reported previously [Freeman et al. (1974 ▶). Chem. Soc. Chem. Commun. pp. 366–367] but the atomic coordinates have not been deposited in the Cambridge Structural Database (refcode DPENPB). Additional details of the hydrogen bonding are presented here.

Theoretical study of electronic structure and complexation of PbII(S2COR)2 [R = Me, Et, Ph] complexes

The complexes of PbII(S2COR)2 [R = Me, Et, Ph] having three possible coordination patterns S,S/S,S or S,O/S,S or S,O/S,O are studied. The formation energy and relative energy of these isomers reveal that the order of stability is S,S/S,S > S,O/S,S > S,O/S,O. Complexation energy of the isomers decreases substantially from gas phase to water. The natural bond orbital (NBO) analysis has been performed to explore the metal–ligand coordination. Similar types of metal–ligand coordinations are observed for methyl and ethyl substituent. In S/S,S/S coordination, lead mainly uses its 6p sub-shells along with 6d and 7s orbitals to coordinate with 3p sub-shells of the surrounding sulfur atoms. Significant mixing of 6p and 6d orbitals of lead is also observed. In contrast, for phenyl substituent, lead uses only its 6p sub-shells to coordinate with sulfur atoms. In S,O/S,O or S,O/S,S coordination, oxygen uses its 2s and 2p sub-shells while sulfur uses its 3p sub-shells to coordinate with 7s and 6p sub-shells of lead. The binding energy and NBO analysis of these chelates indicate that lead–sulfur (soft–soft) coordination is preferred over lead–oxygen (soft–hard). In all these chelates, valence 6s orbital of lead shows inert pair effect with no participation in chelation.

The role of conformational changes in the signal enhancement of a selective chemosensor of Pb2+

In this work, the complexes formed by a fluoroionophore with three dications (M(2+) = Ca(2+), Hg(2+) and Pb(2+)) were studied theoretically, by applying both Density Functional Theory and its Time Dependent version (TDDFT). The experimental information concerning this fluoroionophore-Pb(2+) complex indicates a large increment in the absorption and emission signals. From a theoretical point of view, by using the structure with minimum energy of the fluoroionophore free as reference; distortions in the structure are observed when the complexes are formed and consequently important changes in terms of certain properties occur. Although there are increments in signal intensity in the case of the three ions considered here, TDDFT predicts that the Pb(2+) complex will manifest the most pronounced response and this conforms to experimental observations. Thus, we can associate the experimentally observed behavior with conformational changes. In order to corroborate this hypothesis we have analyzed the fluoroionophore-M(2+) structure complexes, both with ions, without ions and without the ionophore (which binds the ions); it is evident that if the ligand manifests geometric distortions, then the absorption signal will increase. The inert pair effect induces the particular conformation of the complex, when the Pb(2+) is present, acting as a lone pair, which is revealed when the electron localization function is analyzed. If structural distortions are present then the charge distribution and consequently the dipolar moment also present significant changes.

Periodic Mesoporous Hydridosilica − Synthesis of an “Impossible” Material and Its Thermal Transformation into Brightly Photoluminescent Periodic Mesoporous Nanocrystal Silicon-Silica Composite

There has always been a fascination with "impossible" compounds, ones that do not break any rules of chemical bonding or valence but whose structures are unstable and do not exist. This instability can usually be rationalized in terms of chemical or physical restrictions associated with valence electron shells, multiple bonding, oxidation states, catenation, and the inert pair effect. In the pursuit of these "impossible" materials, appropriate conditions have sometimes been found to overcome these instabilities and synthesize missing compounds, yet for others these tricks have yet to be uncovered and the materials remain elusive. In the scientifically and technologically important field of periodic mesoporous silicas (PMS), one such "impossible" material is periodic mesoporous hydridosilica (meso-HSiO(1.5)). It is the archetype of a completely interrupted silica open framework material: its pore walls are comprised of a three-connected three-dimensional network that should be so thermodynamically unstable that any mesopores present would immediately collapse upon removal of the mesopore template. In this study we show that meso-HSiO(1.5) can be synthesized by template-directed self-assembly of HSi(OEt)(3) under aqueous acid-catalyzed conditions and after template extraction remains stable to 300 °C. Above this temperature, bond redistribution reactions initiate a metamorphic transformation which eventually yields periodic mesoporous nanocrystalline silicon-silica, meso-ncSi/SiO(2), a nanocomposite material in which brightly photoluminescent silicon nanocrystallites are embedded within a silica matrix throughout the mesostructure. The integration of the properties of silicon nanocrystallinity with silica mesoporosity provides a wealth of new opportunities for emerging nanotechnologies.

Supramolecular assemblies of metal ions in complex bimetallic and trimetallic salts based on hexacyanoferrate(III) ion

Two complex heterometallic salts with formulae Tl 6[Fe(CN) 6] 1.33(NO 3)(OH) ( 1) and [Co(bpy) 2(CN) 2] 2{[Ag(CN) 2] 0.5[Fe(CN) 6] 0.5}·8H 2O ( 2) have been synthesized and fully characterized. Single crystal X-ray analyses reveal that compound 1 is comprised of discrete Tl + cations and [Fe(CN) 6] 3− anions together with OH − and NO 3 - anions. Compound 2 contains [Co(bpy) 2(CN) 2] + cations and {[Ag(CN) 2][Fe(CN) 6]} − anions together with eight molecules of water of crystallization. Both structures form unprecedented three-dimensional supramolecular networks via non-covalent interactions. Another important observation is that the stereochemically active inert (lone) pair present on Tl + plays little role in controlling the structure of 1. The water molecules in 2 play important roles in providing stability, organizing a supramolecular network through hydrogen bonding. In the syntheses of 1 and 2, Fe(II) is oxidized to Fe(III) and Co(II) to Co(III), respectively, facilitating the formation of the salts that are obtained. Both compounds exhibit photoluminescence emission in solution near the visible region.

Atomic and electronic structures of the ordered23×23and molten1×1phase on the Si(111):Sn surface

In this work, a selection of unresolved topics regarding the electronic and atomic structures of Si and Ge surfaces, both clean ones and those modified by metal adsorbates, are addressed. The results presented have been obtained using theoretical calculations and experimental techniques such as photoelectron spectroscopy (PES), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Si(001) surfaces with adsorbed alkali metals can function as prototype systems for studying properties of the technologically important family of metal-semiconductor interfaces. In this work, the effect of up to one monolayer (ML) of Li on the Si(001) surface is studied using a combination of experimental and theoretical techniques. Several models for the surface atomic structures have been suggested for 0.5 and 1 ML of Li in the literature. Through the combination of experiment and theory, critical differences in the surface electronic structures between the different atomic models are identified and used to determine the most likely model for a certain Li coverage. In the literature, there are reports of an electronic structure at elevated temperature, that can be probed using angle resolved PES (ARPES), on the clean Ge(001) and Si(001) surfaces. The structure is quite unusual in the sense that it appears at an energy position above the Fermi level. Using results from a combined variable temperature ARPES and LEED study, the origin of this structure is determined. Various explanations for the structure that are available in the literature are discussed. It is found that all but thermal occupation of an ordinarily empty surface state band are inconsistent with our experimental data. In a combined theoretical and experimental study, the surface core-level shifts on clean Si(001) and Ge(001) in the c(4×2) reconstruction are investigated. In the case of the Ge 3d core-level, no previous theoretical results from the c(4×2) reconstruction are available in the literature. The unique calculated Ge 3d surface core-level shifts facilitate the identification of the atomic origins of the components in the PES data. Positive assignments can be made for seven of the eight inequivalent groups of atoms in the four topmost layers in the Ge case. Furthermore, a similar, detailed, assignment of the atomic origins of the shifts on the Si surface is presented that goes beyond previously published results. At a Sn coverage of slightly more than one ML, a 2√3 × 2√3 reconstruction can be obtained on the Si(111) surface. Two aspects of this surface are explored and presented in this work. First, theoretically derived results obtained from an atomic model in the literature are tested against new ARPES and STM data. It is concluded that the model needs to be revised in order to better explain the experimental observations. The second part is focused on the abrupt and reversible transition to a molten 1×1 phase at a temperature of about 463 K. ARPES and STM results obtained slightly below and slightly above the transition temperature reveal that the surface band structure, as well as the atomic structure, changes drastically at the transition. Six surface states are resolved on the surface at low temperature. Above the transition, the photoemission spectra are, on the other hand, dominated by a single strong surface state band. It shows a dispersion similar to that of a calculated surface band associated with the Sn-Si bond on a 1×1 surface with Sn positioned above the top layer Si atoms. There has been extensive studies of the reconstructions on Si surfaces induced by adsorption of the group III metals Al, Ga and In. Recently, this has been expanded to Tl, i.e., the heaviest element in that group. Tl is different from the other elements in group III since it exhibits a peculiar behavior of the 6s2 electrons called the “inert pair effect”. This could lead to a valence state of either 1+ or 3+. In this work, core-level PES is utilized to find that, at coverages up to one ML, Tl exhibits a 1+ valence state on Si(111), in contrast to the 3+ valence state of the other group III metals. Accordingly, the surface band structure of the 1/3 ML √3 x √3 reconstruction is found to be different in the case of Tl, compared to the other group III metals. The observations of a 1+ valence state are consistent with ARPES results from the Si(001):Tl surface at one ML. There, six surface state bands are seen. Through comparisons with a calculated surface band structure, four of those can be identified. The two remaining bands are very similar to those observed on the clean Si(001) surface.

Electronic structure of the thallium-induced2×1reconstruction on Si(001)

With a Tl coverage of one monolayer, a $2\ifmmode\times\else\texttimes\fi{}1$ reconstruction is formed on the Si(001) surface at room temperature. In this study, low-temperature angle-resolved photoelectron spectroscopy (ARPES) data reveal four surface state bands associated with this Tl induced reconstruction. Calculated surface state dispersions, obtained using the ``$\text{pedestal}+\text{valley-bridge}$'' model, are found to be similar to those obtained using ARPES. Inclusion of spin-orbit coupling in the calculations is found to be important to arrive at these results. A known effect of the strong spin-orbit coupling is the reluctance of the $\text{Tl}\text{ }6{s}^{2}$ electrons to participate in the bonding, i.e., the inert pair effect. In the calculations, inclusion of spin-orbit coupling results in a $\ensuremath{\sim}5\text{ }\text{eV}$ downshift of the $\text{Tl}\text{ }6{s}^{2}$ electrons.

Surface core-level shifts on clean Si(001) and Ge(001) studied with photoelectron spectroscopy and density functional theory calculations

In this work, a selection of unresolved topics regarding the electronic and atomic structures of Si and Ge surfaces, both clean ones and those modified by metal adsorbates, are addressed. The results presented have been obtained using theoretical calculations and experimental techniques such as photoelectron spectroscopy (PES), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Si(001) surfaces with adsorbed alkali metals can function as prototype systems for studying properties of the technologically important family of metal-semiconductor interfaces. In this work, the effect of up to one monolayer (ML) of Li on the Si(001) surface is studied using a combination of experimental and theoretical techniques. Several models for the surface atomic structures have been suggested for 0.5 and 1 ML of Li in the literature. Through the combination of experiment and theory, critical differences in the surface electronic structures between the different atomic models are identified and used to determine the most likely model for a certain Li coverage. In the literature, there are reports of an electronic structure at elevated temperature, that can be probed using angle resolved PES (ARPES), on the clean Ge(001) and Si(001) surfaces. The structure is quite unusual in the sense that it appears at an energy position above the Fermi level. Using results from a combined variable temperature ARPES and LEED study, the origin of this structure is determined. Various explanations for the structure that are available in the literature are discussed. It is found that all but thermal occupation of an ordinarily empty surface state band are inconsistent with our experimental data. In a combined theoretical and experimental study, the surface core-level shifts on clean Si(001) and Ge(001) in the c(4×2) reconstruction are investigated. In the case of the Ge 3d core-level, no previous theoretical results from the c(4×2) reconstruction are available in the literature. The unique calculated Ge 3d surface core-level shifts facilitate the identification of the atomic origins of the components in the PES data. Positive assignments can be made for seven of the eight inequivalent groups of atoms in the four topmost layers in the Ge case. Furthermore, a similar, detailed, assignment of the atomic origins of the shifts on the Si surface is presented that goes beyond previously published results. At a Sn coverage of slightly more than one ML, a 2√3 × 2√3 reconstruction can be obtained on the Si(111) surface. Two aspects of this surface are explored and presented in this work. First, theoretically derived results obtained from an atomic model in the literature are tested against new ARPES and STM data. It is concluded that the model needs to be revised in order to better explain the experimental observations. The second part is focused on the abrupt and reversible transition to a molten 1×1 phase at a temperature of about 463 K. ARPES and STM results obtained slightly below and slightly above the transition temperature reveal that the surface band structure, as well as the atomic structure, changes drastically at the transition. Six surface states are resolved on the surface at low temperature. Above the transition, the photoemission spectra are, on the other hand, dominated by a single strong surface state band. It shows a dispersion similar to that of a calculated surface band associated with the Sn-Si bond on a 1×1 surface with Sn positioned above the top layer Si atoms. There has been extensive studies of the reconstructions on Si surfaces induced by adsorption of the group III metals Al, Ga and In. Recently, this has been expanded to Tl, i.e., the heaviest element in that group. Tl is different from the other elements in group III since it exhibits a peculiar behavior of the 6s2 electrons called the “inert pair effect”. This could lead to a valence state of either 1+ or 3+. In this work, core-level PES is utilized to find that, at coverages up to one ML, Tl exhibits a 1+ valence state on Si(111), in contrast to the 3+ valence state of the other group III metals. Accordingly, the surface band structure of the 1/3 ML √3 x √3 reconstruction is found to be different in the case of Tl, compared to the other group III metals. The observations of a 1+ valence state are consistent with ARPES results from the Si(001):Tl surface at one ML. There, six surface state bands are seen. Through comparisons with a calculated surface band structure, four of those can be identified. The two remaining bands are very similar to those observed on the clean Si(001) surface.

Rationalizing and functionalizing stannaspherene: Very stable stannaspherene “alloys”

It is illustrated here by ab initio calculations based on density functional theory and other high level methods that the high stability of the icosahedral Sn(12) (2-) dianion known as stannaspherene, reflects stability toward ionization rather than cohesion. This could be also connected with novel fluxional rearrangements and paths of Sn(12) (1-) leading eventually to Sn(12) (2-) involving charge transfer. In view of the very similar structural and electronic properties with the corresponding isovalent borane (B(12)H(12))(2-), it is demonstrated that stannaspherene can be further rationalized and functionalized on the basis of an isolobal analogy between group 14 clusters and isovalent boranes, carboranes, and bisboranes. Such analogy is of the same nature with analogous isolobal and isovalent similarities between silicon, hydrogenated silicon-carbon clusters and deltahedral boranes and carboranes, which the present author, scoptically and synoptically, has described as the "boron connection." It is predicted and verified theoretically: First, that the isovalent Bi(2)Sn(10) and Sb(2)Sn(10) clusters, considered as the microscopic analogs of tin-bismuth alloys, are very stable (more stable than stannaspherene itself) very symmetric and isolobal to Sn(12) (2-); and second, that embedded clusters of the form M@Sn(12) (2-), M@Bi(2)Sn(10), M=Pt,Pd are very stable and highly symmetrical (I(h) and D(5d) respectively) with large highest occupied-lowest unoccupied molecular orbital gaps and very large embedding energies of the order of 5-6 eV. It is furthermore predicted that Pt@Sn(12) (2-) and Pt@Bi(2)Sn(10) can be synthesized in view of their higher stability compared to Pt@Pb(12) (2-) which has already been synthesized. The marginal energy difference of 0.03 eV between the meta- and the para-isomer of Bi(2)Sn(10) indicates a fluxional behavior with respect to Bi-Sn interchange which should be related with the Sn(12) (1-) fluxionality leading eventually to Sn(12) (2-). This rearrangement is also associated with a strange aromatic behavior. The same type of Bi-Sn fluxionality is also encountered in higher energy structures. Due to the "inert pair effect" in tin, the validity of the isolobal analogy is much stronger and fully valid compared to isovalent species based on germanium or silicon, such as Ge(12) (2-), Bi(2)Ge(10), and Ge(10)C(2)H(2) and Si(12) (2-), Bi(2)Si(10), and Si(10)C(2)H(2). The present ideas are in full agreement with available experiments and suggest even further functionalization of stannaspherene, analogous to metaloboranes, metalocarboranes, and stannaboranes with several potential applications.

Silicon−Bismuth and Germanium−Bismuth Clusters of High Stability

Mixed metal-semiconductor clusters of the form Bi2Si(n-2) and Bi2Ge(n-2), n = 3-8, 12, are studied theoretically by ab initio methods including density functional theory with the hybrid B3LYP functional, second-order perturbation, and coupled cluster CCSD(T) theory using the doubly polarized TZV2P basis sets. These clusters are characterized by high stability and symmetry and relatively large highest occupied-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. It is shown that the lower energy structures of these clusters and their bonding and electronic characteristics are fully compatible with very powerful stability rules and structural laws similar to the ones for the corresponding isovalent boranes, carboranes, and bisboranes. This is particularly true for the Bi2Ge(n-2) clusters, the global minima of which are identical to the ones of the corresponding isovalent carboranes and bisboranes, obtained by proper substitutions from the corresponding dianions. The Bi2Si(n-2) clusters for n > 8, although isolobal to the homologous Si(n)(2-) clusters, are not fully isolobal with the corresponding boranes, carboranes, and bisboranes as the n = 3-8 clusters. For the Bi2Si(n-2) clusters with n < or = 8, a strange odd-even effect is observed. For the even-n Bi2Si(n-2) clusters, in contrast to those of odd-n, the structures obtained from the Si(n)(2-) dianions through the so-called "boron connection" compete with those obtained from substitutions on the corresponding neutral cluster structures. The even-n Bi2Si(n-2) clusters are fluxional or isomerizable. The Bi2Si4 cluster in particular is fluxional and structurally similar to the corresponding Si6 cluster. The differences between the Bi2Si(n-2) and Bi2Ge(n-2) clusters are attributed mainly to the inert pair effect in Bi2Ge(n-2). For this reason, although the lowest energy structure of the Bi2Si(10) cluster has relatively low (C(s)) symmetry derived from the corresponding Si(12)(2-) structure, the homologous Bi2Ge10 cluster is characterized by high D5d symmetry obtained from the Ge(12)(2-) dianion of full icosahedral symmetry, similar to the corresponding (B12H12)2- borane. The present rationalization of the silicon/germanium-bismuth clusters is very promising for deeper understanding and future functionalization toward useful chemical and technological applications.

Electronic structure of stannite‐type Cu2ZnSnSe4 by first principles calculations

AbstractThe electronic structure of stannite‐type Cu2ZnSnSe4 (CZTSe) was studied by first‐principles calculations. The valence band maximum (VBM) is an antibonding of Cu 3d and Se 4p orbitals. The conduction band minimum (CBM) is an antibonding Sn 5s and Se 4p orbitals. The Zn atom does not affect the VBM or the CBM in CZTSe. The Cu‐Se is a weak covalent bonding because both occupied, and the bonding or bital of Cu 4s and Se 4p is occupied and the antibonding orbital is unoccupied. The band structure of CZTSe has a partially localized Sn 5s orbital. The electrons of Sn 5s do not greatly contribute to the chemical bonding because of an inert pair effect. The electronic structure of CZTSe is discussed in comparison with that of CuInSe2. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Success and pitfalls of the Sin−2C2H2–C2Bn−2Hn isolobal analogy: Depth and breadth of the boron connection

The extent and depth of the so-called boron connection suggested recently by the present author [J. Chem. Phys. 128, 184305 (2008)] for the Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) isovalent species [following similar connection of (Si(n))(2-)-(B(n)H(n))(2-) dianions] are further investigated by considering larger species up to n=12 and additional isovalent moieties. Here we consider, using density functional and coupled clusters theory, isovalent Si(n-2)C(2)H(2) and Ge(n-2)C(2)H(2) clusters, in comparison to the corresponding C(2)B(n-2)H(n) carboranes and to each other for n=3-8, 12. Special attention is given to the n=8 species, where the corresponding carborane is highly fluxional, and to n=12, where the "parent structures" of the corresponding (Si(12))(2-) and (B(12)H(12))(2-) dianions have drastically different symmetries. The structures generated by substitutions on (Si(12))(2-), as well as (B(12)H(12))(2-), are compared and interrelated for both C(2)B(10)H(12) and Si(10)C(2)H(12). The carborane generated from the C(s) (Si(12))(2-) dianion provides an illustrative example of carborane rearrangements, reverting after geometry optimization to the second lowest C(2)B(10)H(12) meta isomer of C(2v) symmetry, 0.12 eV above the lowest energy para isomer. This demonstrates the amphidirectional character of the boron connection. It is found that n=8 is the upper limit of the n range in which the isolobal Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) analogy is fully operative not only for the lowest but also for the second and third lowest energy states. For n<or=8 all three pairs of structures are isovalent, isostructural, and isolobal. For large values of n (n>8) the boron connection, although not fully isolobal, is still valid in a broader and more general sense, still providing deeper and broader fundamental understanding and insight for both species. It was also found that the Ge(n-2)C(2)H(2) clusters are not fully homologous (isolobal) neither to the isovalent Si(n-2)C(2)H(2) clusters nor to the corresponding C(2)B(n-2)H(n) carboranes, preferring structures in which the two carbon atoms are always in adjacent positions. This is attributed to the relative weakness of the Ge-Ge and Ge-C bonds relative to Si-Si and Si-C bonds and the "inert pair effect."

Autocatalytic degradation of white phosphorus with silylenes

White phosphorus (P(4)) is prone to undergo degradation by nucleophiles and is reluctant to do so with electrophiles. Silylenes possess a strong singlet character but at the same time bear a largely inert lone pair orbital at the silicon atom. Thus they predominantly react in a similar way to electrophilic carbenes. Due to the poor pi-character of the P-P bonds in white phosphorus, the overlap with the empty orbital for the electrophilic silylene is less facile and results in a relatively large barrier for the addition reaction. The electrophilic approach of the silylene to white phosphorus is catalyzed by addition of a second P(4), forming a trigonal bipyramidal transition state geometry. Its stability towards fragmentation is essentially lower than that of the silyl cation. The entropy contributions for bimolecular versus termolecular reactions are discussed.

Preparation and Crystal Structures of Bismuth Technetates: A New Metal Oxide System

Two new oxides have been unambiguously identified as Bi2Tc2O7-delta with delta = 0.14(1) and Bi3TcO8 through X-ray absorption near-edge structure spectroscopy and neutron powder diffraction. The compound Bi2Tc2O7-delta has a cubic pyrochlore-type structure with a = 10.4746(1) A, space group Fd3m (origin choice 2), and Z = 8. The compound Bi3TcO8 is also cubic, a = 11.5749(1) A, space group P2(1)3, Z = 8, and has a fluorite-related crystal structure. In Bi2Tc2O7-delta the Tc(IV) cations are octahedrally coordinated, whereas in Bi3TcO8 the Tc(VII) cations are tetrahedrally coordinated. A third new phase, probably Bi3Tc3O11, could not be obtained pure, but preliminary X-ray powder diffraction data affords a primitive cubic lattice with a = 9.3433(1) A. On the basis of structural similarities between Bi2Tc2O7-delta and closely related oxides, Bi2Tc2O7-delta is expected to be a metallic oxide with Pauli paramagnetism. Electronic structure calculations of both Bi2Tc2O7-delta and Bi3TcO8 further support metallic conductivity in the former and insulating behavior in the latter. The inert pair effect of the Bi cations on the crystal structures of Bi2Tc2O7-delta and Bi3TcO8 is also described. In addition, calculations of the valence electron localization function for Bi2Tc2O7-delta and Bi3TcO8 provide further visualization of the Bi 6s(2) lone pair electrons in the real space of the crystal structures.

Ab initiostudy of thallium nanoclusters onSi(111)−7×7

We have studied the stability of a thallium nanocluster of various numbers of atoms $(N=1,2,\dots{},10)$ on a $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ substrate using density functional theory total energy calculations. We have compared it with clusters of other group III elements (Al, Ga, and In). Thallium is found to be unstable with the triangular cluster, which has been known to be stable for other group III elements. Instead, a slightly different structure, in which Si atop atoms are lower than thallium atoms in height by $2.56\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, was found to be quite stable. Such an abnormal structure originates from the inert pair of $6{s}^{2}$ electrons due to the significant spin-orbit interaction. The initial relaxed $N=6$ Tl cluster continues to grow with increasing $N$ up to $N=9$ in the faulted-half unit cell, which is consistent with experimental observations.

An ab initio investigation of bismuth hydration

A series of geometry, frequency, and energy calculations of aquabismuth(III) complexes were carried out at up to the MP2/CEP-121G* level. A thorough examination of all species up to and including enneacoordinate species was carried out. The structures of the complexes are compared with experimental data where available. Deviations from the most highly symmetric structures are consistent with the inert-pair effect. In addition, protonated bismuth–oxo clusters of the stoichiometry Bi6O8–n(OH)n(2+n)+, n = 0–4 are examined. The Raman spectrum of bismuth perchlorate with added perchloric and hydrochloric acid is measured.Key words: bismuth, aqua complexes, ab initio, Raman spectra.

Atomic and electronic structure ofTl∕Ge(111)−(1×1): LEED and ARPES measurements and first-principles calculations

We have investigated the atomic and electronic structure of the $\mathrm{Tl}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ surface. The Tl adsorption site was determined to be the ${T}_{4}$ site by dynamical low-energy electron diffraction $I\text{\ensuremath{-}}V$ analysis. The surface electronic structure was measured using angle-resolved photoelectron spectroscopy. By comparing the observed band structure with the first-principles calculation, we revealed that only the $\mathrm{Tl}\phantom{\rule{0.2em}{0ex}}6p$ electrons are involved in the Tl--Ge bonding and that hybridization of $\mathrm{Tl}\phantom{\rule{0.2em}{0ex}}6{s}^{2}6p$ valence electrons is negligible. This indicates that the inert-pair effect plays a decisive role in the formation of the $(1\ifmmode\times\else\texttimes\fi{}1)$ surface. On the other hand, a surface band is not fully occupied but forms a small hole pocket around $\overline{\ensuremath{\Gamma}}$. We propose that it is caused by a small amount of remaining defects due to the small energy gap. Our calculation including spin-orbit interaction indicates the existence of a large spin-orbit splitting in the lowest unoccupied surface band of $\mathrm{Tl}\phantom{\rule{0.2em}{0ex}}6{p}_{x}+6{p}_{y}$ character.

A Main‐Group Analogue of Housene: The Subtle Influence of the Inert‐Pair Effect in Group 15 Clusters

The carbon–phosphorus–antimony cage of P2C2tBu2SbCl (see picture) adopts an envelope-type structure which is quite distinct from its phosphorus analogue, the tricyclic compound P2C2tBu2PCl. DFT calculations reveal that subtle changes in electronic structure as a result of the inert-pair effect lead to profound differences in structural and dynamic behavior.

A Main‐Group Analogue of Housene: The Subtle Influence of the Inert‐Pair Effect in Group 15 Clusters

Der Kohlenstoff-Phosphor-Antimon-Käfig von P2C2tBu2SbCl (siehe Bild) liegt in einer Schmetterlingsstruktur vor, während das Phosphoranalogon P2C2tBu2PCl die tricyclische Struktur bevorzugt. DFT-Rechnungen zeigen, dass schon kleine Änderungen der elektronischen Struktur wegen des inerten Ionenpaars zu deutlichen Unterschieden in Struktur und Dynamik führen.