Main Group Atoms Research Articles

Elemental tellurium in organoelement synthesis of tellurium-containing organometallics and telluracycles

An overview presents the known methods of using elemental tellurium in the synthesis of various acyclic and cyclic, saturated and unsaturated compounds containing Te–E bonds (E = main group atom or transition metal atom). These methods include the insertion of tellurium into the C–E (E = halogen, Al, Ga, Sn, Zr) and P–E (E = Li, P, Si, Ge and Sn) and Sn–Sn single bonds and the addition to the E = E (E = Si, Ge and Sn) double bond, and PC triple bond, as well as the breakdown of the CC double bond and the exchange of metal (Hg, Ag) and non-metal (B, I) for Te. Reactions with carbenes, their heavier (Si, Ge, Sn) analogs and free radicals are also discussed, as are the oxidation of organophosphorous (III) compounds, silanes and germanes with Te and the direct cyclisation of alkanes, arenes and heteroarenes containing two halogenomethyl fragments with elemental Te. Three-component heterocyclizations involving elemental tellurium and leading to telluracycles are also discussed. The reactions mentioned above are compared to those involving lighter chalcogens.

Non-covalent interactions and charge transfer in the CO activation by low-valent group 14 complexes.

The CO activation by low-valent group 14 catalysts encompasses the rupture of varied covalent bonds in a single transition state through a concerted pathway. The bond between the central main group atom and the hydride in the complex is elongated to trigger the formation of the C-H bond with CO accompanied by the concomitant formation of the E-O bond between the complex and CO to lead the corresponding formate product. Prior studies have established that besides the apolar nature of CO , its initial interaction with the complex is primarily governed by electrostatic interactions. Notably, other stabilizing interactions and the transfer of charge between catalysts and CO during the initial phases of the reaction have been ignored. In this study, we have quantified the non-covalent interactions and charge transfer that facilitate the activation of CO by group 14 main group complex. Our findings indicate that electrostatic interactions predominantly stabilize the complex and CO in the reactant region. However, induction energy becomes the main stabilizing force as the reaction progresses towards the transition state, surpassing electrostatics. Induction contributes about 50% to the stabilization at the transition state, followed by electrostatics (40%) and dispersion interactions (10%). Atomic charges calculated with the minimal basis iterative stockholder (MBIS) method reveal larger charge transfer for the back-side reaction path in which CO approaches the catalysts in contrast to the front-side approach. Notably, it was discovered that a minor initial bending of CO to approximately initiates the charge transfer process for all systems. Furthermore, our investigation of group 14 elements demonstrates a systematic reduction in both activation energies and charge transfer to CO while descending in group 14. All studied reactions were characterized along the reaction coordinate obtained with the intrinsic reaction coordinate (IRC) methodology at the M06-2X/6-31g(d,p) level of theory. Gibbs free energy in toluene was computed using electronic energies at the DLPNO-CCSD(T)/cc-pVTZ-SSD(E) level of theory. Vibrational and translational entropy corrections were applied to provide a more accurate description of the obtained Gibbs free energies. To better characterize the changes in the reaction coordinate for all reactions, the reaction force analysis (RFA) has been employed. It enables the partition of the reaction coordinate into the reactant, transition state, and product regions where different stages of the mechanism occur. A detailed characterization of the main non-covalent driving forces in the initial stages of the activation of CO by low-valent group 14 complexes was performed using symmetry-adapted perturbation theory (SAPT). The SAPT0-CT/def2-SVP method was employed for these computations. Charge transfer descriptors based on atomic population using the MBIS scheme were also obtained to complement the SAPT analyses.

Antiprotozoal Pt(II) Complexes as Luminophores Bearing Monodentate P/As/Sb-Based Donors: An X-ray Diffractometric, Photoluminescence, and 121Sb-Mössbauer Spectroscopic Study with TD-DFT-Guided Interpretation and Predictive Extrapolation toward Bi.

In this study, it is demonstrated that the radiative rate constant of phosphorescent metal complexes can be substantially enhanced using monodentate ancillary ligands containing heavy donor atoms. Thus, the chlorido coligand from a Pt(II) complex bearing a monoanionic tridentate C^N*N luminophore ([PtLCl]) was replaced by triphenylphosphane (PPh3) and its heavier pnictogen congeners (i.e., PnPh3 to yield [PtL(PnPh3)]). Due to the high tridentate-ligand-centered character of the excited states, the P-related radiative rate is rather low while showing a significant boost upon replacement of the P donor by heavier As- and Sb-based units. The syntheses of the three complexes containing PPh3, AsPh3, and SbPh3 were completed by unambiguous characterization of the clean products using exact mass spectrometry, X-ray diffractometry, bidimensional NMR, and 121Sb-Mössbauer spectroscopy (for [PtL(SbPh3)]) as well as steady state and time-resolved photoluminescence spectroscopies. Hence, it was shown that the hybridization defects of the Vth main-group atoms can be overcome by complexation with the Pt center. Notably, the enhancement of the radiative rate constants mediated by heavier coligands was achieved without significantly influencing the character of the excited states. A rationalization of the results was achieved by TD-DFT. Even though the Bi-based homologue was not accessible due to phenylation side reactions, the experimental data allowed a reasonable extrapolation of the structural features whereas the hybridization defects and the excited state properties related to the Bi-species and its phosphorescence rate can be predicted by theory. The three complexes showed an interesting antiprotozoal activity, which was unexpectedly notorious for the P-containing complex. This work could pave the road toward new efficient materials for optoelectronics and novel antiparasitic drugs.

Atomically Dispersed p-Block Aluminum-Based Catalysts for Oxygen Reduction Reaction.

The main group metals are commonly perceived as catalytically inert in the context of oxygen reduction reactions (ORR) due to the delocalized valence orbitals. Regulating the local environment and structure of metal center coordinated by nitrogen ligands (M-Nx) is a promising approach to accelerate catalytic dynamics. Herein, we, for the first time, report the atomically dispersed Al catalysts coordinated with N and C atoms for 4-electron ORR. The axial coordinated pyrrolyl N group (No) is constructed in the Al-N4-No moiety to regulate the p-band structure of Al center, effectively steering the local environment and structure of the square planar Al-N4 sites, which typically exhibit too strong interaction with ORR intermediates. The dynamic covalency competition of axial Al-No and Al-O bonding could endow the Al center with moderate hybridization between Al 3p orbital and O 2p orbital, alleviating the binding energy of ORR intermediates. The as-prepared Al-N4-No electrocatalyst exhibits excellent ORR activity, selectivity, and durability, along with the rapid kinetics as demonstrated by in situ Raman spectroscopy. This work offers a fundamental comprehension of the fine regulation on p-band and guides the rational design of main-group metal-based single atom catalysts.

Towards Catalytic C–H Activation Using Main Group Elements

AbstractCatalytic C–H activation reactions are now established as a means to directly transform organic molecules and are commonly associated with metals such as palladium, rhodium, ruthenium and iridium. This Account will describe a short number of reports demonstrating that structures containing main group elements can facilitate C–H activation processes. In particular, boron-based catalysts can promote catalytic arene C–H borylation reactions, and an emerging approach using phosphenium ions can also cleave sp2 C–H bonds. These processes use a Lewis acidic main group atom combined with a pendant base to cleave C–H bonds, which compares with metal-catalyzed reactions that proceed via concerted metalation deprotonation mechanisms.1 Introduction2 Metal-Catalyzed C–H Activation via CMD/AMLA Mechanisms3 C–H Borylation via Boron-Based Catalysts4 C–H Activation Using Phosphenium Ions5 Conclusions

Main Group Analogs of Dichalcogeniranes

AbstractThis Mini Review highlights recent advances in the preparation of main group analogs of dichalcogeniranes. The three–membered ring compounds are an intriguing class of compounds in terms of their strained structures and high reactivity. As for three–membered rings composed of a carbon and two group 16 atoms, dioxiranes have long been utilized as oxidants, while some dithiiranes have been synthesized. In contrast, diselenirane and ditellurirane remain elusive. On the other hand, the chemistry of three–membered rings consisting of a non–carbon main group atom and two group 16 atoms has recently gained significant track. These emerging three–membered ring compounds were characterized by X‐ray diffraction analysis on many occasions, revealing unique molecular structures with very long chalcogen–chalcogen single–bond distances in some cases. A common reactivity is dechalcogenation reactions using phosphine reagents, which provide access to the corresponding double‐bonded compounds. This minireview covers recent advances in the synthesis of main group analogs of dioxirane, dithiirane, diselenirane, and even ditellurirane.

Organofunctionalized borotungstate polyoxometalates as tunable photocatalysts for oxidative dimerization of amines.

Organofunctionalized borotungstate Keggin polyoxometalates, ( n Bu4N)3H[HBW11O39(P(O)Ph)2] (PBW11), ( n Bu4N)3H[HBW11O39(As(O)Ph)2] (AsBW11), and ( n Bu4N)4[HBW11O39(PhSiOSiPh)] (SiBW11), were synthesized and structurally characterized. Cyclic voltammetry showed that the electronic properties of the clusters are dependent on the nature of the appended main group atoms (P, As, or Si). The first reduction potentials were found to shift positively with respect to that of the unmodified parent species ( n Bu4N)5[BW12O40], with PBW11 showing the largest shift at +100 mV. All clusters were evaluated as photocatalysts for the oxidative dimerization of amines where the organophosphonate hybrid PBW11 was found to be the most active. This study demonstrates how organofunctionalization of polyoxometalates may be used to tune and improve their performance as photocatalysts for organic reactions.

Functionalised [Ge9Ni] Clusters as Homogeneous Single‐Site Catalysts for Olefin Isomerisation Reactions

AbstractSingle‐site catalysis is an increasingly vital strategy to optimise heterogeneous catalytic reactions. In an ideal case, the nature and the population of catalytically active sites are all identical, which is impossible to realise on a solid support. The idea of single site catalysts is transferred from heterogeneous to homogeneous catalysis by the incorporation of a transition metal with oxidation state 0 in the surface of a small, soluble, molecular, homoatomic germanium atom cluster that also comprises main group atoms with oxidation state close to zero. An optimised synthetic protocol for four cluster compounds Hyp3Et[Ge9Ni]PR3 (R=Ph, ptolyl, iPr, Me) is presented, in which the transition metal is embedded in Ge atoms of a polyhedral cluster. The products were characterised by NMR spectroscopy, LIFDI/MS and elemental analysis and also structurally characterised for R=iPr and Me by single crystal X‐ray structure determination comprising a closo‐[Ge9Ni] core. The catalytic potential is investigated for various olefin isomerisation reactions. The formation of an active metal site is realized by ligand dissociation, which is observed for PPh3. DFT calculations of Hyp3Et[Ge9Ni]PR3 (R=Ph, iPr, Me), Hyp3Et[Ge9Ni](tolyl) and Hyp3Et[Ge9Ni](C6H12) show that ligand exchange with substrate molecules occurs during the catalytic reaction. Since the same principle, namely a bare metal atom site at the cluster surface, applies as in single‐site heterogeneous catalysis and since in contrast to classical homogeneous catalysis the metal atom is embedded in a cluster surface, the abbreviation SSHoC for Single Site Homogeneous Catalyst is proposed for this catalyst type. Since each metal atom is catalytically active, SSHoC enable a considerable increase in efficiency of catalysts and thus allow for sustainable use of expensive and less abundant transition metals.

Graphdiyne/main-group metal elements sandwich structure as a promising single atom electrocatalyst for nitrogen reduction: A theoretical investigation

In an effort to limit global warming and CO2 emission, ammonia has become a viable hydrogen carrier and alternative fuel. Electrocatalytic nitrogen reduction (e-NRR) has gained much attention as a mean to produce ammonia under mild reaction conditions. Consequently, the advancement of highly efficient and selective catalysts for e-NRR is key in the development of clean energy. Based on the "chainmail" protection catalyst proposed by recent studies, we used first-principles calculations to design a catalyst composed of a single main-group metal atom sandwiched between BN-doped graphdiyne and graphdiyne sheets. The structures, labeled BN-M-G (M = Li, Na, K, Mg, Ca and Al), were investigated as potential catalysts for e-NRR. After screening the most favorable configurations for N2 adsorption and activation, we further explored their e-NRR catalytic properties including efficiency and selectivity. Our results show that the optimal e-NRR path on BN-Mg-G and BN-Al-G is the alternating path with a limiting voltage of -0.47 V and -0.67 V, respectively. In contrast to the competing reaction, hydrogen evolution reaction (HER), both catalysts show high selectivity towards e-NRR. Our work shows that through rational design and electronic structure regulation, main-group-metals-based catalysts can have comparable catalytic activity as transition metal catalysts. This present study offers new insights for the development of low-cost and environmetally-friendly main group metal catalysts.

Two-dimensional GeSe monolayer doped with single main-group element atom for hazardous gas detection: A first-principles study

Currently, the identification of sensing materials that can selectively and rapidly respond to hazardous gases is urgent for safeguarding the environment and human life. The adsorption of hazardous gas molecules (NO, NO2, SO2, CO, and H2S) on the GeSe monolayer doped with single main-group element atom is investigated and compared based on first-principles density functional theory (DFT). NO2 has the highest adsorption energy and charge transfer adsorbed on both pristine GeSe and doped GeSe monolayer. At ambient temperature, for any doped GeSe monolayers adsorbing gas molecules (including H2O, O2, and N2), the occupation function value of NO2 is the highest. The analysis of density of states (DOS) and band structure indicates that NO2 interacts with different modified GeSe monolayers in terms of atomic orbitals. Moreover, Al-GeSe has the largest change of work function before and after the adsorption of NO2, meaning that Al-GeSe has the greatest response to NO2. current-voltage (I-V) characteristics demonstrate that Al-GeSe exhibits a remarkable current change after the adsorption of NO2. These results illustrate that Al-GeSe has a strong physical and chemical interaction with NO2, yielding a high response and selectivity, which is anticipated to be the optimal gas sensing material for NO2.

Transforming main-group elements into highly active single-atom electrocatalysts for oxygen reduction reactions

Despite their potential as electrocatalysts, the catalytic mechanisms of main-group single-atom catalysts (MSACs) remain largely unexplored. To address this knowledge gap, we conducted a series of first-principles calculations to investigate a repertoire of MSACs that incorporated 17 active main-group atoms and 8 carbon supports, systematically screening the optimal catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) via activity-adsorption scaling relations. Our findings reveal that as the single-atom in MSACs moves down the periodic table, their adsorption capabilities become even more enhanced. Moreover, MSACs derived from the fifth and sixth periods exhibit mild adsorption capabilities and robust catalytic activity. In addition, our study identifies a linear correlation between the adsorption energy of *OH intermediates, the p-band center, and the electronegativity of the main-group atom in MSACs, serving as an efficacious ORR activity descriptor and allowing for the prediction of novel catalysts. Our analysis further illuminated two distinct interaction patterns of MSACs with oxygenated species, elucidated through studying the px- and pz-band centers and the resulting effects of orbital hybridization. Moreover, we successfully delineated the regulator role of carbon support species in enhancing the adsorption abilities of MSACs. By strategically pairing the appropriate main-group atoms with suitable carbon supports, we propose an innovative approach for the design of high-performance MSAC catalysts.

Metal-metal bonds in Zintl clusters: Synthesis, structure and bonding in [Fe2Sn4Bi8]3– and [Cr2Sb12]3–

We report here the synthesis and characterization of two new members of the M2E12 family of endohedral Zintl clusters, [Fe2Sn4Bi8]3– and [Cr2Sb12]3–, both of which contain open-shell metal dimers encapsulated inside a triple-decker cluster of main-group atoms. The 75-electron [Fe2Sn4Bi8]3– cluster has a D4h-symmetric structure, while [Cr2Sb12]3–, despite having the same 75-electron count, is strongly distorted to a geometry that resembles a CrSb8 crown capped by a CrSb4 unit. The structural differences between the two are driven by the increasing availability of 3d electron density in the earlier transition metal, which leads, ultimately, to different electronic configurations in the two clusters. The trends precisely mirror those observed in the ME10 and ME12 families containing a single transition metal ion.

Icosahedral Pd Nanocrystal Catalysts with Dispersed Bi Atoms via Photochemical Route for Enhanced Formic Acid Oxidation Reaction

AbstractThe decoration of noble metal catalysts with specific main group or transition metal atoms offers an effective way to improve the catalytic performance through electronic structure regulation and active site optimization without losing the active surface area. Herein, icosahedral Pd nanocrystals catalysts with highly dispersed Bi atoms decoration have been realized by a simple photochemical reduction approach. The decoration of Bi atoms maintains the original Pd cyclic penta‐twinned structure. The deposition amount of Bi atoms, which greatly affect the catalytic performance, is fundamentally regulated by the irradiation time. Compared with pure Pd, the Pd−Bi icosahedral nanocrystals show significant improvement in electrocatalytic activity towards formic acid oxidation. Besides, the catalytic performance is correlated with the content of Bi. The optimal performance is occurred at the Bi content of 2.86%.

Theoretical design of Ti2-based magnetic shape memory alloys from first-principles

Magnetic shape memory alloys (MSMAs) typically comprise transition metal and main group atoms. We developed a class of Ti2-based Heusler MSMAs formed by transition metal and metalloid elements using the first-principles calculation method based on density functional theory. For all the studied Ti2YZ (Y = Cr, Mn, Fe, and Co; Z = B, Si, Ge, As, Sb, and Te) Heusler alloys, the calculated formation energies in the austenitic phase show that, except for Ti2CrTe, all alloys have thermodynamic stability. Based on a comprehensive analysis of the total energy difference between austenite and martensite, the density of states (DOS), and the bulk mechanical properties, we predict that of all the Ti2-based alloys studied here, 17 alloys are likely to undergo a martensitic phase transition. The DOS of the cubic structures reveals that the substantial peak structures around the Fermi level, mainly formed by strong hybridization between the Ti atom and its neighbor Y and Z atoms, lead to cubic instability. Compared with the typical MSMA of Ni2MnGa, most Ti2YZ alloys possess a larger martensitic driving force, a higher martensitic transition temperature, and better ductility. Therefore, our study could inspire the development and application of new Ti2-based MSMAs.

Quest of Quadruple Bonding Between Two Main-Group Atoms in AeB- and AeC (Ae=Ca, Sr, Ba) and the Role of d Orbitals of Heavier Alkaline-Earth Atoms in Covalent Interactions.

Quantum chemical calculations using ab initio methods at the MRCI+Q(6,8)/def2-QZVPP and CCSD(T)/def2-QZVPP levels as well as density functional theory are reported for the diatomic molecules AeB- and isoelectronic AeC (Ae=Ca, Sr, Ba). The boride anions AeB- have an electronic triplet (3 Σ- ) ground state. The quintet (5 Σ- ) state is 5.8-12.3 kcal/mol higher in energy and the singlet (1 Δ) state is 13.1-15.3 kcal/mol above the triplet. The isoelectronic AeC molecules are also predicted to have a low-lying triplet (3 Σ- ) state but the quintet (5 Σ- ) state is only 2.2 kcal/mol (SrC) and 2.9 kcal/mol (CaC) above the triplet state. The triplet (3 Σ- ) and quintet (5 Σ- ) states of BaC are nearly isoenergetic. All systems have rather strong bonds. The calculated bond dissociation energies of the triplet (3 Σ- ) state are between De =38.3-41.7 kcal/mol for AeB- and De =49.4-57.5 kcal/mol for AeC. The barium species have always the strongest bonds whereas the calcium and strontium compounds have similar BDEs. The bonding analysis indicates that there is little charge migration in AeB- in the direction Ae→B- where the alkaline earth atoms carry positive charges between 0.09 e-0.22 e. The positive charges at the Ae atoms are much larger in AeC where the charge migration Ae→C is between 0.90 e-0.91 e. A detailed analysis of the interatomic interactions with the EDA-NOCV method shows that all diatomic species AeB- and AeC are built from dative interactions between Ae (1 S, ns2 ) and B- or C (3 P, 2 s2 2pπ 1 2pπ' 1 ). The eventually formed bonds in AeC are better described in terms of interactions between the ions Ae+ (2 S, ns1 )+C- (4 S, 2 s2 2pπ 1 2pπ' 1 2pσ 1 ). Inspection of the orbital interactions suggests that the alkaline earth atoms Ca, Sr, Ba use mainly their (n-1)d AOs besides the (n)s AOs for the covalent bonds. This creates a second energetically low-lying σ-bonding MO in the molecules, which feature valence orbitals with the order ϕ1 (σ-bonding)<ϕ2 (σ-bonding)<ϕ3 (degenerate π-bonding). All four occupied valence MOs of AeB- and AeC are bonding orbitals. Since the degenerate π orbitals ϕ3 are only singly occupied, the formal bond order is three.

F2BMF (M = B and Al) Molecules: A Matrix Infrared Spectra and Theoretical Calculations Investigation.

Reactions of laser-ablated B and Al atoms with BF3 have been explored in the 4 K excess neon through the matrix isolation infrared spectrum, isotopic substitutions and quantum chemical calculations. The inserted complexes F2BMF (M = B, Al) were identified by anti-symmetric and symmetric stretching modes of F-B-F, and the F-11B-F stretch modes are at 1336.9 and 1202.4 cm-1 for F211B11BF and at 1281.5 and 1180.8 cm-1 for F211BAlF. The CASSCF analysis, EDA-NOCV calculation and the theory of atoms-in-molecules (AIM) are applied to investigate the bonding characters of F2BBF and F2BAlF molecules. The bonding difference between boron and aluminum complexes reveals interesting chemistries, and the FB species stabilization by a main group atom was first observed in this article.

Genuine quadruple bonds between two main-group atoms. Chemical bonding in AeF- (Ae = Be-Ba) and isoelectronic EF (E = B-Tl) and the particular role of d orbitals in covalent interactions of heavier alkaline-earth atoms.

Quantum chemical calculations of anions AeF- (Ae = Be-Ba) and isoelectronic group-13 molecules EF (E = B-Tl) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 various basis sets. The equilibrium distances, bond dissociation energies and vibrational frequencies are reported. The alkali earth fluoride anions AeF- exhibit strong bonds between the closed-shell species Ae and F- with bond dissociation energies between 68.8 kcal mol-1 for MgF- and 87.5 kcal mol-1 for BeF- and they show an unusual increasing trend MgF- < CaF- < SrF- < BaF-. This is in contrast to the isoelectronic group-13 fluorides EF where the BDE continuously decreases from BF to TlF. The calculated dipole moments of AeF- are very large between 5.97 D for BeF- and 1.78 D for BaF- with the negative end always at the Ae atom (Ae→F-). This is explained by the location of the electronic charge of the lone pair at Ae, which is rather distant from the nucleus. The analysis of the electronic structure of AeF- suggests significant charge donation Ae←F- into the vacant valence orbitals of Ae. A bonding analysis with the EDA-NOCV method suggests that the molecules are mainly covalently bonded. The strongest orbital interaction in the anions comes from the inductive polarization of the 2pσ electrons of F-, which leads to a hybridization of the (n)s and (n)pσ AOs at Ae. There are two degenerate π donor interactions Ae←F- in all anions AeF-, which provide 25-30% to the covalent bonding. There is another σ orbital interaction in the anions, which is very weak in BeF- and MgF-. In contrast, the second stabilizing σ orbital interaction in CaF-, SrF- and BaF- yields a strongly stabilizing σ orbital, because the Ae atoms use their (n - 1)dσ AOs for bonding. The energy lowering of the second σ interaction in the latter anions is even stronger than the π bonding. The EDA-NOCV results suggest that BeF- and MgF- have three strongly polarized bonds, whereas CaF-, SrF- and BaF- have four bonding orbitals. The quadruple bonds in the heavier alkaline earth species are made possible because they use s/d valence orbitals like transition metals for covalent bonding. The EDA-NOCV analysis of the group-13 fluorides EF gives a conventional picture with one very strong σ bond and two rather weak π interactions.

Unified classification of non-covalent bonds formed by main group elements: a bridge to chemical bonding.

Using correlation plots of binding energy and electron density at the bond critical point, we investigated the nature of intermolecular non-covalent bonds (D-X⋯A, where D = O/S/F/Cl/Br/H, mostly, X = main group elements (except noble gases), A = H2O, NH3, H2S, PH3, HCHO, C2H4, HCN, CO, CH3OH, and CH3OCH3). The binding energies were calculated at the MP2 level of theory, followed by Atoms in Molecules (AIM) analysis of the ab initio wave functions to obtain the electron density at the bond critical point (BCP). For each non-covalent bond, the slopes of the binding energy versus electron density plot have been determined. Based on their slopes, non-covalent bonds are classified as non-covalent bond closed-shell (NCB-C) or non-covalent bond shared-shell (NCB-S). Intriguingly, extrapolating the slopes of the NCB-C and NCB-S cases leads to intramolecular "ionic" and "covalent" bonding regimes, establishing a link between such intermolecular non-covalent and intramolecular chemical bonds. With this new classification, hydrogen bonds and other non-covalent bonds formed by a main-group atom in a covalent molecule are classified as NCB-S. Atoms found in ionic molecules generally form NCB-C type bonds, with the exception of carbon which also forms NCB-C type bonds. Molecules with a tetravalent carbon do behave like ions in ionic molecules such as NaCl and interact with other molecules through NCB-C type bonds. As with the chemical bonds, there are some non-covalent bonds that are intermediate cases.

On the instability of iodides of heavy main group atoms in their higher oxidation state.

The inert pair effect-the tendency of the s orbital of heavy atoms to stay unreactive, is a consequence of the relativistic contraction of the s orbitals. While the manifestations of this on the reactivity depend on the nature of the substituents, this aspect is often overlooked. Divalent Pb prefers inorganic substituents, whereas tetravalent Pb prefers organic substituents. Among the inorganic substituents, again there are specific preferences-tetravalent Pb prefers F and Cl more than Br and I. It is as though the relativistic contraction of the s orbital of Pb is more significant with Br and I substituents than with Cl, F, and alkyl substituents. Herein, we address this problem using the molecular orbital approach and support it with quasi-relativistic density functional computations. We explain why typical hypervalent systems, like 12-X-6, and 10-X-5 (X is a heavy atom, the number preceding X is the number of valence electrons surrounding X, and the number after X is the coordination number) with less electronegative substituents carrying a lone pair (such as iodine), and Lewis octet molecules like PbI4 are unstable, but their dianions (14-X-6, 12-X-5, PbI42-) are not. For heavy atoms, the relativistic contraction of the s orbital renders the antibonding combination of s with ligand orbitals (σ1*) very low-lying, making it a good acceptor of electrons. Thus, compounds where σ1* is empty are kinetically unstable when an electron donor with appropriate energy (such as the lone pair on iodine or bromine) is present in the vicinity. Donor-acceptor interaction between σ1* and the lone pair on I or Br (F and Cl lone pairs are energetically far away from σ1*) is responsible for the instability of such compounds. The kinetic stability of tetraalkyl lead compounds is due to the absence of lone pairs on the alkyl substituents. This work illustrates the key factor responsible for the instability of heavy element iodides by taking into consideration the covalent nature of the bonds, while the existing explanations assume a purely ionic bonding, which is an oversimplification.

Bridges and Vertices in Heteroboranes.

A number of (hetero)boranes are known in which a main group atom X 'bridges' a B-B connectivity in the open face, and in such species X has previously been described as simply a bridge or, alternatively, as a vertex in a larger cluster. In this study we describe an approach to distinguish between these options based on identifying the best fit of the experimental {Bx} cluster fragment with alternate exemplar {Bx} fragments derived from DFT-optimized [BnHn]2- models. In most of the examples studied atom X is found to be better regarded as a vertex, having 'a 'verticity' of ca. 60-65%. Consideration of our results leads to the suggestion that the radial electron contribution from X to the overall skeletal electron count is more significant than the tangential contribution.