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The open circuit voltage (VOC) deficit of Cu2ZnSn(S,Se)4 (CZTSSe) kesterite solar cells is higher than that of the closely related Cu(InGa)Se2 solar cells. One of the most promising strategies to overcome the large VOC deficit of kesterite solar cells is by reducing the recombination losses through appropriate cation substitution. In fact, replacing totally or partially Zn or Cu by an element with larger covalent radius one can significantly reduce the concentration of I–II antisite defects in the bulk. In this study, an investigation of the impact of partial substitution of Cu by Ag in CZTSSe solid solution monograins is presented. A detailed photoluminescence study is conducted on Ag-incorporated CZTSSe monograins and a radiative recombination model is proposed. The composition and structural quality of the monograins in dependence of the added Ag amount are characterized using Energy Dispersive X-ray Spectroscopy and X-Ray Diffraction method, respectively. The Ag-incorporated CZTSSe monograin solar cells are characterized by temperature dependent current-voltage and electron beam induced current methods. It was found, that low Ag contents (x ≤ 0.02) in CZTSSe lead to higher solar cell device efficiencies.

We have performed density functional theory calculations to study the binding and charge transfer at interfaces between blue phosphorene and various metal substrates. The substrates considered are Al(111), Ag(111), Au(111), Pt(111), Zr(0001) and Sc(0001). The substrates suitable for use for synthesis and exfoliation of blue phosphorene, or instead for use as electrodes for either making contacts or charge doping blue phosphorene, are identified. We find that both the charge transfer and strength of binding correlate excellently with a simple descriptor D = Δ χ / Δ R , where Δχ is the difference in Pauling electronegativities between the metal and phosphorus, and Δ R the corresponding difference in covalent radii. The predictions of the descriptor are validated by subsequent calculations on Pd(111), Cu(111) and Ir(111) substrates. This easily computed descriptor should help in the rapid identification of two-dimensional material + metal combinations suitable for specific applications.

The reactions of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), towards the zinc and cadmium halides, MX2 (M = Zn, Cd; X = Cl, Br, I), afford the adducts, [H(sebenzimMe)]2MX2, which have been structurally characterized by X-ray diffraction. The halide ligands of each of these complexes participate in hydrogen bonding interactions with the imidazole N–H moieties, although the nature of the interactions depends on the halide. Specifically, the chloride and bromide derivatives, [H(sebenzimMe)]2ZnX2 and [H(sebenzimMe)]2CdX2 (X = Cl, Br), exhibit two intramolecular N–H⋯X interactions, whereas the iodide derivatives, [H(sebenzimMe)]2ZnI2 and [H(sebenzimMe)]2CdI2, exhibit only one intramolecular N–H⋯I interaction. Comparison of the M–Se and M–Cl bond lengths of the chloride series, [H(sebenzimMe)]2MCl2 (M = Zn, Cd, Hg), indicates that while the average M–Cl bond lengths progressively increase as the metal becomes heavier, the variation in M–Se bond length exhibits a non-monotonic trend, with the Cd–Se bond being the longest. These different trends provide an interesting subtlety concerned with use of covalent radii in predicting bond length differences. In addition to tetrahedral [H(sebenzimMe)]2CdCl2, [H(sebenzimMe)]3CdCl2·[H(sebenzimMe)]4CdCl2, which features both five-coordinate and six-coordinate coordinate centers, has also been structurally characterized. Finally, the reaction between CdI2 and H(sebenzimMe) at elevated temperatures affords the 1-methylbenzimidazole complex, [H(sebenzimMe)][H(benzimMe)]CdI2, a transformation that is associated with cleavage of the C–Se bond.

Metal–metal multiple bonding is common amongst the d-block transition metals; but intermetallic multiple bonding involving f-block metals, such as uranium, is extremely rare. This is largely due to the difficulties in synthesis and characterization of the relevant compounds. Here, we report a set of heterometallic clusters featuring uranium-metal triple bond s. X-ray diffraction reveals that the uranium-iron (1.9689(13) A) and uranium-cobalt (2.0926(7) A) bond lengths are shorter than the sum of the triple bond covalent radii for corresponding atoms. These complexes represent the first structurally authenticated example of multiple bonding between f-block elements and first-row transition metals. Theoretical studies reveal the high bond orders ( up to 2.93 ) and the significant degree of covalency in these uranium-metal triple bonds. These studies demonstrate that the uranium-metal multiple bonds are no longer an elusive bonding motif, and this opens an avenue to investigation of the potential applications in catalysis and small-molecule activation of heterometallic clusters with actinide-metal multiple bonds .

Impurities are often driven to segregate to grain boundaries, which can significantly alter a material's thermal stability and mechanical behavior. To provide a comprehensive picture of this issue, the influence of a wide variety of common nonmetallic impurities (H, B, C, N, O, Si, P and S) incorporated during service or materials processing are studied using first-principles simulations, with a focus on identifying changes to the energetics and mechanical strength of a Cu Σ5 (310) grain boundary. Changes to the grain boundary energy are found to be closely correlated with the covalent radii of the impurities and the volumetric deformations of polyhedra at the interface. The strengthening energies of each impurity are evaluated as a function of covalent radius and electronegativity, followed by first-principles-based tensile tests on selected impurities. The strengthening of a B-doped grain boundary comes from an enhancement of the charge density among the adjacent Cu atoms, which improves the connection between the two grains. Alternatively, the detrimental effect of O results from the reduction of interactions between the Cu atoms. This work deepens the understanding of the possible beneficial and harmful effects of impurities on grain boundaries, providing a guide for materials processing studies.

The hexagonal boron-noble gas compounds B 6 Ng n 4+ (n = 1–6, Ng = He Rn) have been studied theoretically. The geometry, bond energies and thermodynamics properties are reported. For Ar Rn, the B Ng bond length is close to the sum of the covalent radii of B and Ng, the B Ng average bond energy is large up to 59.61–169.43 kcal/mol. There is large charge transfer from Ng to the boron ring, the Ng → boron σ-donation originating from the interaction between the valence p orbital of Ng and the LUMO of the boron ring is the predominant contribution towards the Ng B bond stabilization. The NICS analysis shows that B 6 Ng n 4+ have aromaticity.

It is demonstrated that the effective atomic radii suitable for identification of covalent bonding can be revealed from the parameters of machine-learning classifier trained to distinguish between bonded and non-bonded pairs of atoms in 28 710 molecules. The proposed way cardinally differs from the conventional technique of finding the atomic radii by solving a regression problem and approximating the bond lengths by the sum of adjustable parameters. Despite the contrasting approach, the proposed 'machine-learned atomic radii' obtained for H, B, C, N, O, F, Si, P, S, Cl, Ge, As, Se, Br elements are shown to be in close agreement with the conventional 'regression-based' covalent radii. We find this example appealing in demonstrating how some of the concepts peculiar to chemical intuition can be re-discovered by using a suitable machine-learning model. The boundaries between ‘definitely bonded’ and ‘likely bonded’ interatomic distances introduced naturally in the proposed formulation may prove useful for molecular modeling software.

A new series of isostructural rare earth compounds R1+xZr1−xNi (R = Er, Tm, Lu; x ~ 0.5) was synthesized from the elements by arc melting and subsequent annealing at 870 K for 1400 h. The crystal structures of the intermetallic compounds were investigated by means of single-crystal X-ray diffraction. They all crystallize in the TiNiSi structure type (space group Pnma, No. 62, oP12). In R1+xZr1−xNi, the R/Zr statistical mixture leading to nonequiatomic compositions occupies the position corresponding to the nickel site of the TiNiSi structure type. The calculated shortest interatomic distances are close to the sums of the single-bond covalent radii of respective elements. Electronic structure calculations performed with the tight-binding LMTO method revealed the non-zero density of states at the Fermi level and suggest metallic character. R1+xZr1−xNi (R = Er, Tm, Lu; x ~ 0.5) undergoes no long-range magnetic ordering down to 2 K.

Bi containing III/V semiconductors are frequently mentioned for their importance as part of the next generation of optoelectronic devices. Bi containing ternary and quaternary materials like Ga(AsBi), Ga(NAsBi) or Ga(PAsBi) are promising candidates to meet the requirements for new laser structures for telecommunications and solar cell applications. However, in previous studies it was determined that the incorporation of sufficient amounts of Bi still poses a challenge, especially when using MOVPE (metalorganic vapour phase epitaxy) as the growth technique. In order to figure out which mechanisms are responsible for the limitation of Bi incorporation, this work deals with the question of whether there is a relationship between strain, induced by the large Bi atoms, and the saturation level of Bi incorporation in Ga(AsBi). Ga(NAsBi) structures were grown by MOVPE at a low temperature, 400 °C, and compared to Ga(PAsBi) as well as Ga(AsBi) growth. By using the two group V atoms P and N, which have a smaller covalent radius than Bi, the effect of local strain compensation was investigated systematically. The comparison of Bi incorporation in the two quaternary materials systems proved the importance of local strain for the limitation of Bi incorporation, in addition to other effects, like Bi surface coverage and hydrocarbon groups at the growth surface. This, of course, also opens up ways to strain-state-engineer the Bi incorporation in semiconductor alloys.

The organo-montmorillonites are highly attractive materials because of their wide application potential. In this work the basic structural, vibrational and thermal characteristics of the samples from two series of structurally analogous alkylammonium- and alkylphosphonium-montmorillonites are reported for the first time. The first series consisted of the organo-montmorillonites prepared from commercially available tetramethyl-(Me4-N/P), tetrabutyl-(Bu4-N/P) and tetraoctyl-(Oc4-N/P) analogues, while the second series was prepared from tributylalkyl cations with the fourth chain of the increasing length (Bu3-N/P-CX, X = 4, 8, 12, 16). The XRD analysis revealed that for both series, when comparing the equivalent pairs of the surfactants, slightly higher expansion of the interlayer space was observed for phosphonium-based than for ammonium-based montmorillonites. Difference in the d001 values was in accordance with a higher covalent radius of P than N. The impact of the different central atom (N/P) of the cation head-group was also studied by IR spectroscopy in the middle and near region. As a result of differences in the charge density of the nitrogen and phosphorus cationic centers the positions of the C-H bands (stretching and overtones) of phosphonium-Mt were shifted to lower wavenumbers compared to their ammonium counterparts. The effect of the central atom of the head-group was significant for smaller cations, but gradually decreased with increasing size of the ammonium and phosphonium surfactant. DTG profiles showed a significant step above 200°C attributed to the release of the organic phase. The temperature of maximal decomposition rate was higher for phosphonium cations compared to ammonium analogues. The mass profiles and temperature-dependent IR spectra showed the evolution of CO2 and H2O species as the main products. In addition, the C-H stretching and bending bands of the unspecified organic phase were detected in the spectra.

It is predicted that the oxygen behaviour in A-site may be different from that of B-site. The magnetic behaviour of oxygen in alkaline ferrites having the composition AFe2O4 (A = Mg, Ca and Sr) has been discussed using electronic structure calculations. The calculations have shown that the physical properties of the studied compounds are exclusively sensitive to the magnetic moment carried by the 2p-electrons of the oxygen anions. The compounds, with A = Ca, Sr and Ba, have a closely related electronic structures different from MgFe2O4. Fe with oxygen is more active in Ca, Sr and Ba than in the Mg compound this means that Oxygen atom is inactive in alkaline ferrites when carrying no moment. The calculations have shown that the alkaline impurities, with relatively large covalent radius, can form an excitonic state near the band edge of the ferrite. The electron pockets formed at the high symmetry point N in up and down channel for MgFe2O4, but these pockets not appear for the other studied compounds. MgFe2O4 has shown a lower current carrier density than the others compounds. The spin relaxation is expected to be suppressed as moving from Mg to Ba. It is speculated that as u increases, the ferrite becomes more efficient for the electronics that depends mainly on the electron spin.

Dual-doped graphene is synthesized by a facile solvothermal method with the assistance of ionic liquids containing both N and X (X = B, P or S) atoms. All three types of co-doped graphene present excellent catalytic activity, demonstrating preferred four-electron selectivity and low peroxide yields toward oxygen reduction reaction in alkaline solution. Particularly, N, P-graphene exhibits superior catalytic activity to its counterparts in terms of half-wave potential (ΔE 1/2 = −70 mV relative to commercial Pt/C), methanol tolerance and long-term stability. This could be attributed to the unique porous nanostructure, change of charge density and high distortion of carbon structures originating from the combination of large electronegativity of N element and big covalent radius of P atoms.

In the present work, experimental values of several physical properties for hydrogen halides (including At) are used to obtain, by extrapolation, values of physical properties for tennessine (element 117, Ts) and tennessine hydride (TsH). Using experimental data of Cl, Br, I and At halides, two empirical equations are obtained, allowed us to calculate the ionic and covalent contribution to the bond energy, respectively: E ionic = [μ 2 h/d 3 ]c 2 10 −12 and E covalent = [−22.78P + 400.09] + ν (H X). Using both equations, as well some empirical trends in the Cl, Br, I and At halides, the values of some physical properties of Ts and TsH are estimated. The frequency energy of the Ts H vibration, ν (Ts H), value is calculated as 20.24 kJ mol −1 . For the same bond: μ = 0.24 D; bond distance = 196.76 pm; bond energy = 163.03 kJ mol −1 . The polarizability and covalent radius for Ts are estimated as 11.31/10 −24 cm 3 and 159.76 pm, respectively.

This study examines the mid and near infrared spectra of a suite of natural and synthetic powdered minerals of the isostructural barite mineral group. The mid-infrared (MIR) spectra are wholly compliant with anhydrous high purity anglesite, celestine and barite. The complementary near-infrared (NIR) spectra contain bands at ca 1.9 μm (5263 cm−1) and ca 1.4 μm (7140 cm−1) assigned to the (H2O) v2 (antisymmetric stretch) + v3 (bending vibration) combination band and overtones of the fundamental OH stretching modes respectively. Peak-fitting of the ca 4660-6000 cm−1 region of interest reveals four shoulders to the (H2O) (v2 + v3) combination which are systematically ordered with respect to interstitial metal covalent radius: [PbSO4] (lowest wave number) → [SrSO4] → [BaSO4] (highest wave number). These bands are assigned to (M2+−O–H) surface complexation of water with the sulfates. It is further interpreted that the two good-to-perfect cleavages in the mineral group with resultant variation in (M2+ - O - H) bond lengths accounts for the existence of the four bands and explain why absorption strength at ca 1.91 μm (5236 cm−1) is not a reliable indication of the overall abundance of water in sulfate minerals. Spectral interpretation within the vicinity of the water combination ca 1.91 μm (5236 cm−1) is further complicated by the presence of a higher wavenumber (v1 + 4v3) sulfate combination and the appearance of a band interpreted as the (5v3) overtone of [SO42−]. The results are discussed in the light of these findings.

p-Type Zn doped (AlxGa1-x)0.5In0.5P layers have been grown on (001) GaAs substrates by metalorganic vapor phase epitaxy. The lattice constants have been carefully measured by X-ray double crystal diffraction. Mismatches between the substrates and epitaxial layers are observed in the direction corresponding to lattice contraction. The variation of fully-relaxed lattice constants calculated from symmetric (004) and asymmetric (115) diffraction shows that the contraction is related to both increasing inlet dopant flows and increasing growth temperatures. Secondary ion mass spectrometry is employed to analyze the elements in the epitaxial layers. The variations of Al, Ga, In and Zn components indicate that a decrease of In incorporation during the growth leads to the contracted lattice, although Zn atoms, which incorporate into the sites of In atoms during the doping, are smaller in covalent radius. The decrease is supposed to be caused by the competition between In and Zn atoms on the growth surface, which is well fit by the surface adsorption-trapping model. In addition, the suppression of Al and Ga incorporation by inlet dopant flows is observed. The possible cause might be the ethane generated by the pyrolysis of dopant source, which impedes the decomposition process of metalorganic sources of Al and Ga atoms.

Ce 23 Ru 7 Zn 4 was obtained by melting of the elements in a sealed niobium ampoule in an induction furnace and subsequently by slow cooling in a muffle furnace. The structure was refined from X-ray single crystal diffractometer data at room temperature and can be described in the Pr 23 Ir 7 Mg 4 type structure: P 6 3 mc , a = 985.7(1), c = 2215.0(3) pm, wR = 0.0577, 2481 F 2 values, 75 variables. Like often described for intermetallic Ce–Ru compounds, also for Ce 23 Ru 7 Zn 4 Ce–Ru distances (252 pm) far below the sum of the covalent radii (289 pm) can be observed. Such a behavior can only be explained by partly tetravalent cerium and this is confirmed by susceptibility measurements. According to a Curie–Weiss fit a cerium valence of 3.11(3) could be calculated and no magnetic ordering down to 2.5 K could be found.

III/V semiconductors with cubic zinc-blende crystal structure, for example GaAs, GaP or InP, become metastable if atoms with significantly smaller or larger covalent radius than the matrix atoms are alloyed. Examples are the incorporation of Boron, Nitrogen and Bismuth in the above-mentioned materials. The resulting multinary compound semiconductors, like for example (Ga,In)(N,As), Ga(N,As,P) and Ga(As,Bi), are extremely interesting for several novel applications. The growth conditions, however, have to be adopted to the metastability of the material systems. In addition, structure formation can occur which is different from stable materials. This paper summarizes our current knowledge on growth characteristics of several metastable materials. Mainly examples for Metal Organic Vapor Phase Epitaxy (MOVPE) are given. The MOVPE growth characteristics are compared to selected examples using Molecular Beam Epitaxy growth to highlight that the observed growth characteristics are intrinsic for the studied metastable material systems. Furthermore, structural peculiarities of dilute borides, nitrides and bismides occurring during growth as well as in growth interruptions are summarized and correlated to the growth conditions.

Structure and bonding analysis of the Fe O bonds in the model heme-imidazole and heme-thiolate complexes [(por)Fe(Im)(R)] (R = HC(O)O − , 1FeIm , R = H 3 CC(O)O − , 2FeIm , R = NO 3 − , 3FeIm ) and [(por)Fe(SMe))(R)] − (R = HC(O)O − , 4FeSMe , R = H 3 CC(O)O − , 5FeSMe , R = NO 3 − , 6FeSMe ) were investigated at the DFT/TZP level of theory using density functionals BP86, PW91, PBE, TPSS and B3LYP. The Fe O bond distances at BP86/TZP in the complexes [(por)Fe(Im)(R)] (1.858 A in 1FeIm , 1.868 A in 2FeIm and 1.919 A in 3FeIm ) and [(por)Fe(SMe)(R)] − (1.955 A in 4FeSMe , 1.981 A in 5FeSMe and 1.991 A in 6FeSMe ) are longer than those expected for a Fe O single bond estimated on the basis of covalent radii predictions (Fe O = 1.79 A). The Fe O bond distances in [(por)Fe(Im)(R)] 1FeIm – 3FeIm are shorter than those in [(por)Fe(SMe)(R)] − 4FeSMe – 6FeSMe . The thiolate is more trans-directing ligand than the imidazole ligand. The results of optimized geometries, NPA charges, Wiberg bond indices, NHO analysis and BDEs clearly indicate that the formate, acetate and nitrate ligands bind more weakly in heme-thiolate complexes than in heme-imidazole complexes. The reaction intermediate of chloroperoxidase enzyme would be more reactive with organic substrates than the myeloperoxidase (MPO) enzyme. The quartet and the sextet spin states are less stable than the doublet spin state. The optimized bond distances are shortest for doublet spin state.

The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)3AlC2 was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)3AlC2 is determined to be P63/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)3AlC2, Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M3AX2 lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)3AlC2 becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)3AlC2 were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)3AlC2 with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms.

The metathesis of CpMn(CO)(NO)SnCl3 (I) and anionic carbonyls (ML−) Mn ( CO ) 5 − , Co ( CO ) 4 − , Cp ′ Mo ( CO ) 3 − , Cp ′ W ( CO ) 3 − , Cp ″ Mo ( CO ) 3 − provided trinuclear mixed-metal complexes of general formula CpMn(CO)(NO)SnCl2ML (VIII–XII; Cp = C5H5, Cp′ = C5H4CH3, Cp″ = C5H3(C4H9)2). Along with XII, the product of transmetallation Cp″Mo(CO)3SnCl3 (XIII) was separated. Interaction of I with Fe 2 ( CO ) 8 2 − gave the mixed-metal cluster Cp(CO)(NO)Mn–SnCl2–Fe(CO)4–SnCl2–MnCp(CO)(NO) (XIV) which contain the pentanuclear metal-chain. As determined by the single crystal XRD analysis, the Mn–Sn distance in CpMn(CO)(NO)Sn fragment is 0.2 A shorter than the expected value based on the Mn/Sn sum of covalent radii (SCR). From the other hand, the shortening of Sn–ML distances is in an accord with the donor ability of an ML fragment in the row Co(CO)4 ∼ Mn(CO)5

The impact of doping on the lattice constants of 4H–silicon carbide (4H–SiC) is an important material aspect influencing several steps of material and device production. Dopant incorporation in 4H–SiC causes misfit between the highly N-doped substrate and differently doped epilayers and hence, wafer bowing and the existence of a critical epilayer thickness. In this paper, the wafer bow is determined by geometrical measurements of the substrate prior to and after the epitaxial growth of single epilayers with different epilayer thicknesses and doping states, i.e. with nitrogen (N) or aluminum (Al) doping and different dopant concentrations. The misfit between substrate and epilayer is deduced from these bow measurements based on a model by Stoney. For highly Al-doped epilayers grown on highly N-doped substrates, the misfit is determined additionally by HRXRD measurements. The doping-dependent misfit, obtained from bow and HRXRD measurements, is compared to theoretical values calculated based on a model by Jacobson. For Al-doped epilayers, the experimental and theoretical values of misfit agree well. For N-doped epilayers, an effective covalent radius of N in 4H–SiC of 66 pm has to be introduced to match the theoretical misfit to the experimental one. The critical epilayer thickness is calculated based on the models by Matthews and Blakeslee and by People and Bean. The comparison of calculated values and experimental findings with respect to dislocation behavior at the substrate–epilayer interface and the dislocation densities of epilayers proves that the model by Matthews and Blakeslee underestimates the critical epilayer thickness for 4H–SiC homoepitaxy.

In this work, the pressure induced phase transition of InAs is investigated by density functional theory. The first-order phase transition of InAs from zinc-blende (ZB) to the rocksalt (RS) structure occurs at 4.9 GPa accompanies by a 26% volume collapse. It is found that the nearest In and As atoms bonded as covalent bond, but there is no strong interaction between the nearest In–In or As–As atoms. Crystal space of ZB structure is occupied by tetrahedrons (4 In–As covalent bonds) partly with many interstice, and crystal space of RS is fulfilled by close-packed octahedrons (6 In–As covalent bonds). With increasing pressure, rebuild of covalent bond due to variations of electronic structure causes phase transition from ZB to RS structure. Furthermore, directional changes of covalent bond along [100] and [110] bring evident variation of shear on the {100} and {110} planes.

Quantum chemical calculations have been performed to investigate the interaction of the neutral H 2 B radical with hydrogen cyanide (HCN) and hydrogen iso-cyanide (HNC) at the MP2/aug-cc-pVTZ level. The results show that a weak hydrogen bond and a strong covalent interaction can be formed between the two molecules. In H 2 B HCN HCN and H 2 B HNC HNC trimers, there is enhancing interplay between the single-electron hydrogen bond and another type of hydrogen bond. Although the covalent interaction in H 2 B NCH and H 2 B CNH dimers is also enhanced due to presence of the second NCH (CNH) molecule in H 2 B NCH NCH and H 2 B CNH CNH trimers, there exists a negative cooperativity between the covalent interaction and the hydrogen bond. The complexes have been analyzed with natural bond orbital (NBO) and atoms in molecules (AIM) theories.

This review summarizes structural parameters for forty five Cu 4 (μ 4 -O)(η-X) 6 (L 4 ) tetramers. There are four types of structurally equal core units, CuCl 3 ON, CuCl 3 OO, CuCl 3 OCl and CuBr 3 ON. There are also tetramers which contain structurally unequal core units: CuCl 3 ON (x2) and CuCl 2 BrON (x2); CuCl 3 ON (x1), CuCl 2 BrON (x2) and CuCl 3 OCl (x1). There is a tendency for an elongated Cu⋯Cu separation as well as Cu–L bond distances with increase of the covalent radius of the coordinating atom(s). Tetrahedral distortion around oxygen atom (OCu 4 ) increases in the order: 1.67° (CuCl 3 OCl) 3 ON) 3 OO′) 3 ON). The mean Cu–Cl–Cu bridge angle of 80.5° is about 4.0° more open than that of a Cu–Br–Cu (76.5°). The cluster Cu 16 O 4 Br 7 Cl 17 (4-Mepy) 16 (4-Mepy = 4-methylpyridine) contains four crystallographically independent tetramers: Cu 4 OBrCl 5 (4-Mepy) 4 (1), Cu 4 OBrCl 5 (4-Mepy) 4 (2) Cu 4 OBr 2 Cl 4 (4-Mepy) 4 (3) and Cu 4 OBr 3 Cl 3 (4-Mepy) 4 (4), which is a unique example of stereoisomerism. There are other examples which exist in two isomeric forms. Another contains two or even four crystallographically independent tetramers within the same crystal, differing mostly by degree of distortion and is an examples of distortion isomerism. Pairs of Cu 4 OCl 6 ( n -Meim) 4 ( n -Meim = n -methylimidazole) ( n = 1 or 2) are examples of ligand isomerism.

The structures of Ca 60 Mg 15 Cu 25 , Ca 60 Mg 20 Cu 20 and Ca 60 Mg 25 Cu 15 metallic glasses have been investigated by neutron and X-ray diffraction. The correlation functions show a peak manifold in the region 2.2–4.5 A, arising from the various atom pairs for these glasses. The results show clearly that there are contacts between the solute atoms (Cu and Mg) which agree with a simple estimate of the maximum solute atom fraction beyond which solute–solute contact becomes topologically necessary. The Cu–Mg and Cu–Ca distances are consistent with the sum of covalent radii, whereas all other interatomic distances are consistent with the sum of metallic radii. The neutron and X-ray diffraction correlation functions were simultaneously fitted with a series of symmetric peaks to obtain coordination numbers and interatomic distances, but only the results from the first two peaks, Cu–Cu and Cu–Mg, are of reasonable reliability. The Percus–Yevick approximation for binary hard sphere systems has been used to simulate the results. This shows that the results of fits to the Cu–Cu and Cu–Mg correlations give a reasonable description for atoms which are in close contact, but do not include an additional contribution at longer distance due to atoms which are almost in contact. The fits to the Cu–Ca peak are strongly affected by overlaps with both the longer distance peaks used in the fit, and the broad trailing edge in the distributions of interatomic distances that is not taken into account by fitting with symmetric peaks. Final results show that copper has a total of 6.5 neighbours which are in close contact, but has a total coordination number of about 12–13, when atoms which are almost in contact are included.

η5-2,5-Diphenylphosphacymantrene (1) in benzene or CH2Cl2 solution does not react with amines. However, with amines NHEt2, NEt3, NEt(i-Pr)2 in excess and in the presence of small amounts of water 1 reacts to form anionic complexes [(CO)3Mn(η4-Ph2H2C4P( O)H]− A+, where cation A = H 2 NEt 2 + (2a), HNEt 3 + (2b), HN(Et) ( Pr i ) 2 + (2c). Probably, first an unstable intermediate with Р–ОН bond is formed as a result of the attack by the activated H2O molecule at the P atom. Afterwards, the rapid rearrangement occurs with migration of H from O to P which leads to the ligand 2,5-diphenyl-1-Н-phosphol-1-oxide, Ph2H2C4P( O)H. Salts 2а–с have been characterized by 1H, 31P, 13C NMR- and IR-spectra and the structures of 2a and 2c established by single crystal X-ray diffraction analyses. The phosphoryl ligand in anions 2a and 2c has the “envelope” conformation and is η4-coordinated with Mn(CO)3. The phosphorus atom is not involved in the coordination with Mn because the Mn–P distances (2.7670(4) and 2.7732(8) A) are greater than the sum of covalent radii of P and Mn.

The mechanical force to polymeric materials in vacuum at 77 K produces mechano radicals, mechano anions and mechano cations due to homogeneous and heterogeneous scissions of the covalent bonds comprising polymer main chain. The ionic degree of the covalent bond was estimated by calculating the “absolute ΔMulliken atomic charge,” which was defined as the difference between the Mulliken atomic charges of the two adjacent atoms comprising the covalent bond of the polymer main chain. The ionic yield of the covalent bond increased with increasing the absolute ΔMulliken atomic charge. The empirical formula for the ionic yield was obtained with the absolute ΔMulliken atomic charge, and indicates that the ionic yield could be estimated from its chemical structure.

The surface segregation trend of trace elements in pure aluminum foils was investigated by density functional theory. The model of nine-layer Al(1 0 0) slab substituted partially by trace element atoms was proposed for calculating surface segregation energy. The calculating results show that (i) B, Mg, Si, Ga, Ge, Y, In, Sn, Sb, Pb and Bi exhibit negative segregation energy and possibly move to the surface, while Be, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zr exhibit positive segregation energies and migrated into the bulk; (ii) the segregation energy was found to be related with the covalent radius, the relaxed position at the surface of the substituting atom and the surface energy; (iii) the segregation behavior of trace element generates lots of defects and dislocation, which can increase the initial pitting nucleation sites in the surface of aluminum foils; (iv) the impurity atom concentration was tested with Pb-doped surfaces, the calculated negative segregation energies in all coverage increases rapidly with the Pb coverage. These conclusions are helpful for designing of the chemical composition and to advance the tunnel etching of aluminum foils.

Reaction of K2[PtCl4] or Na2[PdCl4] with 6,6′-diphenyl-2,2′-bipyridine, L, gives the cyclometallated species [Pt(L–H)Cl], 1, and [Pd(L–H)Cl], 2, respectively, where L–H is a terdentate N^N^C anionic ligand originated by direct activation of a C(sp2)–H bond. The crystal structure of 2 has been solved by X-ray diffraction and compared to that of the analogous complex [Pd(L′–H)Cl] L′ = 6-phenyl-2,2′-bipyridine. The second phenyl ring in 2 entails a considerable distortion of the coordination around the metal. A similar distortion is also to be expected in the analogous compound 1, due to the almost equal covalent radii of palladium(II) and platinum(II). From the complexes 1 and 2 the chloride can be displaced with AgBF4 and substituted by CO or PPh3 to give the corresponding cationic species. By reaction of 1 with Na[BH4] substitution of H− for Cl− can be achieved: the rare hydrido complex [Pt(L–H)H], stabilized only by nitrogen ligands, was isolated in the solid state and fully characterized in solution. It is noteworthy that in the case of the 6-phenyl-2,2′-bipyridine the analogous terminal hydride [Pd(L′–H)H] is unstable. In platinum chemistry the reaction of 6-substituted 2,2′-bipyridines is known to give either N^N^C or N′^C(3) rollover cyclometallation, depending on the nature of the metal precursor. In the case of 6,6′-Ph2-2,2′-bipy cyclometallation was also shown to undergo multiple C–H activation giving the C^N^C pincer complex [Pt(L-2H)(DMSO)]. The latter species can be related to complex 1: indeed its reaction with HCl produces complex 1 and [Pt(L–H)(DMSO)Cl], a rollover species with a pendant phenyl substituent.

Gordy identified electronegativity of an atom with the electrostatic potential felt by one of its valence electrons and suggested an ansatz, χ = Zeff/rcov where χ is the electronegativity, Zeff is the effective nuclear charge and rcov is the radial distance equal to atom’s single bond covalent radius. In this report we have elucidated that the use of covalent radius in evaluating the electronegativity, (χ), of atoms through Gordy’s formula, χ = Zeff/rcov, is not justified. Armed with this knowledge and relying upon the electronegativity equalization principle, we have argued that the most probable radius (Slater radius), and not the covalent radius, is the fundamental size descriptor of atoms in mathematical formulation of electronegativity property of atoms. We have also suggested that the electronegativity, χ, is not equal to Zeff/r according to Gordy’s original formula rather it is proportional to Zeff/r and the new electronegativity ansatz is χ = [a(Zeff/r) + b], where a and b are constants and r is the Slater (most probable) radius of atoms. We have taken care so that the electronegativity is evaluated in proper unit i.e. energy unit. We have computed the electronegativity of 103 elements of the periodic table through the new suggested ansatz and using Slater radii and effective nuclear charge computed by us. The evaluated new electronegativities are found to satisfy the sine qua non of a reasonable scale of electronegativity by exhibiting periodicity of periods and groups, and observing silicon rule and correlating many physico-chemical properties of elements. The intrinsic inertness of Hg atom is revealed in the present scale.

In-related nitride semiconductors exhibit characteristic behavior which can be ascribed to the large difference in the covalent radius between In and N atoms. We consider atomic and electronic structures of N mono-vacancies (V N ) in InGaN in detail by the first principles calculations. We find that the second nearest neighbor In–In interactions, which are not important in conventional semiconductors such as Si and InAs, are as crucial as the nearest neighbor In–N interaction in In-related nitride semiconductors. Moreover, we clearly show that the strong second nearest neighbor In–In interactions in InN are the physical origin of the unusually narrow band gap of InN.

Synthesis, structure and bonding in transition metal complexes with borane ligands HBcat’ (catecholborane or substituted catecholborane), HBpin (pinacolborane), HBCy2 (dicyclohexylborane), HBEt2 (diethylborane) and HBMe2 (dimethylborane) have been discussed. The review summarizes the results and progress in the understanding of the nature of the metal–borane bonds, which has been gained in recent theoretical studies. Geometrical data (both experimental and theoretically calculated) show M–B, M–H and B–H bond distances longer than the sum of covalent radii. These results are consistent with the interaction of both boron and hydrogen with metal central, while preserving B–H bond character. The nature of the metal–borane interactions is quantitatively analyzed with an energy decomposition analysis. The [LnM]-[η2-BHR2] bonding in metal–borane complexes is more than half electrostatic.

Using first-principles total energy calculations within density functional theory, we investigated adsorption and diffusion of various elements (S, Ge, Pd, Ag and Pt) on the Ru(0 0 0 1) surface. It was found that the hcp hollow site is the preferred adsorption site for all atoms considered (S, Ge, Pd and Pt) except Ag which has equal probability to be adsorbed at the hcp hollow site, fcc hollow site, and the bridge site. Furthermore, S and Ge addlayers become unstable when the coverage goes beyond 0.25 monolayer (ML) due to the repulsive interaction between adsorbates. In contrary, the binding energies of the metal adsorbates Pd, Ag and Pt increase with coverage, all the way up to full coverage (1 ML). The diffusion barriers of the five adsorbates on Ru(0 0 0 1) are investigated and their relationships with the covalent radii of the adsorbates are discussed.

Structural characteristics of the novel [fac-Re(CO)3L]Cl complex, where L denotes the N-methyl-2-pyridinecarboxyamide, are presented. Molecular structure of the complex has been established by means of X-ray single crystal diffraction and compared with quantum mechanical calculations. It has been shown that pyridinethioamide is softer base than pyridineamide. As a result of different ReI–S and ReI–O bond covalent shortenings (reflecting their diverse ionic/covalent character ratio) difference of the bond lengths decreases from a theoretical value of 0.45 A to the value of 0.28 A.

A theoretical study of the stretched σ -borane complexes [( η 5 -C 5 H 5 )Rh(HBpin) 2 ] 1Rh and [( η 5 -C 5 H 5 )Rh(HBpin)(Bpin) 2 ] 2Rh (HBpin, pinacolborane) at the DFT BP86 levels has been carried out. In contrast to the previously optimized geometry of [( η 5 -C 5 H 5 )Rh(HBO 2 C 2 H 4 ) 2 ], the optimized structure of 1Rh contains almost similar HBpin ligands. The B–H bond distances B1–H1, 1.696 A and 1.747 A in 1Rh and 1.686 A in 2Rh are longer than expected for a single bond based on covalent radius predictions (1.19 A) The B1–Rh–H1 bond angles 55.6° and 52.6° in 1Rh and 2Rh , respectively and B2–Rh–H2 bond angle in 1Rh are larger than the B–M–H bond angles in σ -borane complexes (37–41°) and are consistent with rhodium– σ -borane bonding, preserving some B–H bond character. The Rh–B and Rh–H bonding orbitals are strongly polarized towards rhodium.

Organosilicon backbone containing ligands 1,2-bis(dimethyl(2-pyridyl)silyl)ethane (L1) and 1,2-bis(dimethyl(3-quinolyl)silyl) ethane (L2) have been synthesized by treating 2-bromopyridine and 3-bromoquinoline with n-butyllithium and reacting the resulting lithiated products with 1,2-bis(chlorodimethylsilyl)ethane. The ligation of L1 and L2 with Pd(II), Ag(I) and Cu(II) has been investigated. The single crystal structures of L2, [Pd(L1)Cl2] (1), [Cu(L1)Br2] (3) and [PdCl2(L2)]2 (4) have been solved. All the three complexes are metallamacrocyclic in nature. The last one is 22-membered and the first example which has ligands containing organosilicon backbone. The geometry of Pd as well as Cu is very close to square planar. The Pd–N, Pd–Cl, Cu–N and Cu–Br bond distances (2.010(1)–2.027(3), 2.3063(10)–2.3114(4), 2.004(4)–2.018(5) and 2.4137(10)–2.4172(10) A) are very close to sum of covalent radii, indicating strong ligation of L1 and L2 with the metal ions.

Structural and electronic properties of self-assembled monolayer with 4-(4-amino-phenylazo) benzoic acid (APABA) on the Si(001)(4 x 2) surface are investigated by ab initio calculation based on density functional theory. For the APABA chemisorption on the silicon surface, we have assumed two different binding sites: (i) amino group of molecule and (ii) carboxyl group of molecule. Considering amino-site, we have assumed two possible models for the chemisorption of molecules on the Si(001)-(4 x 2) surface: (i) an intrarow position between two neighboring Si dimers in the same dimer row (Model 1), (ii) on-dimer position (Model 11). We have found that Model 11 is 1.10 eV energetically more favorable than Model I. The Si-N bond length was calculated as 1.85 angstrom which is in excellent agreement with the sum of the corresponding covalent radii of 1.87 angstrom. Considering carboxyl-site, we have assumed exactly the same model as mentioned above. Again we have found that Model 11 is energetically favorable than Model I. The calculated bond lengths for Si-O and O-C are 1.76 and 1.35 angstrom, respectively. (c) 2007 Elsevier B.V. All rights reserved.

The electronic and molecular structures of the complexes [(MeC 5 H 4 )Re(CO) 2 (σ-HBcat)] 1 , [(MeC 5 H 4 )Re(CO) 2 (σ-HBpin)] 2 and [(MeC 5 H 4 )Re(CO) 2 (σ-HBMe 2 )] 3 have been investigated at the DFT BP86/TZ2P levels in order to understand the structures, bonding and energetic of the interactions between a transition metal and a σ-HBR 2 ligand. The nature of the Re–η 2 -HBR 2 interactions was analyzed with charge and energy decomposition methods. The Re–B, B–H, Re–H bond distances are longer than that expected for single bond based on covalent radius predictions. The B–Re–H1 bond angles in the complexes [(η 5 -MeC 5 H 4 )Re(CO) 2 (η 2 -HBR 2 )]: 40.8° in 1 , 41.5° in 2 and 38.5° in 3 are small and are consistent with B–H bonding. These results are consistent with the description of [(MeC 5 H 4 )Re(CO) 2 (η 2 -HBR 2 )] as a Re(I) complexes in which both hydrogen and boron of the [HBR 2 ] ligands have a bonding interaction with the rhenium and B–H bond character is preserved. Upon coordination of [HBR 2 ], reduction in B–H bond order about 1/2 was calculated. The theoretically predicted bond dissociation energies of the borane ligands are −40.5 kcal/mol for [(η 5 -MeC 5 H 4 )Re(CO) 2 (HBcat)], −39.7 kcal/mol for [(η 5 -MeC 5 H 4 )Re(CO) 2 (HBpin)] and −43.3 kcal/mol for [(η 5 -MeC 5 H 4 )Re(CO) 2 (HBMe 2 )]. The [(η 5 -MeC 5 H 4 )Re(CO) 2 ]–[η 2 -H-BR 2 ] bonding in borane complexes 1 – 3 is more than half electrostatic. All three complexes exhibit about 42–44% covalent bonding of the borane ligand to the metal fragment. Indeed the three center-two electron bond in the Re–H–B bridge may be regarded as a “protonated π-bond”. The metal–η 2 -H-BR 2 moiety in borane complexes is, therefore, analogous to metal boryl complexes with M–B σ and π bonds with a boron atom lying in the plane defined by the metal and two R substituents.

New linear dependences are presented here for the volumes of alkali halides in crystals and in aqueous solutions: (1) the molar volumes in solids ( V B o ) and the partial molal volumes ( ϕ V o ) in aqueous solutions on the sums of crystal ionic volumes, V (M + , X − ) cr , (2) ϕ V o on V B o , (3) ϕ V o on the sum of aqueous ionic volumes, V (M + , X − ) aq and (4) ϕ V o , V B o , V (M + , X − ) cr , V (M + , X − ) aq and volume sum of the hydrated ions, ϕ V o ( M + , X - ) h on the sum of atomic volumes V (M, X) cov based on covalent radii. All ionic radii are based on the Golden ratio. The volumes due to electrostriction and ionic hydration have also been estimated.

The recent discovery of the magnesium diboride superconductor, a system characterized by a remarkably high transition temperature ( T c = 39 K ), has renewed interest towards experimental and theoretical research of the superconductivity in isostructural systems, such as transition-metal diborides. Here we present an X-ray-absorption fine-structure investigation in AlB 2 -type 4d transition-metal diborides TMB 2 (TM=Y, Zr, Nb) at the transition-metal K-edge. The vibrational behavior of these transition-metal diborides has also been investigated by analyzing the extended X-ray-absorption fine-structure (EXAFS) spectra. The change of the bond length and Debye–Waller factors of TM–B, TM–TM (in-plane) and TM–TM (out-plane) with the covalent radii of the transition-metal atoms has been shown. Data clearly demonstrate that the chemical stability is in the following order for the 4d transition-metal diborides: ZrB 2 >NbB 2 >YB 2 . The chemical stability of YB 2 must be much less than that of the two other compounds. It suggests that the species difference of the transition metals influences the vibrational behavior of TM–B and TM–TM bonds. In particular, a clearly different behavior of the vibration is observed for the transition-metal atoms in and out of plane. The results suggest the importance of the metal atoms in the diborides for superconductivity of these compounds and an intimate relationship between the local atomic displacements and the superconductivity.

The title systems, including EThE′, are treated at DFT level using a B3LYP functional and small-core quasirelativistic pseudopotentials. Most of the studied systems are bent, like their isoelectronic ThO 2 analogue, except for some anionic systems containing Ir. The bond lengths vary considerably and can lie above or below the sum of triple-bond covalent radii. Among the studied systems, the iridium-containing species show the strongest back-donation to Th. The bonding can be simply understood and could theoretically go up to a ‘24-electron principle’ limit at the actinide.

This Letter shows that the ion–water (oxygen) distances for ions of monovalent, divalent and trivalent elements including lanthanides increase linearly with the covalent radii of atoms. The slopes and intercepts of the lines are found to depend on the Golden ratio. The ionic radii in water have been evaluated and ion/water boundaries located. The hydration distances of the oxygen of water to the peripheries of the cations are of four types and are constants. The distance of the hydrogen of water to the peripheries of halogen anions, which is the length of the ‘hydrogen bond’, is also a constant.

By use of synchrotron radiation, X-ray diffraction of filled skutterudite YbFe4P12 has been studied at ambient and high pressures. The crystal structure of this compound was refined by the Rietveld analysis of powder X-ray diffraction data at ambient pressure. The positional parameters, bond distances and bond angles are obtained for YbFe4P12. The distance between Yb and P atoms in YbFe4P12 is 2.973 A, and almost agrees with the sum of the atomic radius (1.92 A) of Yb and the covalent radius (1.06 A) of P. Powder X-ray diffraction patterns of this phosphide were measured with a diamond-anvil cell and an imaging plate at room temperature and high pressures. The volume (V) vs. pressure (P) curve for YbFe4P12 is also fitted by the Birch equation of state. Bulk modulus (B0) and its pressure derivative ( B 0 ′ ) are 167±6 GPa and 2±1, respectively. The cell volume of YbFe4P12 decreased monotonically with increasing pressure up to 10 GPa. The V–P curve for YbFe4P12 suggests that the valence state in Yb ions does not change at high pressures. The electrical resistivity of the good sample of YbFe4P12 prepared at high pressure was measured at low temperatures. The resistivity minimum is observed at around 50 K. The physical properties of YbFe4P12 with a 4f-hole are compared to those of CeFe4P12 with a 4f-electron.

The local structures of molten BiCl 3 and its mixtures in LiCl–KCl eutectic melt were investigated by X-ray absorption fine structure (XAFS) technique. The first Bi–Cl correlation in molten pure BiCl 3 shows covalent nature, since the distance was almost the same as sum of the covalent radii of Bi and Cl and the coordination number was almost 3. The similar property was also observed in the mixture of 75% BiCl 3 with LiCl–KCl eutectic melt. Drastic change was detected in 25% BiCl 3 mixture melt. The first Bi–Cl distance was the sum of the ionic radii in molten 25% BiCl 3 melt. The results suggest that BiCl 3 changes from molecular liquid to ionic by mixing with alkali chlorides.

Measurements were made regarding the adsorption of lead, cadmium, copper, zinc and barium to freshwater surface coatings (biofilms and associated minerals), which were collected in Nanhu Lake in Jilin Province, PR China, in order to investigate the variability of adsorption capacities of these heavy metals mentioned in the above surface coatings. The adsorption of lead and other heavy metals to the biofilms was observed to decrease in the following order: copper, lead, zinc, cadmium, and barium. Generally, the values of Gamma(max) (the maximum adsorption, micromol/m(2)) increased with the standard electrode potential of metal elements used and were recorded as 166.7, 40.0, 29.4, 10.8, and 1.8 for copper, lead, zinc, cadmium and barium, respectively. The values of 1/Gamma(max) increased linearly with the decrease in values of the standard electrode potential of metal elements with a significant correlation (n=5, p=0.01) and increased linearly with the increase in values of covalent radius of metal elements with a significant correlation (n=5, p=0.05). This indicates that standard electrode potential and covalent radius were two of the principal characteristics of metals employed, causing the variation of lead and other heavy metal adsorption to the surface coatings.

A new correlation between first derivative of 1-normalized Shannon entropy ð›S h =›NÞ Z and gradient expansion parameter(a) in the extended version of the Fukui function (FF) is proposed in the present study. The calculated avalues from this studyis used to construct the corresponding FFs and is applied for calculation of hardness and covalent radii of some atomic system.Calculated hardness compares favorably with experimental values, and predicted covalent radii are in agreement with existingtheoretical values and experimental data. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Information theory; Shannon entropy; Fukui function 1. IntroductionFor a homogeneous system of N electronsconstrained to move in a volume V, the Fukui function(FF) can be written as:fðrÞ¼›rðrÞ›N n ¼rðrÞNð1Þfor homogeneous case r(r)andfðrÞ of course areconstant. For inhomogeneous case they are not,but Eq. (1) provides the natural local densityapproximation (LDA) for fðrÞ: It has been pointedout [1] that this definition of fðrÞ can be improvedby incorporating appropriate inhomogeneity cor-rections. Recently a specific form for this correc-tion obtained using gradient correction based onLDA formula as follow [2]fðrÞ¼rðrÞN½1 þaFðr;r;7r;7

The analysis of Mn–chalcogen bond length and of tetrahedral covalent radius of Mn (rtcMn) in AII1−xMnxBVI and in Mn chalcogenides, performed in the previous paper [Chem. Phys. Lett. 350 (2001) 577], was supplemented here by the case of AIII1−xMnxBV (AIII=In, Ga; BV=As, N) compounds. It was found that for chalcogen anions the dependence of rtcMn vs anion atomic number (Aa) can be linearly approximated. Inclusion of the respective rtcMn values for the group-V anions (As, N) reveals their consistency with the above rtcMn(Aa) dependence thus suggesting its more `universal' character.

As a continuation of our systematic investigation of the effect of substituents on the ring geometry and dynamics in silacyclobutanes and in order to explore the role of the silicon atom as a mediator for electronic interactions between the attached fragments, we studied the molecular structure of 1,1-diethynylsilacyclobutane (DESCB) by means of gas-phase electron diffraction and ab initio calculations. The structural refinement of the electron diffraction data yielded the following bond lengths ( r a ) and bond angles (uncertainties are 3σ): r( Si – C )=1.874(2) A , r( Si – C )=1.817(1) A , r(– C C –)=1.209(1) A , r( C – C )=1.563(2) A , ∠(C–Si–C)=79.2(6)°, ∠( C–Si–C )=106.5(6)°. The geminal Si–C C moieties were found to be bent outwards by 3.1(15)° and the puckering angle was determined to be 30.0(15)°. The evidently short Si–C bond length, which was also reproduced by the ab initio calculations, could be rationalized as being the consequence of the electronic interaction between the outer π charges of the triple bond and the 3pπ orbitals at the silicon atom. It is also likely that the conjugation of the geminal ethynyl groups leads to an enhancement of this bond contraction. Electrostatic interactions and the subsequent reduction of the covalent radius of the silicon atom may also contribute to this bond shortening. It has been found that the endocyclic Si–C bond length fits nicely within a scheme describing a monotonous decrease of the Si–C bond length with the increase of the electronegativity of the substituent in various geminally substituted silacyclobutanes. A series of related silacyclobutanes and acyclic diethynylsilanes have been studied by applying various ab initio methods and their optimized structures were compared to the structure of DESCB. Among these compounds are 1,1-dicyanosilacyclobutane (DCYSCB), which is isoelectronic to DESCB, 1,1-diethynylcyclobutane (DECB) which is isovalent to DESCB, monoethynylsilacyclobutane (MESCB) and monocyanosilacyclobutane (MCYSCB). Searching for reasonable support for the explanation of the structural results of DESCB we performed detailed natural population analysis as well as Mulliken population analysis (MPA) on DESCB and other related molecules. In contrast to the Mulliken charges, the natural atomic charges provided helpful information concerning the bonding properties in DESCB and the corresponding compounds. By varying the size of some basis sets, we could demonstrate the validity of the repeatedly discussed dependency of the Mulliken MPA on the basis set. For the performance of the quantum mechanical calculations we employed the following methods and basis sets: HF/6-31G(d,p), DFT/B3PW91/6-31G(d), DFT/B3PW91/6-311++G(d,p), MP2/6-31G(d,p) and MP2/6-311++G(d,p).

The analysis of the Mn–chalcogen atom (Te, Se, S) bond lengths in Mn-based AII–BVI and AIV–BVI metal chalcogenides (derived from X-ray absorption fine structure studies) as well as in Mn chalcogenides (from X-ray diffraction (XRD)) enabled the author to introduce certain self-corrections into the papers [Chem. Phys. Lett. 283 (1998) 313; Chem. Phys. Lett. 336 (2001) 226]. In particular, it was found that both the tetrahedral- and octahedral covalent radii of manganese depend on a choice of the anion species in the Mn–chalcogen atom bond, and therefore they cannot be considered as the element constants for Mn.

Two different formulations of electronegativity are discussed and briefly reviewed; one focuses upon the atom in a molecule, while the other is in terms of the chemical potential. Our analysis emphasizes the need for the latter approach to take account of the external potential. We also examine two proposed procedures for approximating empirical covalent radii, as the radial distances at which an atom's electrostatic potential V ( r ) equals (a) its ionization energy I , and (b) its chemical potential as given by 0.5( I + A ), where A is the electron affinity. Effective nuclear charges are computed for both distances for a group of 16 atoms.