Semiempirical study of electronic and bonding properties of iron silicide clusters

Summary Molecular orbital calculations of iron, silicon, and iron silicide clusters have been carried out using the UHF-MINDO/SR method. The nature of the bonding in these compounds has been investigated by analyzing the importance of bonding indexes and diatomic components of the total energy. It has been found that in iron silicide the strongest bond is formed between Fe-Si and that it arises mainly as the result ofsp-sp type orbital interactions. Althoughd orbitals show very little overlap withs-p orbitals, they do contribute significantly to bonding through electrostatic type diatomic interactions. By means of a detailed analysis ofsp, andd orbitals and total density of states (DOS) of Fe₇Si₇, Si₇Fe₇, Fe₁₅, and Si₁₇ clusters, the present calculations have permitted us to explain the origin of the iron silicide UPS experimental peaks.     

The Geometric Structure of Silver‐Doped Silicon Clusters

Cationic silver‐doped silicon clusters, Si_nAg⁺ (n=6–15), are studied using infrared multiple photon dissociation in combination with density functional theory computations. Candidate structures are identified using a basin‐hopping global optimizations method. Based on the comparison of experimental and calculated IR spectra for the identified low‐energy isomers, structures are assigned. It is found that all investigated clusters have exohedral structures, that is, the Ag atom is located at the surface. This is a surprising result because many transition‐metal dopant atoms have been shown to induce the formation of endohedral silicon clusters. The silicon framework of Si_nAg⁺ (n=7–9) has a pentagonal bipyramidal building block, whereas the larger Si_nAg⁺ (n=10–12, 14, 15) clusters have trigonal prism‐based structures. On comparing the structures of Si_nAg⁺ with those of Si_nCu⁺ (for n=6–11) it is found that both Cu and Ag adsorb on a surface site of bare Si_n⁺ clusters. However, the Ag dopant atom takes a lower coordinated site and is more weakly bound to the Si_n⁺ framework than the Cu dopant atom.     

Structure and properties of small silicon and aluminum clusters

Small Si_n and Al_n clusters (n = 3–10) were studied with the semiempirical molecular orbital method (MO) method SINDO1. For each n, various structures were optimized to determine the most stable structure. To obtain good qualitative agreement with available ab initio calculations d orbitals had to be omitted from the basis set. Both silicon and aluminum tend to build three-dimensional structures rather than two- or one-dimensional structures, except for n = 3 or 4. The structure growth was studied by approaching various sites of stable structures with one or more atoms. It was found that silicon and aluminum exhibit different structure growth, and consequently, different most-stable structures. Ionization potentials, HOMO -LUMO energy differences, binding energies per atom, and average atomic valencies are presented.     

Structures,magnetic properties,and electronic counting rule of metals‐encapsulated cage‐like M2 Si18 (M = Ti‐Zn) clusters

The geometries, magnetic properties and stabilities of the transition metal (TM) atoms encapsulated M₂Si₁₈ (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) clusters have been systematically calculated by using the density function theory with generalized gradient approximation. Only when the doping metal atom has no more than half‐full d electronic shell, a double hexagonal prism cage‐like M₂Si₁₈ structure could form. The total moments of M₂Si₁₈ are either 0 or 2μ_B. Co₂Si₁₈ is the most stable cluster among all 3d doped M₂Si₁₈ clusters. The model of shell closure at the TM atom may be helpful to understand the stability of M₂Si₁₈ clusters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

One-pot synthesis to engineer a Co-KIT-6 catalyst for the ring opening of methylcyclopentane

Co-KIT-6 mesoporous materials with Ia3d symmetry and Si/Co ratios of 50, 25, and 10 were prepared using hydrothermal one-pot synthesis. The Co-KIT-6 mesoporous materials were characterized by X-ray diffraction, N₂ adsorption–desorption isotherms, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy. The physicochemical characterization results show that all of the samples have well-ordered cubic mesostructures and that the structural integrity is preserved for n_Si/n_Co ratios as high as 10. It was found that most of the cobalt ions exist as isolated framework species, but for Co-KIT-6 with an n_Si/n_Co ratio of 10, the presence of extra-framework species/small cobalt oxide clusters cannot be excluded. The ability of these catalysts was tested by examining the conversion of the reaction of methylcyclopentane with hydrogen at atmospheric pressure and temperatures between 200 and 450 °C. The catalytic results show that their catalytic activity increases significantly with increasing cobalt content. The active sites, tetrahedrally coordinated Co and isolated atomic Co sites, were responsible for the endocyclic CC bond rupture between substituted secondary–tertiary carbon atoms, whereas the small clusters serve as active sites for the successive CC bond rupture. The ring-opening selectivity can be improved by increasing the density of isolated cobalt atom sites at low reaction temperatures.     

The Silicon Carbonyls Revisited: On the Existence of a Planar Si(CO)4

Recently, some works have focused attention on the reactivity of silicon atom with closed-shell molecules. Silicon may form a few relatively stable compounds with CO, i.e. Si(CO), Si(CO)₂, Si[C₂O₂], while the existence of polycarbonyl (n>2) silicon complexes has been rejected by current literature. In this paper, the reaction of silicon with carbonyl has been reinvestigated by density functional calculations. It has been found that the tetracoordinated planar Si(CO)₄ complex is thermodynamically stable. In Si(CO), silicon carbonyl, and Si(CO)₂, silicon dicarbonyl, the CO are datively bonded to Si; Si(CO)₄, silicon tetracarbonyl, may be viewed as a resonance between the extreme configurations (CO)₂Si + 2CO and 2CO + Si(CO)₂; while Si[C₂O₂], c-silicodiketone, is similar to the compounds formed by silicon and ethylene. A detailed orbital analysis has shown that the Si bonding with two CO is consistent with the use of sp ²-hybridized orbitals on silicon, while the Si bonding with four CO is consistent with the use of sp ² d-hybridized orbitals on silicon, giving rise to a planar structure about Si.     

Electronic structure of clusters modeling silica

Self-consistent-field-Xα-scattered wave calculations on clusters Si₂O₇^6? and H₆Si₂O₇ modeling silica have been performed. Incorporation of Si 3d orbitals produces significant changes in the overall valence structure. In addition to σ Si — O bonds, there exists a bonding π character due to the participation of O 2p and Si 3d. Hydrogen terminators do not seem to correct edge effects for these π states.     

High Magnetic Moments in Manganese‐Doped Silicon Clusters

We report on the structural, electronic, and magnetic properties of manganese‐doped silicon clusters cations, Si_nMn⁺ with n=6–10, 12–14, and 16, using mass spectrometry and infrared spectroscopy in combination with density functional theory computations. This combined experimental and theoretical study allows several structures to be identified. All the exohedral Si_nMn⁺ (n=6–10) clusters are found to be substitutive derivatives of the bare Si_n+1⁺ cations, while the endohedral Si_nMn⁺ (n=12–14 and 16) clusters adopt fullerene‐like structures. The hybrid B3P86 functional is shown to be appropriate in predicting the ground electronic states of the clusters and in reproducing their infrared spectra. The clusters turn out to have high magnetic moments localized on Mn. In particular the Mn atoms in the exohedral Si_nMn⁺ (n=6–10) clusters have local magnetic moments of 4 μ_B or 6 μ_B and can be considered as magnetic copies of the silicon atoms. Opposed to other 3d transition‐metal dopants, the local magnetic moment of the Mn atom is not completely quenched when encapsulated in a silicon cage.     

CNDO/2 calculations of relaxation and reconstruction of diamond and silicon [111] surfaces

CNDO /2 calculations have been performed on the clusters X₄H₉ and X₄Y₉ modeling the [111] diamond and silicon surfaces. The X is either carbon or silicon atom and the Y is a pseudoatom containing one sp³ hybrid orbital. It is shown that in the CNDO /2 approximation in the foregoing pseudoatom models, the charge distribution of the cluster is better than the hydrogen atom, because the electronegativity of the hydrogen differs significantly from the electronegativity of the sp² orbital of the silicon atom. Using the CNDO /2 parametrization, the electronegativity of the hydrogen is very near to the electronegativity of the sp³ orbital of the carbon atom, thus the hydrogen can be used for the saturation of the carbon clusters.     

Sc4Pt7Si2 – An Intergrowth Structure of ScPtSi and ScPt Related Slabs

The metal‐rich silicide Sc₄Pt₇Si₂ was synthesized by arc‐melting. Sc₄Pt₇Si₂ crystallizes with its own structure type, space group Pbam. The structure was refined from single‐crystal X‐ray diffractometer data: a = 647.6(1), b = 1617.1(3), c = 398.96(9) pm, wR2 = 0.0495, 807 F² values and 42 variables. Sc₄Pt₇Si₂ is an intergrowth structure of slightly distorted ScPtSi (TiNiSi type) and ScPt (CsCl type) related slabs. The silicon atoms have the typical coordination number 9 (4 Sc + 5 Pt) in the form of a tricapped trigonal prism. Together, the platinum and silicon atoms build up a complex three‐dimensional [Pt₇Si₂] network with short Pt–Si (238–246 pm) and Pt–Pt (282–303 pm) distances. The scandium atoms fill distorted square prismatic or pentagonal prismatic voids within this network, also with short Sc–Pt distances (276–308 pm). The structural difference of these two scandium species is reflected by substantial discrepancies in ⁴⁵Sc chemical shifts. The quadrupolar interaction parameters that were estimated from the nutation behavior of the two signals were used for an assignment to the two sites.     

Chemical Bonding in Silicon Carbonyl Complexes

Although silylene-carbonyl complexes are known for decades, only recently isolable examples have been accomplished. In this work, the bonding situation is re-evaluated to explain the origins of their remarkable stability within the Kohn-Sham molecular orbital theory framework. It is shown that the chemical bond can be understood as CO interaction with the silylene via a donor-acceptor interaction: a σ-donation from the σ_CO into the empty p-orbital of silicon, and a π-back donation from the sp² lone pair of silicon into the π*_CO antibonding orbitals. Notably, it was established that the driving force behind the surprisingly stable Si−CO compounds, however, is another π-back donation from a perpendicular bonding R−Si σ-orbital into the π*_CO antibonding orbitals. Consequently, the pyramidalization of the central silicon atom cannot be associated with the strength of the π-back donation, in sharp contrast to the established chemical bonding model. Considering this additional bonding interaction not only shed light on the bonding situation, but is also an indispensable key for broadening the scope of silylene-carbonyl chemistry.     

Molecular shape,shape of the geometrically active atomic states,and hybridization

In a recent publication [C. A. Nicolaides and Y. Komninos, Int. J. Quant. Chem. 67 , 321 (1998)], we proposed that in certain classes of molecules the fundamental reason for the formation of covalent polyatomic molecules in their normal shape is to be found in the existence of a geometrically active atomic state (GAAS) of the central atom, whose shape, together with its maximum spin‐and‐space coupling to the ligands, predetermines the normal molecular shape (NMS). The shape of any atomic state was defined as that which is deduced from the maxima of the probability distribution ϱ(cos θ₁₂) of the angle formed by the position vectors of two electrons of an N‐electron atom. Because the shape of the GAAS determines the NMS and because the NMS allows the construction of corresponding hybrid orbitals, we examined and discovered the connection between the GAAS shape and Pauling's function for the strength of two equivalent orthogonal orbitals at angle θ₁₂ with one another. It is shown that the computed ϱ(cos θ₁₂) of the GAAS can be cast in a form which allows the deduction of the composition of the hybrid orbitals of maximum spin states with configurations sp³, sp³d⁵, sp³d⁵f⁷, slⁿ, s²lⁿ and the demonstration of the central atom's tendency to form bonds in directions which coincide with the nodal cones of the hybrid bond orbitals. These results not only reinforce the validity of the theory as to the fundamental “mechanism” for the formation in the normal shape of coordination compounds and covalently bonded polyatomic molecules, but also provide the justification for the relevance and importance of the hybridization model. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 25–34, 1999     

A theoretical study of the dependence of the ASxSi6−x cluster structures and properties on composition

The effect of the composition ratio between arsenic and silicon atoms on the structures and properties of As_xSi_6?x (x = 0–6) have been systematically investigated using the density functional theory at the B3LYP/6‐311+G* level. The As_xSi_6?x clusters prefer substitutional rather than attaching structures; the Si‐rich clusters favor Si₆‐like structures, whereas the As‐rich clusters prefer As₆‐like structures. The As atoms locating at the framework may explain the difficulty of removal of arsenic impurities from polycrystalline silicon. In general, the average binding energies gradually decrease, implying the As_xSi_6?x clusters become increasingly unstable as x increases. Both the HOMO‐LUMO gaps and the As‐dissociation energies present a strong even–odd alternation, implying alternating chemical stability, with the even x members being more stable than the odd ones. The dissociation energies of an As atom from As_xSi_6?x are: 3.07, 2.84, 1.84, 2.52, 1.86, and 2.85 eV, for x = 1–6, respectively, and 3.80, 3.08, 2.64, 3.01, 2.93, 3.16 eV for Si (x = 0–5). These dissociation energy results should provide a useful reference for further experimental investigations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Grown from lithium flux,the ErCo5Si3.17 silicide is a combination of disordered derivatives of the UCo5Si3 and Yb6Co30P19 structure types

A ternary hexaerbium triacontacobalt enneakaidecasilicide, ErCo₅Si_3.17, crystallizes as a combination of disordered variants of the hexagonal UCo₅Si₃ (P6₃/m) and Yb₆Co₃₀P₁₉ (P) structure types and is closely related to the Sc₆Co₃₀Si₁₉ and Ce₆Rh₃₀Si₁₉ types. The Er, Co and three of the Si atoms occupy sites of m.. symmetry and a fourth Si atom occupies a site of .. symmetry. The environment of the Er atom is a 21‐vertex pseudo‐Frank–Kasper polyhedron. Trigonal prismatic coordination is observed for the Si atoms. The Co atoms are enclosed in heavily deformed cuboctahedra and 11‐vertex polyhedra. Crystallochemistry analysis and the data from electronic structure calculations (TB–LMTO–ASA) suggest that the Er atoms form positively charged cations which compensate the negative charge of the [Co₁₂Si₉]^m− polyanions.     

The Binary Silicides Eu5Si3 and Yb3Si5 – Synthesis,Crystal Structure,and Chemical Bonding

The binary silicides Eu₅Si₃ and Yb₃Si₅ were prepared from the elements in sealed tantalum tubes and their crystal structures were determined from single crystal X-ray data: I4/mcm, a = 791.88(7) pm, c = 1532.2(2) pm, Z = 4, wR2 = 0.0545, 600 F² values, 16 variables for Eu₅Si₃ (Cr₅B₃-type) and P62m, a = 650.8(2) pm, c = 409.2(1) pm, Z = 1, wR2 = 0.0427, 375 F² values, 12 variables for Yb₃Si₅ (Th₃Pd₅ type). The new silicide Eu₅Si₃ contains isolated silicon atoms and silicon pairs with a Si–Si distance of 242.4 pm. This silicide may be described as a Zintl phase with the formula [5 Eu²⁺]¹⁰⁺[Si]^4–[Si₂]^6–. The silicon atoms in Yb₃Si₅ form a two-dimensional planar network with two-connected and three-connected silicon atoms. According to the Zintl-Klemm concept the formula of homogeneous mixed-valent Yb₃Si₅ may to a first approximation be written as [3 Yb]⁸⁺[2 Si^–]^2–[3 Si^2–]^6–. Magnetic susceptibility investigations of Eu₅Si₃ show Curie-Weiss behaviour above 100 K with a magnetic moment of 7.85(5) μ_B which is close to the free ion value of 7.94 μ_B for Eu²⁺. Chemical bonding in Eu₅Si₃ and Yb₃Si₅ was investigated by semi-empirical band structure calculations using an extended Hückel hamiltonian. The strongest bonding interactions are found for the Si–Si contacts followed by Eu–Si and Yb–Si, respectively. The main bonding characteristics in Eu₅Si₃ are antibonding Si1₂-π* and bonding Eu–Si1 states at the Fermi level. The same holds true for the silicon polyanion in Yb₃Si₅.     

Li8Si8, Li10Si9, and Li12Si10: Assemblies of Lithium-Silicon Aromatic Units

We report the global minima structures of Li₈Si₈, Li₁₀Si₉, and Li₁₂Si₁₀ systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters T_d−Li₄Si₄ and C_2v−Li₆Si₅. Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit‘s aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks.     

Structures and electronic properties of the small rubidium‐doped silicon RbSin (n = 1–12) clusters

The geometries, relative stabilities, and electronic properties of small rubidium‐doped silicon clusters RbSi_n (n = 1–12) have been systematically investigated using the density functional theory at the B3LYP/GENECP level. The optimized structures show that lowest‐energy isomers of RbSi_n are similar with the ground state isomers of pure Si_n clusters and prefer the three‐dimensional for n = 3–12. The relative stabilities of RbSi_n clusters have been analyzed on the averaged binding energy, fragmentation energy, second‐order energy difference, and highest occupied molecular orbital‐lowest unoccupied molecular orbital energy gap. The calculated results indicate that the doping of Rb atom enhances the chemical activity of Si_n frame and the magic number is RbSi₂. The Mulliken population analysis reveals that the charges in the corresponding RbSi_n clusters transfer from the Rb atom to Si atoms. The partial density of states and chemical hardness are also discussed. © 2014 Wiley Periodicals, Inc.     

Two‐Orbital Three‐Electron Stabilizing Interaction for Direct Co2+?As3+ Bonds involving Square‐Planar CoO4 in BaCoAs2O5

The quest for new oxides with cations containing active lone‐pair electrons (E) covers a broad field of targeted specificities owing to asymmetric electronic distribution and their particular band structure. Herein, we show that the novel compound BaCoAs₂O₅, with lone‐pair As³⁺ ions, is built from rare square‐planar Co²⁺O₄ involved in direct bonding between As³⁺E and Co²⁺ d_z2 orbitals (Co As=2.51 Å). By means of DFT and Hückel calculations, we show that this σ‐type overlapping is stabilized by a two‐orbital three‐electron interaction allowed by the high‐spin character of the Co²⁺ ions. The negligible experimental spin‐orbit coupling is expected from the resulting molecular orbital scheme in O₃AsE–CoO₄ clusters.     

Two‐Orbital Three‐Electron Stabilizing Interaction for Direct Co2+As3+ Bonds involving Square‐Planar CoO4 in BaCoAs2O5

The quest for new oxides with cations containing active lone‐pair electrons (E) covers a broad field of targeted specificities owing to asymmetric electronic distribution and their particular band structure. Herein, we show that the novel compound BaCoAs₂O₅, with lone‐pair As³⁺ ions, is built from rare square‐planar Co²⁺O₄ involved in direct bonding between As³⁺E and Co²⁺ d_z2 orbitals (Co? As=2.51 Å). By means of DFT and Hückel calculations, we show that this σ‐type overlapping is stabilized by a two‐orbital three‐electron interaction allowed by the high‐spin character of the Co²⁺ ions. The negligible experimental spin‐orbit coupling is expected from the resulting molecular orbital scheme in O₃AsE–CoO₄ clusters.     

Mg15Ir5Si2, ein Magnesium‐Iridiumsilicid mit isolierten Ir5Si2‐Baugruppen

Mg₁₅Ir₅Si₂ a Magnesium Iridium Silicide with Isolated Ir₅Si₂ Building Groups Mg₁₅Ir₅Si₂ (tetragonal, P4₂/n, a = 1371.7(1) pm, c = 873.0(2) pm, Z=4, 1497 reflections, 103 parameters, R1 = 0.048) was prepared by reaction of the elements at 900 °C in sealed tantalum ampoules. The compound is the silicide with the highest alkaline earth metal content known so far. It is the first example of a silicide with an isolated transition metal silicon building group embedded in a matrix of non‐transition metal atoms. The structure contains planar Ir₂SiIrSiIr₂ groups with silicon atoms in nearly trigonal planar coordination of three iridium atoms (d_Ir‐Si = 235 and 236 pm).