Structural,Electronic and Tunable Magnetic Properties of Transition Metal Doped Rh<Subscript>8</Subscript> Cluster from First Principles Calculation

The configurations, electronic and magnetic properties of the Rh₇M (M?=?3d, 4d transition metal) are systematically investigated within the framework of the generalized gradient approximation density-functional theory (DFT-GGA). The results indicated that the ground state structures of Rh₇M (M?=?3d) clusters prefer to a bicapped octahedron configuration, while the Rh₇M (M?=?4d) clusters present a different degree of geometry reconstruction relative to the perfect cubic structure of Rh₈ cluster. In most cases, the doped clusters show relatively higher stability, indicating that impurity atoms could stabilize the pure Rh₈ cluster; the Rh₇M (M?=?3d, 4d) have smaller frontier orbital energy gaps in comparison to the host. The magnetic moments of Rh₇M (M?=?3d, 4d) exhibit a tunable magnetism with range from 3μ_B to 13μ_B and the Fe atom doping enhances the magnetic moment of mixed cluster.     

The local electronic and magnetic properties of Fe impurity in Al clusters

The local magnetic property,d electronic structure and the charge transfer effect of Fe impurity in Al clusters have been studied by using a tight-binding model Hamiltonian in the unrestricted Hartree-Fock approximation, which includes intra-atomic and interatomic Coulomb interactions. We have obtained that local magnetic moment of Fe impurity in FeAl _N clusters decreases with increasing cluster size and convergences to zero (that of bulk given by Anderson) withN larger than 12, meanwhile, the local magnetic moment for smaller clusters depends on the clusters size and it is a monotonous descent function of cluster size. We have also found that the spin splitting of the localizedd states decreases as the cluster size increases, which mainly results from the interaction between the localized electrons of Fe atom and the delocalized electrons of Al atoms.     

Cluster molecular orbitals and an orbital symmetry view of solid phase transitions in perovskites

The molecular orbitals, normalization constants and energies of the M₈(O_h), M₄(T_d) and M₆(O_h) clusters are derived and tabulated through the d-atomic orbitals. A vector method, adapted to computer application, is devised to compute s, p and d overlap between variously oriented orbitals at atoms that do not have co-directional local axes. Mixing of σ, π and δ orbitals to give the same irreducible representation is also included. As illustrations, the orbitals of Sr₈, La₈, TiO₆ and AlO₆ clusters are computed by the Mulliken—Wolfsberg and Helmholz approximations. During solid phase transitions in the perovskite structures of SrTiO₃ and LaAlO₃, the TiO₆ octahedron rotates about the C₄ axis whereas the AlO₆ octahedron rotates about the C₃ axis. This difference is explained qualitatively in terms of the relative symmetries of the cluster HOMOs and LUMOs using the second-order Jahn—Teller effect. Allusions are made to the application of this cluster symmetry approach to other systems.     

Preparation and properties of the systems Fe2−xCrxWO6, Fe2−xRhxWO6, and Cr2−xRhxWO6

Solid solutions of the end members Fe₂WO₆, Cr₂WO₆, and Rh₂WO₆ have been prepared and their crystallographic and magnetic properties studied. All solid solutions crystallize with the trirutile structure, and their magnetic behavior is characterized by the existence of antiferromagnetic interactions and effective molar Curie constants corresponding to those expected from contributions of the spinonly moments of high-spin Fe³⁺, Cr³⁺, and diamagnetic low-spin Rh³⁺ ions. Fe₂WO₆ crystallizes with the tri-α-PbO₂ structure and is antiferromagnetic and conducting. The random rutile Rh₂WO₆ is conducting, and the difference between its magnetic and electric properties and those of the inverse trirutile Cr₂WO₆ are discussed in terms of possible interactions between Cr³⁺(3d) or Rh³⁺(4d) orbitals and W⁶⁺(5d) orbitals.     

Synthesis, structure, and catalytic properties of polymer-immobilized rhodium clusters

Immobilized Rh₆ clusters (cyclohexene hydrogenation catalysts) were prepared by the polymer-analogous transformations or copolymerization of cluster-containing monomers and characterized. Intermediates formed in the course of a catalytic reaction were studied using IR spectroscopy, XPS, and atomic force microscopy. It was found that the relative intensity of a low-energy line in the Rh3d _5/2 spectrum of the initial polymer-immobilized cluster in the XPS spectrum of Rh₆ increased in the course of hydrogenation. The catalytic activity of the immobilized complex changed symbatically with both the number of Rh atoms bound to the H(CO) group and the number of Rh atoms, the charge on which was greater than that in the parent cluster. Some experimental evidence was obtained in favor of the hypothesis of cluster fragmentation in the course of hydrogenation with the formation of highly active, most likely, nanosized particles, which are true catalysts, in low concentrations. The surface of macrocomplex particles after hydrogenation became more homogeneous and hydrophilic; this fact is also indicative of an increase in the concentration of polar functional groups in surface layers. This was likely due to Rh-Rh bond cleavage in the polymer-immobilized cluster.     

Trends in Geometric,Energetic, Electronic,and Magnetic Properties of Vanadium–Copper Clusters Cu<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>V with <Emphasis Type="Italic">n</Emphasis> = 1–12: Density Functional Calculations

A systematic quantum chemical investigation on the geometric, energetic, electronic and magnetic properties of vanadium-copper nanoalloy clusters (n = 1–12) is performed by using BPW91/LanL2DZ calculations. The calculated results show that the structural evolution of Cu _n V clusters favors a compact and icosahedral growth pattern and V atom favors occupying the most highly coordinated position. Energetic properties show that doping of one V atom contributes to strengthening the stability of the copper clusters with the growth of the clusters. The stacking mode of clusters apparently has a more important effect on the clusters stability than the electronic structure. However, electronic structures have some contribution to the stability of Cu _n V clusters as well. The electronic properties of Cu _n V are analyzed through vertical ionization potential (VIP), vertical electron affinity (VEA) and chemical hardness (η). The magnetism calculations show that when doping V atom in copper clusters, the cluster system generate a very large magnetic moment and its contribution mainly comes from the 3d orbital of doping-V atom.     

Shell structure of electronic states of Cu clusters

Electronic states of icosahedral Cu₁₃ and Cu₁₂ clusters were calculated by the DV-Xα method which took into account of many-electron effects. The valence 3d orbitals are localized within atom and isolated from the valence 4s orbitals extended over the whole cluster. The characteristic feature of the states ofs valence electrons of Cu₁₃ cluster well corresponds to that of one-electron picture i.e. the shell model of the 3-dimensional isotropic harmonic oscillator potential. Electronic states of valence levels of Cu₁₂ cluster which have no central atom fairly well correspond to those of a combined potential of the harmonic oscillator and a 3-dimensional Gaussian potential barrier at the center of the cluster.     

Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements

Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant d_π orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled d_π orbitals. Both ligands readily form bridges that span M? M bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)₃], [CpW(CO)₂], [Co(CO)₃], and CH are related by the isolobal analogy. Thus the compounds [(RO)₃W ? W(OR)₃], [Cp(CO)₂W?W(CO)₂Cp], hypothetical [(CO)₃Co?Co(CO)₃], and HC?CH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W₃(μ₃-CR)(OR′)₉] versus [Co₃(μ₃-CR)(CO)₉] and [W₄(C)(NMe)(OiPr)₁₂] versus [Fe₄(C)(CO)₁₃]—but differ with respect to M? M bonding. The carbonyl clusters use e_g-type orbitals for M? M bonding whereas the alkoxide clusters employ t_2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or M? M bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M₂(OR)₆] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.     

ESR investigation of paramagnetic carbonyl-metal clusters of high nuclearity

An ESR study has been made of the high nuclearity paramagnetic metal cluster anions [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3-, [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- and [Co₆(CO)₈(μ₂-CO)₆C]^-. The assignment of the HOMO is based on a mixed valence model which relates the g tensor components of cluster systems to those of an appropriate conventional paramagnetic center. With this model the HOMOs of [Rh₁₂(CO)₁₃(μ₂-CO)₁₀(C)₂]^3- and of [Co₁₃(CO)₁₂(μ₂-CO)₁₂(C)₂]^4- are found to be mainly comprised of metal d_z² atomic orbitals, while for [Co₆(CO)₈(μ₂-CO)₆C]^- a large overlap between d atomic orbitals and ligand orbitals is suggested. The occupation of the valence molecular orbitals deduced from the ESR data is consistent with the variations in MM bond distance observed by X-ray analysis.     

Complexes of Rh(III), Ir(III), and Pt(IV) with metallated 2-(n-tolyl)pyridine,ethylendiamine, acetate,and diethyldithiocarbamate ligands

cis-C,C Isomers of the [M(ptpy)₂(L∧L)](PF₆)_Z complexes [M = Rh(III), Ir(III), Pt(IV); ptpy^? = deprotonated form of 2-(n-tolyl)pyridine, (L∧L) = acetate, trifluoroacetate, or diethyldithiocarbamate anions, or ethylenediamine; z = 0, 1, 2] were prepared and characterized by ¹H and ¹⁹F NMR, IR, electronic absorption and emissions spectroscopy, and by voltammetry methods. The highest occupied and the lowest unoccupied molecular orbitals were assigned to d _π and π*_ptpy orbitals of the metal and the metallated ligand. Luminescence of the complexes in the visible spectral region was assigned to the spin-forbidden optical transition from the lowest energy state of the mixed nature (π_ptpy-π*_ptpy/d _x -π*_ptpy).     

Synthesis of [M(NH3)5Cl](ReO4)2 (M = Cr, Co, Ru, Rh, Ir) and investigation of thermolysis products. Crystal structure of [Rh(NH3)5Cl](ReO4)2

Complex salts [M(NH₃)₅Cl](ReO₄)₂, where M = Cr, Co, Ru, Rh, Ir, have been prepared. The crystal structure of [Rh(NH₃)₅Cl](ReO₄)₂ was determined by single crystal X-ray diffraction. Crystal data: a = 17.369(4) Å, b = 7.7990(16) Å, c = 11.218(2) Å, V = 1430.5(5) ų, space group C2/m, Z = 4, d _calc = 3.19 g/cm³, R = 0.0447. Complex salts from the above series are shown to be isostructural; they were defined by X-ray crystallography. Thermal decomposition of the compounds in an inert atmosphere and under hydrogen has been studied. According to X-ray phase analysis (XRPA) data, the M_0.33Re_0.67 (M = Co, Ru, Rh, Ir) monophase solid solutions are the products of reduction of the salts under hydrogen.     

Studies of the spin-Hamiltonian parameters and the Jahn–Teller distortions for tetragonal Cu(H2O)62+ clusters in trigonal A2Mg3(NO3)12·24H2O (A = La,Bi) crystals

The anisotropic and isotropic spin-Hamiltonian parameters (g factors and hyperfine structure constants) of tetragonal Cu(H₂O)₆²⁺ clusters due, respectively, to the static and dynamic Jahn–Teller effects for Cu²⁺ in trigonal A₂Mg₃(NO₃)₁₂·24H₂O (A = La, Bi) crystals are calculated from the high-order perturbation formulas based on the cluster approach. In the approach, the admixture between the d orbitals of 3dⁿ ion and the p orbitals of ligand ion via covalence effect is considered. All of the calculated results are in agreement with the experimental values. The tetragonal elongations (characterized by ΔR = R_// ? R_⊥) of Cu(H₂O)₆²⁺ cluster due to the Jahn–Teller effect in A₂Mg₃(NO₃)₁₂·24H₂O crystals are acquired from the calculations. The results are discussed.     

Electronic Structure and Formation Heat of Pu3M and PuM3 (M=Ga, In, Sn and Ge) Compounds

The equilibrium structure, electronic structure, and formation heat of Pu₃M (PuM₃) (M=Ga, In, Sn, and Ge) compounds with AuCu₃ structure have been calculated using full potential linear augmented plane wave (FPLAPW) method within generalized-gradient approximation (GGA) including spin-orbit coupling (SOC) and spin polarization (SP). The calculated lattice parameters are in good agreement with the experimental values. Density of state analysis shows hybridization effects between Pu and M are governed by the competitions depending on the M amount: Pu 6d-Pu 5f, M p-Pu 6d, and M sp-M sp interactions. Electronegativity difference and electronic hybridization effect are two important factors to influence the formation heat and stability of Pu₃M (PuM₃) compounds. The larger is the electronegativity difference and the lower is M s-band or p-band center relative to the Fermi energy, the more negative is the formation heat and the more stable are Pu₃M (PuM₃) compounds.     

Chemical Research Faculties: An International Directory : Edited by G.L. Pollock. The American Chemical Society, Washington, 1984, xxxv + ca. 530 pages. US $129.95 (U.S.A. and Canada); US $155.95 (elsewhere). ISBN 0-8412-0817-4.

Heterobinuclear complexes of formula [LMCl₂(pz)M′(tfb)] (M = Ru, L = p-cymene, M′ = Rh; M = Ir, L = C₅Me₅, M′ = Rh; M = Rh, L = C₅Me₅, M′ = Ir) and [(C₅Me₅)IrCl(pz)₂Rh(tfb)] (tfb = tetrafluorobenzo[5.6]bicyclo[2.2.2]octan-2,5,7-triene) have been prepared. The molecular structure of [(p-cymene)Ru(μ-Cl)2(μ-pz)Rh(tfb)] has been determined by X-ray diffraction. It consists of two moieties, (p-cymene)Ru and (tfb)Rh, triply-bridged by a pyrazolate group and two chlorine atoms.     

Structure, stability and electronic property of the gold-doped germanium clusters: AuGe n (n = 2–13)

The structure, stability and electronic property of the AuGe _n (n = 2–13) clusters with different spin configurations are systematically investigated with density-functional theory approach at UB3LYP/LanL2DZ level. In examining the lowest energy structures, it is found that the growth behaviors for the small-sized AuGe _n (n = 2–9) clusters and relatively large-sized AuGe _n (n = 10–13) clusters are different. As the number of Ge atom increases, the Au atom would gradually move from convex to surface and to interior sites. For the most stable structures of AuGe _n (n = 10–13) clusters, the Au atom would be completely surrounded by the Ge atoms to form Au-encapsulated Ge _n cages. Natural population analysis shows that the charges always transfer from the Au atom to the Ge _n framework except for the AuGe₂ cluster. This indicates that the Au atom acts as electron donor even the 5d orbitals of the Au atom are not significantly involved in chemical bonding. The analyses of the average atomic binding energies as well as the dissociation energies and the second-order differences of total energy show that the AuGe _n clusters with n = 5, 9 and 12 are more stable than their neighboring ones, in which the bicapped pentagonal prism AuGe₁₂ in D _2d symmetry is most stable. The highest occupied molecular orbital–lowest unoccupied molecular orbital gaps are explored to be in the region of semiconductors and the more stable clusters have slightly smaller gaps. It could be expected that the stable clusters might be considered as the novel building blocks in practical applications, e.g., the cluster-assembled semiconductors or optoelectronic material.     

Organic Reactions : Volume 23, W.G. Dauben,editor-in-chief,John Wiley & Sons,New York, 1976, vii + 520 pp., $32.45, £ 19.75.

M(R)(CO)(PPh₃)₂ compounds (M = Rh, Ir; R = Me, p-tolyl, p-methoxyphenyl, C₆F₅, Ph₃C and Ph) have been prepared by oxidative addition to a p⁸ halogen or a d¹⁰ anionic complex followed by electrochemical reduction.     

Gold and platinum molecular cluster compounds studied by 197Au Mössbauer Spectroscopy

The technique of ¹⁹⁷Au Mössbauer absorption spectroscopy has been used to study four varieties of the high nuclearity gold molecular cluster compounds abbreviated as Au₅₅, all having a cuboctahedral structure, with 12 PPh₃-ligands or modifications of PPh₃. The technique of emission spectroscopy developed in our laboratory [1] has been applied to four different molecular platinum carbonyl cluster compounds of varying cluster nuclearity, abbreviated as Pt₃₈, Pt₂₆, Pt₂₄, and Pt₁₉ [2]. The four Au55 compounds were studied, both as dry materials and as frozen solutions. The M6ssbauer parameters of the chlorine ligated site of the water soluble version differ strongly due to the effect of Na⁺ on the Au-Cl distances. In the frozen solution, the effect of the ionic charge on this cluster can be clearly seen. The similarity of the spectra of all the Pt compounds, especially the occurrence of a substantial singlet contribution in all spectra, is explained by a coalescing mechanism for the low-nuclearity clusters, induced by the neutron irradiation damage inflicted while preparing the sample as M6ssbauer sources. The observed decrease of the absorption intensity with increasing temperature is evidence for a cluster character of the sample remaining after neutron irradiation.     

Structural, Electronic, and Magnetic Properties of MB n (M?=?Y, Zr, Nb, Mo, Tc, Ru, n?≤?8) Clusters

The geometries, stabilities, electronic, and magnetic properties of MB _n (M?=?Y, Zr, Nb, Mo, Tc, Ru, n????8) clusters have been systematically investigated by density functional theory. It is shown that the lowest energy structures of MB _n (n????3) clusters can be obtained by substituting one B atom in the lowest energy structures of B _n+1 clusters in most cases. After n????8, the 3D configurations prevail and become the lowest-energy structures. The second-order energy difference and the dissociation energy show YB₇, ZrB₇, NbB₆, MoB₆, TcB₆, RuB₆ clusters possess relatively higher stabilities. The doped-M atoms improve the chemical activity of the host clusters in most cases; but different M atom has different effect on B atom??s electronic structure. The binding strengths are strong between M and B _n , which plays an important role in the M?CB growth mechanisms. It is interesting that the relative orientation between the magnetic moments of the M (M?=?Zr, Nb, Mo, Tc, Ru) atoms and those of its neighboring B atoms exhibits ferromagnetic or antiferromagnetic alignment in contrast to the ferromagnetic alignment of YB _n .     

Transition-element hexafluoride systems in ionic lattices. A Suhf molecular orbital study

An unrestricted Hartree-Fock SCFMO method, based on the MCZDO method of Brown and Roby, suitable for computing spin densities in transition-element compounds, is described. The method is used to study spin densities on fluorine in Cs₂MnF₆, K₂NaCrF₆, K₂MnF₆, K₂NaFeF₆, KMnF₃, RbMnF₃ and KNiF₃ using a “cluster” approximation in which a MF ₆ ^n? unit is explicitly considered. Excellent agreement is obtained between calculated and experimental spin parameters. The effect of the lattice is incorporated by using the electrostatic approximation of Brown, O'Dwyer and Roby. The lattice potential for these highly symmetric systems is found to have little effect on spin densities and charge distributions, but it effects substantial stabilization of the anion molecular orbitals. A general feature of the results is that central-atom 4p orbitals are scarcely involved in bonding, this being confined to the 3d and to some extent the 4s orbitals. Comments are offered on the lack of spin symmetry in the unrestricted Hartree-Fock wavefunctions of these systems, and the need to evaluate the core hamiltonian as accurately as possible.     

General properties of the electronic structure of alkali metal clusters and Ia-IIa mixed clusters

The results of the systematic ab-initio CI investigation of neutral and charged Li _n , Na _n , BeLi _k and MgNa _k clusters are summarized and analyzed. The general characteristic features of the electronic structure are pointed out:a) The participation of the atomic orbitals, which are empty in Ia and IIa metal atoms, allows for a higher valency of these atoms in clusters.b) Jahn-Teller and pseudo-Jahn-Teller effects strongly influence the electronic and geometric structure of clusters.c) Deformations of cluster geometry can lead to biradicaloid structures with higher spin multiplicity in their ground states.d) The peculiarities of the electronic structures of clusters can be deduced from the presence of many “surface” atoms. The theoretical results agree with experimental data presently available and they are useful for interpretation of the experimental findings.