Ligand-Free Metal Clusters

Recent advances in the synthesis and spectroscopic characterization of ligand-free metal clusters immobilized in cryogenic rare gas matrices have contributed greatly to the understanding of electronic, geometric, dynamic, and chemical bonding properties of a wide range of uni- and bimetallic clusters as a function of nuclearity and metal type. The knowledge accumulated on molecular metal aggregates devoid of ligands and isolated on various supports will form an important data base for gauging the reliability of quantum chemical calculations on metal clusters, as well as for comprehending certain aspects of chemisorption on, and catalysis by, supported metal clusters. It can be envisaged that information on ligand-free metal clusters entrapped in a wide range of matrix environments in conjunction with the data for these same metal clusters in the gas phase and in molecular beams will probably contribute towards understanding metal-support interactions and to the designed synthesis of a new breed of high-technology heterogeneous catalysts in the not too distant future.     

Catalysis by Molecular Metal Clusters

Molecular metal clusters form a very large and diverse family. They present the opportunity of modeling the intermediates involved in surface mediated catalytic reactions, of providing a source of very reactive mononuclear metal fragments, and of effecting catalytic cycles in which the cluster remains intact. The last mentioned aspect is the subject of this review article. The state-of-the-art of cluster catalysis is critically analyzed. The possibilities offered by molecular metal catalysts of performing catalytic reactions at multimetal atom sites are also discussed.     

Condensed Metal Clusters

The chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal-rich compounds of transition metals with p-elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal-metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.     

Crystal Structure of BaNb6.3(1)Ti3.6(1)O16 containing Nb6O12, fused Ti3O13 Clusters and Ti3O10 Units

A new phase, BaNb_6.3(1)Ti_3.6(1)O₁₆, has been synthesised. Electron diffraction studies indicate an hexagonal substructure with unit cell parameters a ≈ 8.9 Å and c ≈ 9.5 Å. In some of the ED patterns superstructure reflections are present, indicating a supercell with a = √3 · a_sub and c = c_sub. However, X‐ray single‐crystal diffraction studies of a crystallite yielding reflections corresponding to the supercell revealed it to be monoclinic, with the unit cell parameters a = 26.811(2) Å, b = 15.4798(2) Å, c = 9.414(2) Å, β = γ = 90° and α = 90.0(3)°. The average crystal structure was refined, using the subcell with a = 8.937(2) Å, b = 15.479(2) Å, c = 9.414(2) Å, β = γ = 90° and α = 90.0(3)°, space group Cm11, and Z = 4, to R_I = 3.24% and R_wI = 3.44%. The structure can be described as an hexagonal close packing layers of Nb₆ octahedra, Ba, and O atoms (A1, A2) and layers of O atoms (B1, B2), appearing in the packing sequence: A1B1A2B2. The Nb₆ octahedra are found in isolated Nb₆O₁₂O₆ clusters, and the Ti atoms in Ti₃O₁₃ and Ti₃O₁₀ units in octahedral and tetrahedral voids formed by O atoms, respectively. The Ti positions were found to be only partly occupied. Microanalysis indicates that some Nb atoms are located in the Ti₃ triangles. A model is presented that interprets these not fully occupied Ti₃ triangles as a result of a superimposing of three different structures. Two of these consist of two fused Ti₃O₁₃ units, forming an Ti₆O₁₉ unit, and a Ti₃O₁₀ unit, while the third consists of alternating Ti₃O₁₃ units.     

Octahedral Niobium Thiocyanato Complexes Containing [Nb6Cl9O3] Cluster Core: Syntheses, Crystal Structures and Evidences of NCS Ligand Exchange

Two isomers (cis and trans) of [Nb₆Cl₉O₃(NCS)₆]^5– niobium cluster complexes characterized by an ordered arrangement of oxygen and chlorine ligands over the 12 inner positions in the cluster core were prepared by reaction of Cs₂LaNb₆Cl₁₅O₃ and ScNb₆Cl₁₃O₃ solid state compounds with aqueous solution of potassium thiocyanate. The cis and trans isomers (idealized C _{2 v} and D ₃ symmetry, respectively) crystallize with counter cations and water molecules to form the following salts: Cs_4.75K_0.25[Nb₆Cl₉O₃(NCS)₆] · 5.5H₂O (1) and Cs_2.5K_2.5[Nb₆Cl₉O₃(NCS)₆] · 2H₂O (2), respectively. The trans isomer is ordered in structure 1 (s.g.: C2/c), while in the structure of 2 (s.g.: Pnma), the cis isomer is randomly disordered over two positions that correspond one to each other by a mirror plane. Interestingly, the treatment of thiocyanato complexes cis and trans with hydrochloric acid led to substitution of (NCS)^– ligands by Cl^– and to formation of soluble complexes [Nb₆Cl₉O₃Cl₆]^5–. The cesium salt of trans-[Nb₆Cl₉O₃Cl₆]^5– was crystallized as Cs₅[Nb₆Cl₁₅O₃] · CsCl · 7H₂O (3) and structurally characterized. Indeed, the formation of soluble [Nb₆Cl₉O₃(NCS)₆]^5– intermediates is a necessary step towards the formation of soluble anions.     

RP-Bridged Metal Carbonyl Clusters: Synthesis,Properties, and Reactions

Metal carbonyl clusters possess a complicated chemistry that is only beginning to be understood. One of the main current goals in this area is thus an understanding of their reactivity. This article describes the syntheses and reactions of clusters that contain metal carbonyl fragments bridged by a main-group element. But what is the sense of making such clusters still more complicated by the incorporation of main-group elements? The example of μ₃-bridged carbonyl clusters will serve to show that the main-group element plays an important role in the study of reaction paths; it holds the metal carbonyl fragments together even when the bonds between them are broken in the course of a reaction. Trinuclear μ₃-bridged clusters prove to be small enough to allow the analysis of typical cluster reactions (such as the reversible breaking of metal-metal bonds) in terms of single reaction steps. They are also large enough to provide surprises by their multifaceted reactivity. It will be shown that a detailed study of trinuclear RX-bridged metal carbonyl clusters (X ? N, P, As, Sb, Bi)—a very small part of carbonyl cluster chemistry—can lead to a better understanding of the general reaction principles involved.     

Tantalum‐Niobium Mixing in Ta3–xNbxTeI7 (0 ≤ x ≤ 3)

A substitutional study of the layered, trinuclear metal cluster system, Ta_3–xNb_xTeI₇ (0 ≤ x ≤ 3), has been performed. Synthetic, crystallographic, and spectroscopic results are presented for starting compositions corresponding to the x values: 1, 1.5, and 2. For the entire composition range studied, Ta(Nb) could readily substitute into the Nb(Ta)₃TeI₇ structure, but with changes in the observed stacking arrangements of the layers as x varies. For tantalum‐rich (x ≤ 1.8) phases, the structure conformed to the Nb₃SeI₇ structure type, also adopted by Ta₃TeI₇ and one polytype of Nb₃TeI₇. Niobium‐rich (i. e. x ≥ 1.7) phases were observed to adopt two structure types according to X‐ray powder diffraction, but crystals could only be obtained for the Nb₃SBr₇ structure type, which is a second modification of Nb₃TeI₇. Extended Hückel calculations are used to discuss the distribution of metal clusters in this system.     

Formation of Ad-Layers and Clusters on Condensation of Metal Vapors on Solid Surfaces

Synthesis of organometallic materials can be accomplished in many cases by cocondensation of metal atoms and organic molecules at low temperatures. The reaction kinetics is determined by the competition between metal cluster growth and formation of the organometallic compound. Interesting compounds may contain one or more metal atoms; the latter type could be obtained by reaction between a cluster containing the desired number of metal atoms and an organic molecule. A precise knowledge of the events occurring on condensation of metal atoms and cluster formation can therefore be of value in the control of chemical synthesis. These phenomena have been investigated in connection with the study of the growth of thin metallic films, both experimentally and theoretically. Direct observation of the formation of very small clusters is difficult. The good agreement between experimental results and recent calculations for the development of large clusters, however, allows reliable theoretical conclusions for the first stages of adsorption and cluster formation. The present contribution describes experimental work on film growth and relevant theoretical concepts, and an attempt is made to develop applications to organometallic synthesis.     

Tetrahedral Carbonylcobalt Clusters

A variety of general methods is available for the synthesis of tetrahedral heteronuclear cobalt clusters of type (CO)₉Co₃E(L)_n (E=hetero element). The size of E has a limiting effect on the existence of such complexes. No compounds (CO)₉Co₃E(L)_n have so far been prepared in which the covalent radius of E exceeds 1.30 Å;. Elements such as In, Sn, Pb, Sb, or Bi do not fit into the tetrahedral Co₃E cluster unit and form open complexes E[Co(CO)₄]_n (n=3, 4). Structural variations among the heteronuclear clusters are encountered in the presence or absence of carbonyl bridges between the cobalt atoms: hetero atoms E bearing several ligands L favor bridging, whereas those bearing only one ligand or none at all permit terminal bonding of all the CO groups.     

New Transition Metal Clusters with Ligands from Main Groups Five and Six

In both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three- or four-membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe₃)₂ (E = S, Se, Te) and E′R(SiMe₃)₂ (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.     

Reversible Lithium Insertion into LiNb6Cl19

Crystals of LiNb₆Cl₁₉ were obtained as black needles by solid state reaction of Nb powder, NbCl₅, and Li₂C₂ at 530 °C. The structure contains ladder‐like motifs built of edge and face sharing [NbCl₆] octahedra. The parallel alignment of infinite ladders yields a one‐dimensional structure, with lithium ions occupying voids in a linearly aligned manner. Each ladder combines characteristic fragments from the niobium chloride structures NbCl₄, A₃Nb₂Cl₉ (A = Rb, Cs), and Nb₃Cl₈. Lithium insertion was achieved electrochemically by slow scan cyclic voltammetry in PC (1 M LiClO₄) electrolyte. 3.73 moles of lithium per formula unit could be intercalated, with a high degree of reversibility, to a composition close to Li₅Nb₆Cl₁₉ stoichiometry.     

Synthesen und Kristallstrukturen neuer chalkogenido‐verbrückter Niob‐Kupfer‐Cluster

Syntheses and Crystal Structures of Novel Chalcogenido‐bridged Niobium Copper Clusters In the presence of tertiary phosphines, the reaction of NbCl₅ and Copper(I) salts with Se(SiMe₃)₂ (E = S, Se) affords the new chalcogenido‐bridged niobium‐copper cluster compounds ( 1 ) and [NbCu₄Se₄Cl (PPh₃)₄] ( 2 ). Using E(R)SiMe₃ (E = S, Se, R = Ph, ⁿPr) instead of the bisilylated selenium species leads to the compounds [NbCu₂(SPh)₆(PMe₃)₂] ( 3 ), [NbCu₂(SPh)₆(PⁿPr₃)₂] ( 4 ), [NbCu₂(SePh)₆(PMe₃)₂] ( 5 ), [NbCu₂(SePh)₆(PⁿPr₃)₂] ( 6 ), [NbCu₂(SePh)₆(PⁱPr₃)₂] ( 7 ), [NbCu₂(SePh)₆(P^tBu₃)₂] ( 8 ), [NbCu₂(SePh)₆(PⁱPr₂Me)₂] ( 9 ), [NbCu₂(SePh)₆(PPhEt₂)₂] ( 10 ), [Nb₂Cu₂(SⁿPr)₈(PⁿPr₃)₂Cl₂] ( 11 ) and [Nb₂Cu₆(SⁿPr)₁₂(PⁱPr₃)₂Cl₄]·2 CH₃CN ( 12 ·2 CH₃CN). By reacting Cu^I salts and NbCl₅ with the monosilylated selenides Se(^tBu)SiMe₃ and Se(ⁱPr)SiMe₃ which have a weak Se–C bond the products [Nb₂Cu₆Se₆(PⁱPr₃)₆Cl₄] ( 13 ), [Nb₂Cu₄Se₂(SeⁱPr)₆(PⁿPr₃)₄Cl₂] ( 14 ) and [Nb₂Cu₆Se₂(SeⁱPr)₁₀(PEt₂Me)₂Cl₂]·DME ( 15 ) are formed which contain selenide as well as alkylselenolate ligands. The molecular structures of all of these new compounds were determined by single crystal X‐ray diffraction measurements.     

Towards the Synthesis of a Metal Hamburger Complex: The Interaction of the PCy2(CH2)3Ph Ligand with Ruthenium Clusters

The reactions of [Ru _m C(CO) _n ] (m = 5, n = 15; 3 m = 6, n = 17; 4) with PCy₂(CH₂)₃Ph afforded mono- and bis-substituted derivatives [Ru₅C(CO)₁₄PCy₂ (CH₂)₃Ph] 13, [Ru₅C(CO)₁₃{PCy₂(CH₂)₃Ph}₂] 14, [Ru₆C(CO)₁₆PCy₂(CH₂)₃Ph] 15 and [Ru₆C(CO)₁₅{PCy₂(CH₂)₃Ph}₂] 16. Compounds 13–16 were characterised spectroscopically and the molecular and crystal structures of compound 13, 13a ([Ru₅C(CO)₁₄PPh₂(CH₂)₃Ph]), 14 and 16 were determined by single crystal X-ray crystallography. Compound [Ru₆C(CO)₁₅{PCy₂(CH₂)₃Ph}₂] 16 was subjected to a thermal treatment in dibutylether, however no reaction was observed.     

Trinuclear Clusters of the Early Transition Elements

Trinuclear clusters of the early transition elements, i.e. those elements situated on the left-hand side of the transition series, represent the simplest types of clusters. They characteristically have a pronounced formation tendency and high stability; they are therefore produced under a wide variety of conditions and their triangular M₃ skeleton is conserved in ligand exchange reactions. These clusters play a very important role in the chemistry of the respective elements, and especially those of the 4 d and 5 d series. Biochemical implications are of interest in connection with Mo^IV. This progress report presents a systematic account of the chemistry and the molecular and electronic structure of such compounds. A connection is also established with the crystal field treatment of mononuclear metal complexes.     

Die Clusterazide M2[Nb6Cl12(N3)6]·(H2O)4—x (M = Ca,Sr, Ba)

The Cluster Azides M₂[Nb₆Cl₁₂(N₃)₆]·(H₂O)_4—x (M = Ca, Sr, Ba) The isotypic cluster compounds M₂[Nb₆Cl₁₂(N₃)₆] · (H₂O)_4—x (M = Ca (1) , M = Sr (2) and M = Ba (3) ) have been synthesized by the reaction of an aequeous solution of Nb₆Cl₁₄ with M(N₃)₂. 1 , 2 and 3 crystallize in the space group Fd3¯ (No. 227) with the lattice constants a = 1990.03(23), 2015.60(12) and 2043, 64(11) pm, respectively. All compounds contain isolated 16e— clusters whose terminal positions are all occupied by orientationally disordered azide ligands.     

铌团簇和配合物的多面体分子轨道理论研究

应用多面体理论方法研究铌纯金属团簇和配合物的电子结构成键性质,并将理论预测结果用密度泛函方法验证.证实多面体理论是简便预测过渡金属簇,尤其是簇骨架电子结构的有效方法.铌的小团簇总是以低自旋密堆积结构为稳定构型,当体系的价电子数满足轨道成键数时,用该方法可较准确地推断成键性质(如Nb₄和[Nb₆Cl₁₂]⁺⁴);而对于不满足成键数的体系(如Nb₅和Nb₆),则可利用该理论分析畸变趋势.