Molecular Cluster Compounds Research Articles

From the Metal to the Molecule—Ternary Bismuth Subhalides

Subvalent compounds, that is, metal-rich substances in which the average oxidation state of the cation is smaller than would be expected from the (8-N) rule, have proved to be a rich source of unexpected structural and physical features. The extraordinary structural chemistry generally observed in subvalent compounds is a consequence of the low and often non-integer oxidation states of the metal atoms coupled with the low concentration of valence electrons. Both factors can lead to a wide-range of bonding types within the same compound. A characteristic of these compounds is the interplay between "metallic" regions, with delocalized electrons and mainly nonpolar bonds between the metal atoms, and "saltlike" regions, which are characterized by strong localization of the electrons and heteropolar exchange between the metal and nonmetal atoms. The volumes of the different structural regions as well as the extent to which they interpenetrate can vary from compound to compound. The ternary subhalides of bismuth belong to a new class of substances which cover the whole spectrum from partially oxidized "porous" metals, through one- and two-dimensional metals, up to semiconducting ionic or molecular cluster compounds. These subvalent compounds with their unusually high chemical stabilities provide excellent vehicles for further research and their potential is described in the following article.

Rhenium sulfide cluster chemistry †

Rhenium sulfides are still very rare except for octahedral cluster compounds. The chemistry of trinuclear, tetranuclear and octahedral cluster compounds is reviewed in comparison with that of molybdenum sulfide clusters and in the light of the relationships between molecular and solid state cluster compounds. A promising outlook is expected.

Nanoparticles: Uses and Relationships to Molecular Cluster Compounds

Nanoparticles: Uses and Relationships to Molecular Cluster Compounds

Frequency and Temperature Scaling in the Conductivity and Its Structural Consequences

We have studied the temperature (T) and frequency (ω/2π) dependence of the complex conductivity σ(ω, T) of several systems where hopping conduction is dominant: conducting carbon-black/polymer composites as a function of carbon-black (CB) concentration, aggregates of molecular cluster compounds as a function of the size and of the distance between the metal cores and polymers as a function of degree of doping. For all compounds the conductivity as a function of ω and T can be scaled onto single curves. Using an extension of the anomalous diffusion model the specific details of these curves for the CB composites are shown to be characteristic of the underlying fractal structure. The main features of the scaled dependence of σ(ω, T) of the randomly packed cluster compounds are well described by effective medium theories, except at low frequencies where percolation effects become important. Similar features are seen in polythiophene derivatives at doping levels above the percolation threshold.

Synthesis, structure and properties of [Cr2(PS4)4]6−; the first discrete transition metal cluster from thiophosphate flux reactions

The molecular cluster compound [Cr2(PS4)4]6– forms by the reaction of Cr metal with thiophosphate fluxes.

Gold and platinum molecular cluster compounds studied by $^{197}$ Au Mössbauer Spectroscopy

The technique of 197Au Mossbauer 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.

M4(Se2)2(PSe4)4 ]8−: a novel, tetranuclear, cluster anion with a stellane-like core

The molecular cluster compounds [M 4 (Se 2 ) 2 (PSe 4 ) 4 ] 8- (M=Cd, Hg) form by the reaction of Cd and Hg with basic polychalcophosphate fluxes.

Angle-resolved X-ray photoelectron spectroscopic experiments on the full series of molecular [Au55(PR3)12Cl6] clusters

Core and valence spectra have been measured in a series of angle-resolved high-resolution X-ray photoelectron spectroscopic experiments on the molecular metal cluster compounds [Au55(PR3)12Cl6] with R3= Ph2(C6H4SO3Na-m), (C6H4Me-p)3, (C6H4OMe-p)3 or Ph3 which represent all such compounds synthesized to date. Only a limited number of samples satisfies the nominal composition. In some of the samples the presence of gold particles, larger than the nominal Au55 clusters, is observed. Gold monomers bound to P and Cl are shown to be present in a number of samples.

Hypothetical C(60) Metal-Cluster Fullerides and General Aspects of Tetrahedral Cluster Bonding.

It is geometrically feasible to insert metal-metal-bonded M(4) tetrahedra and M(6) octahedra into the tetrahedral and octahedral holes, respectively, of the fcc C(60) lattice. The electronic structure of the hypothetical tetrahedral variants C(60)(M(4))(2), M = Rh, Co, is analyzed with approximate molecular orbital methods and band structure calculations. These compounds feature M-M and M-C(60) bonding and a variable degree of electron transfer to or from C(60). The C(60)(M(4))(2) phases should be metallic, but we have no way of predicting if they will be superconducting. A number of discrete molecular tetrahedral cluster compounds which serve as models for the solid state materials are analyzed. There is a clear indication that tetranuclear and even mononuclear Rh, Ir, and Co arene complexes should be relatively unstable.

On the binding-energy shifts in molecular metal cluster compounds

Binding-energy shifts of core levels, valence bands and Auger spectra of the molecular metal cluster compounds Cu 146Se 73(PPh 3) 30 and Au 55(P( 6H 4OCH 3) 3) 12Cl 6 are interpreted in terms of a final-state relaxation, involving a Coulomb charge on the surface of the clusters and a polarization of the surrounding.

Photoemission from large-nuclearity copper-selenide clusters

XPS spectra of core levels, valence bands and Auger transitions have been measured on the molecular cluster compounds Cu 146Se 73(PPh 3) 30, Cu 70Se 35(PEt 3) 22 and Cu 30Se 15(P iPr 3) 12. A correlation of photoemission results with clusters nuclearity is presented.

Hexakis(triethylphosphine)octatelluridohexachromium and a molecule-based synthesis of chromium telluride, Cr3Te4

The molecular cluster compound Cr6Te8(PEt3)6 (1) is formed by the reaction of TePEt3 with either (Et3P)2Cr(allyl)2 or Cr(2,4-dimethylpentadienyl)2. This compound can be converted to the extended inorganic solid state compound Cr3Te4 by simple thermolysis. We have determined the structure of the title compound crystallographically (monoclinic; a = 13.076(5) A, b = 21.194(7) A, c = 23.694(7) A, β = 105.21(5)°, V = 6276(10) A3, Z = 4). The molecule is formed by a Cr6 octahedron, a Te8 cube, and a (PEt3)6 octahedron, all of which are concentric. We compare and contrast the structure and properties of the cluster with those of related solid-state compounds.

Electronic spectra of metal cluster molecules

The electronic absorption spectra (UV-visible-NIR) of a range of molecular metal cluster compounds, including new spectra of Pt309(phen*)36O28 in solution and Au55(PPh3)12Cl6 in the solid state, are discussed and compared with the spectra of colloidal particles of the corresponding metals. We consider frontier orbital separations, the development of ‘interband’ absorptions, the possible appearance of molecular plasma resonances, and charge-transfer in the solid state.

A study of chemically prepared small metal clusters in the nonmetallic regime

Small gold clusters [mean diameter (d)≲ 1.4 nm], unlike larger clusters, show a higher Au(4f) binding energy relative to the bulk value and the presence of a conductance gap in tunnelling measurements, just as the molecular cluster compound, Au55(PPh3)12Cl6; small platinum clusters show similar nonmetallic features.

Physical properties of metal cluster compounds VI: The influence of the ligands on the 197Au Mössbauer spectra of three different Au 55 molecular clusters

The results of Mössbauer Effect measurements on the methylated gold molecular cluster compound Au 55(P(C 6H 4CH 3) 3) 12Cl 6 and the water soluble compound Au 55(PPh 2C 6H 4SO 3Na·2H 2O) 12Cl 6 are compared to those on the original gold cluster compound Au 55(PPh 3) 12Cl 6. A new procedure for the determination of the f-factor of the source is presented. The differences between the spectra for the three versions of this molecular cluster compound are discussed. An attempt is made to draw some preliminary conclusions.

Ion chromatographic analysis of inorganic molecular metal-oxygen cluster compounds

An ion chromatographic technique for the measurement of the concentrations of metal-oxygen cluster anions and the determination of their elemental compositions has been developed. The method depends on the decomposition of the heteropoly anions into their simpler constituent ions by the addition of a basic hydroxide and the subsequent separation and analyses of these constituent ions. Although the susceptibility of the metal-oxygen cluster anions to decomposition in basic solutions is well known and ion chromatography has been supplied to the analysis of tungstate, molybdate and phosphate ions the application of the former observation and the latter method in combination for the analyses of metal-oxygen cluster anions has not been reported previously. The analytical method described here thus constitutes the first reported application of ion chromatography for the determination of concentration and elemental composition of metal-oxygen cluster anions. The method additionally offers the possibility of application in the study of the mechanism of the synthesis and decomposition of the metal-oxygen cluster anions.

Magnetic properties and EPR spectroscopy of molecular metal clusters

The magnetic properties of molecular metal cluster compounds resemble those of small metal particles in the metametallic size regime. Even-electron metal carbonyl clusters with 10 or more metal atoms are paramagnetic, because their frontier orbital separations of less than 1 eV lead to high-spin electronic configurations. The rhodium cluster [Rh17S2(CO)32]3− gives EPR below 200 K with g = 2.04, the first example of this type of paramagnetism in an even-electron carbonyl cluster of this 4d metal. Its spectral parameters are compared with those of osmium carbonyl clusters and some significant differences highlighted. Attempts have also been made to generate radical cations from lowernuclearity, diamagnetic molecular clusters such as Rh6(CO)16 by chemical oxidation in sulphuric acid. An EPR active species (g = 2.09) believed to be [Rh6(CO)16]+ has been obtained.

Osmium carbonyl clusters: XPS characterization of some catalyst precursors

An XPS study on selected Os cluster compounds is reported. Investigated compounds include: Os 3(CO) 12, Os 6(CO) 13, H 2Os 3(CO) 10, HOs 3(CO 11)(OC 6H 5), [(C 3H 5) 3P) 2N] + [HOs 3(CO) 11] − and the Os(II) species Os(CO) 3Cl 2 and Os(CO) 2Cl 2. Os 4 f binding energies (b.e.) are discussed in terms of a quantitative empirical model already proposed by Feltham and Brant, to describe observed b.e. shifts for substituted Os carbonyls in terms of parameter values to be assigned to the ligands. New parameter values, which, in some cases, differ considerably from previously reported ones, are proposed to account for the observed b.e. shifts. The larger role played by constitutive besides additive effects on the experimental b.e. trends is also discussed with reference to the particular electronic structures represented by molecular cluster compounds.

First example of monodispersed (Mo 3Se 3) 1∞ clusters

The close relationship of the structure of ternary molybdenum chalcogenides to molecular cluster compounds suggests that these chalcogenides might be synthesized from such clusters or be solubilized as clusters. However, in contrast to molybdenum halide clusters which can be obtained in solution from the solid phase, many attempts to solubilize these chalcogenide clusters were unsuccessful. This is the first report that some of the pseudo-one-dimensional compounds ( M 2Mo 6 X 6, X = Se, Te; M = Li, Na) can be dissolved when exposed to highly polar solvents such as dimethylsulfoxide or N-methylformamide. We show using optical microscopy, TEM, and light scattering that some of these solutions contain individual (Mo 3 X 3) 1 ∞ chains. This is also the first example of a purely inorganic transition-metal polymer solution. The behavior with respect to flocculation is consistent with the double layer theory. We show that the (Mo 3 X 3) 1 ∞ chains can be oriented in solution.

Gold in bimetallic molecular clusters

Gold-containing mixed-metal clusters and cluster compounds have been studied increasingly over the past few years. In this article the synthesis, structure and chemical reactivity of molecular cluster compounds in which gold is associated with one or more transition metals are reviewed. The bimetallic associations encountered at the molecular level are also examined in view of their importance in homogeneous, supported, and heterogeneous catalysis. The improved catalytic properties of gold-containing bimetallic over monometallic clusters have been described elsewhere in this issue by J. Schwank.