Evaporation of atoms from metal clusters

The evaporation of atoms from metal clusters following photon absorption has been investigated by comparing the predictions of the statistical Weisskopf approach with the one of macroscopic kinetic theory. The consequences on the evaporation rate due to finite size effects in the separation energy and to the initial temperature of the cluster are also discussed.     

Hexa- and triosmium carbonyl clusters bearing bridging dppm and capping sulfido ligands

Treatment of [Os₃(CO)₇(μ₃-S)₂(μ-dppm)] (1) with Me₃NO in toluene at 80 °C affords the trinuclear cluster [Os₃(CO)₆(μ₃-S)₂(NMe₃)(μ-dppm)] (2) and the hexanuclear cluster [Os₆(CO)₁₂(μ₃-S)₄(μ-dppm)₂] (3) in 30% and 51% yields, respectively. The reaction of 1 with [Os₃(CO)₁₀(MeCN)₂] in refluxing benzene at 80 °C gives the hexanuclear cluster [Os₆(CO)₁₄(μ₃-S)₂(μ-dppm)] (4) in 15% yield. Compound 2 reacts with CO, PPh₃ and P(OMe)₃ at room temperature to give 1, [Os₃(CO)₆(μ₃-S)₂(μ-dppm)(PPh₃)] (5) and [Os₃(CO)₆(μ₃-S)₂(μ-dppm){P(OMe)₃}] (6), respectively; in high yields indicating that the NMe₃ ligand is weakly bound. Compound 1 reacts with PPh₃ in presence of Me₃NO to afford 5, 2 and 3 in 53%, 6% and 18% yields, respectively, whereas with P(OMe)₃1 gives only 6 in 84% yield. Compound 3 reacts with CO at 98 °C to regenerate 1 by the cleavage of the three unsupported osmium-osmium bonds. The molecular structures of 4 and 6 have been unambiguously determined by single crystal X-ray diffraction studies. The hexanuclear compound 3 appears to be a64-electron butterfly core with four triply bridging sulfido ligands and two bridging dppm ligands based on the spectroscopic and analytical data. The metal core of 4 can be described as a central tetrahedral array capped on two faces with two additional osmium atoms. The triply bridging sulfido ligands face cap the two tetrahedral arrays formed by metal capping of the two faces of the central tetrahedron. The dppm ligand bridges one edge of one of the external tetrahedral arrays. Compounds 5 and 6 are formed by the displacement of equatorial carbonyl group of 1 by a PPh₃ and P(OMe)₃ ligand respectively and their structures are comparable to that of 1.     

Characterization of oxide supported metal carbonyl clusters

The chemisorption of [Ma₃(CO)_{1 2}] on silica (M = Ru and Os) and alumina (M = Os) has been studied by vibrational and X-ray absorption spectroscopies making close comparisons with model compounds. The results indicate that the first chemisorption species observed has the form [M₃H(CO)₁₀(O---O)]; the bridging hydride was observed directly for the silica systems as evidenced by the M-H-M bending vibration in the i.r. Also consistent with this structure are the EXAFS analysis of the Ru/SiOz material. This indicated an essentially equilateral ruthenium triangle and coordination to oxygen. The published low frequency Raman data on the Os/Al2Oa product was shown to match most closely with that of model compounds with a bidentate oxygen donor ligand (acac or O2CR). The tethered cluster [O_s3H₂(CO)₉(PPh₂C₂H₄SIL)] was found to be a relatively short lived species on a silica surface. Under ambient conditions it reacts further and the i.r., EXAFS and ³¹P NMR data of this species suggest that the two osmium atoms not coordinated to the tethering phosphine become involved with a bidentate site from the surface.     

Tetranuclear planar metal clusters with two capping ligands

Background information and current research activity dealing with tetrametal planar clusters containing two capping ligands is briefly reviewed. The synthetic methods leading to these clusters and their chemical properties are presented. The structure and bonding in lo some representative example is discussed. Their application as models of heterogeneous catalysts, homogeneous catalysts, and precursors to materials is highlighted. Such information can have important implications in the design of a more sophisticated tetranuclear cluster systems for improved process applications.     

Vibrational spectra of bridging carbonyl groups in transition metal carbonyl clusters

The spherical harmonic model (SHM), previously used for the analysis of the terminal nu(CO) vibrations of transition metal carbonyl clusters, is applied to the corresponding bridging CO modes. The model is applicable, although the spectra show a greater sensitivity to the molecular geometry than is the case for their terminal counterparts. The reasons for this sensitivity are discussed. When both micro(2) and micro(3) CO groups are present in a molecule, a spectral distinction may not be apparent.     

Ionic and electronic structure of metal clusters

Structure and electron dynamics of sodium clusters are investigated within the local-density approximation for the electrons. We compare results from detailed ionic structure with those from a structure averaged jellium model and find that the dominance of the electron cloud overlays most of the differences in the background. Ionic structure is indispensable, however, to compute the surface energy of clusters and to provide an unprejudiced picture of cluster fission. For all cases, we compute the resonance spectra associated with electron dynamics. In particular, the very strong deformations during fission deliver unusual resonance modes with a broad spectral fragmentation.     

Delocalized electronic behavior observed in transition metal oxide clusters under strong-field excitation

Heterogeneously composed clusters are exposed to intensity resolved, 100 fs laser pulses to reveal the energy requirements for the production of the high charge states of both metal and nonmetal ions. The ionization and fragmentation of group V transition metal oxide clusters are here examined with laser intensities ranging nearly four orders in magnitude (～3 × 10(11) W/cm(2) to ～2 × 10(15) W/cm(2)) at 624 nm. The ionization potentials of the metal atoms are measured using both multiphoton ionization and tunneling ionization models. We demonstrate that the intensity selective scanning method can be utilized to measure the low ionization potentials of transition metals (～7 eV). The high charge states demonstrate an enhancement in ionization that is three orders of magnitude lower in laser intensity than predicted for the atomic counterparts. Finally, the response from the various metals and the oxygen is compared to elucidate the mechanism of enhanced ionization that is observed. Specifically, the sequence of ion appearances demonstrates delocalized electron behavior over the entire cluster.     

Regularities of thermal decay of carbonyl chalcogenide metal clusters

The thermal decay of 19 individual carbonyl homo- and heterochalcogenide clusters with different M/X ratios (M = Fe, Mn, Pt, Cr, W, Mo, Re, Ru; X = S, Se, Te) was studied by differential scanning calorimetry and thermogravimetry. The process is stepwise and occurs at relatively low temperatures (100—350 °C). The general fact of incomplete removal of carbon monoxide (due to the formation of carbide and oxide impurities) during thermolysis of narbonyl chalcogenide clusters with the M : X ratio greater than 1 was elucidated. Conversely, when M : X 1 (or at any M/X ratio for clusters containing methylcyclopentadienyl groups), pure metal chalcogenides are formed.     

Structural and electronic properties of compound metal clusters

The equilibrium geometries, relative stabilities, and vertical ionization potentials of compound clusters involving Li _n , Na, Mg, and Al atoms have been calculated using ab initio self-consistent field linear combination of atomic orbitals — molecular orbital (SCF-LCAO-MO) method. The exchange energies are calculated exactly using the unrestricted Hartree-Fock (UHF) method whereas the correlation correction is included within the framework of configuration interaction involving pair excitations of valence electrons. While the later correction has no significant effect on the equilibrium geometries of clusters, it is essential for the understanding of relative stabilities. Clusters with even numbers of electrons are found to be more stable than those with odd numbers of electrons regardless of their charge state and atomic composition. The equilibrium geometries of homo-nuclear clusters can be significantly altered by replacing one of its constituent atoms with a hetero-nuclear atom. The role of electronic structure on the geometries and stabilities of compound clusters is discussed.     

The melting behavior of small clusters of atoms

The melting transition of small clusters composed of 13 particles interacting via the Lennard-Jones 6-12 potential has been investigated by means of extensive Metropolis Monte Carlo simulations. The results indicate that whereas the cluster evaporates in vacuum, in a confined pore the cluster undergoes a smooth melting transition from an icosahedral microcrystal to an inhomogeneous liquid phase, with a specific heat peak centered at 39 K. No evidence was found to support the suggestion of a solid-liquid transition in vacuum at 29 K.     

Tuning aromaticity in trigonal alkaline earth metal clusters and their alkali metal salts

In this work, we analyze the geometry and electronic structure of the [X_nM₃]^n?2 species (M = Be, Mg, and Ca; X = Li, Na, and K; n = 0, 1, and 2), with special emphasis on the electron delocalization properties and aromaticity of the cyclo‐[M₃]^2? unit. The cyclo‐[M₃]^2? ring is held together through a three‐center two‐electron bond of σ‐character. Interestingly, the interaction of these small clusters with alkali metals stabilizes the cyclo‐[M₃]^2? ring and leads to a change from σ‐aromaticity in the bound state of the cyclo‐[M₃]^2? to π‐aromaticity in the XM₃^? and X₂M₃ metallic clusters. Our results also show that the aromaticity of the cyclo‐[M₃]^2? unit in the X₂M₃ metallic clusters depends on the nature of X and M. Moreover, we explored the possibility for tuning the aromaticity by simply moving X perpendicularly to the center of the M₃ ring. The Na₂Mg₃, Li₂Mg₃, and X₂Ca₃ clusters undergo drastic aromaticity alterations when changing the distance from X to the center of the M₃ ring, whereas X₂Be₃ and K₂Mg₃ keep its aromaticity relatively constant along this process. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

The indirect location of hydride ligands in metal carbonyl clusters

A semi-empirical method of predicting the location of hydride ligands in metal carbonyl cluster compounds using X-ray structural information is described. The hydride positions thus derived are compared with those obtained from neutron diffraction experiments and predictions made for previously ambiguous cases.     

Solid state13C NMR spectra of metal carbonyl clusters

The solid state¹³C NMR spectra of four¹³CO enriched carbonyl clusters having a tri-iron metallic core have been analyzed to provide structural and dynamic information. In Fe₃(CO)₁₂ (1), the high temperature spectra suggest the occurrence of large amplitude motions of the CO groups around their position at the vertexes of the coordination polyhedron in addition to the motion involving the Fe₃-triangle previously detected in the VT-¹³C MAS spectra.¹³C and³¹P NMR data of Fe₃(CO)₁₁PPh₃ (2) indicates the presence of one molecule in the asymmetric unit in apparent disagreement with the previously reported X-ray data. Furthermore, we show that structural information can be obtained from the chemical shift tensor components readily available from the analysis of the spinning sideband manifold.     

The sphericity of the diverse 10-vertex polyhedra found in bare post-transition metal clusters: germanium clusters with interstitial magnesium atoms as model systems

Theoretical studies show that pendant dimethylamino groups can play a significant role in the chemistry of unsaturated binuclear dimethylaminoborole iron carbonyls. For [C₄H₄BN(CH₃)₂]₂Fe₂(CO)₅, the lowest energy structures have single CO bridges and Fe?CFe single bonds of lengths ~2.8 ?. The lowest energy [C₄H₄BN(CH₃)₂]₂Fe₂(CO) _n (n?=?4, 3) structures have two bridging CO groups with Fe=Fe double bonds of lengths ~2.5 ? for n?=?4 and three bridging CO groups with Fe??Fe triple bonds of lengths ~2.2 ? for n?=?3. These structures are similar to structures previously found for the corresponding methylborole derivatives (C₄H₄BCH₃)Fe₂(CO) _n . However, slightly higher energy [C₄H₄BN(CH₃)₂]₂Fe₂(CO) _n (n?=?4, 3) structures are found in which dimethylaminoborole is a six-electron donor bridging ligand using electron pairs from the nitrogen atom as well as from the two C=C double bonds. For the more highly unsaturated [C₄H₄BN(CH₃)₂]₂Fe₂(CO) _n (n?=?2, 1), low energy singlet (n?=?2) and triplet (n?=?1) perpendicular structures are also found with similar bridging six-electron donor dimethylaminoborole ligands. In addition, highly unsaturated [C₄H₄BN(CH₃)₂]₂Fe₂(CO) _n (n?=?3, 2, 1) structures are found with agostic hydrogen atoms bridging an iron?Ccarbon bond.     

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals

Summary Quantitative and semiquantitative data for reactions of M₂(CO)_10-2nL_2n (M=Mn or Re; L=P-donor ligand; n=0, 1) are analyzed to provide information regarding the mechanisms and relative rates of substitution and oxidation reactions of the [M(CO)_5-nL_n] radicals. Both associative and dissociative paths for substitution can be inferred, but the dissociative paths are generally much slower and occur only at high temperatures and/or when associative substitution is sterically disfavoured. Quantitative comparison of the reactivities of various radicals is made.     

Tuning the electronic structure of Mo-Mo quadruple bonds by N for O for S substitution

A series of quadruply bonded dimolybdenum compounds of form Mo(2)(EE'CC≡CPh)(4) (EE' = {NPh}(2), Mo(2)NN; {NPh}O, Mo(2)NO;{NPh}S, Mo(2)NS; OO, Mo(2)OO) have been synthesised by ligand exchange reactions of Mo(2)(O(2)CCH(3))(4) with the acid or alkali metal salt of {PhC≡CCEE'}(-). The compounds Mo(2)NO, Mo(2)NS and Mo(2)OO were structurally characterised by single crystal X-ray crystallography. The structures show that Mo(2)NO adopts a cis-2,2 arrangement of the ligands about the Mo(2)(4+) core, whereas Mo(2)NS adopts the trans-2,2 arrangement. The influence of heteroatom substitution on the electronic structure of the compounds was investigated using cyclic voltammetry and UV-Vis spectroscopy. Simple N for O for S substitution in the bridging ligands significantly alters the electronic structure, lowering the energy of the Mo(2)-δ HOMO and reducing the Mo(2)(4+/5+) oxidation potential by up to 0.9 V. A different trend is found in the optoelectronic properties, with the energy of the Mo(2)-δ-to-ligand-π* transition following the order Mo(2)OO > Mo(2)NO > Mo(2)NN > Mo(2)NS. Electronic structure calculations employing density functional theory were used to rationalise these observations.     

Electronegativity equalization and the electronic structure of polyhedral clusters of main-group atoms

The concept of the equalization of atomic electronegativities accompanving molecule formation is applied to a study of the electronic structure of polyhedral clusters of main-group atoms such as Ge, Sn, Pb, Tl, and Bi. Emphasis is placed upon charged clusters such as Sn_9?x Pb(x = 0 → 9), Sn_9-xGe, Sn_8?xPb_x Tl^5?, Sn₂Bi, SnTe, etc. The role of the relativistic spin-orbit splitting of an np shell into np_1/2 and np_3/2 subshells in modifying atomic and hence molecular electronegativities is discussed. Correlations are made between calculated charge distributions and observed¹⁹⁹ Sn NMR chemical shifts for clusters of a given size and charge. It is concluded that a useful picture of charge distributions in these clusters may be obtained from electronegativity equalization considerations.