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The acetylene to vinylidene isomerization on several Ru(II) d(6) metal fragments with different electron richness of the metal center has been investigated by means of density functional theory calculations. We considered the [(eta(5)-C(5)Me(5))Ru(dippe)](+), [(eta(5)-C(5)Me(5))Ru(dmpe)](+), [(eta(5)-C(5)H(5))Ru(PMe(3))(2)](+), [(eta(6)-C(6)Me(6))(PMe(3))ClRu](+), [(eta(5)-C(5)H(5))Ru(CO)(PPh(3))](+) and [eta(6)-C(6)H(6))(PMe(3))ClRu](+), species which are quite common in the chemistry of cationic Ru(II) complexes and span a wide range of electron-richness. For each of the considered fragments, the minima on the potential energy surfaces for the two possible isomerization mechanisms, i.e. through a direct 1,2-hydrogen shift or through a hydrido-alkynyl intermediate, have been localized. A linear correlation has been found between the C=C stretching frequencies of the vinylidene complexes, as an estimate of the electron richness, and the stability of the corresponding hydrido-alkynyl intermediates. For the most electron-rich among the considered fragments, [(Cp*)(dippe)Ru(HCCH)](+), the hydrido-alkynyl species has been found essentially isoenergetic with the alkyne complex (only 1.9 kcal mol(-1) higher), in agreement with the experimental evidence showing for this system an equilibrium between these two species. For the same [(Cp*)(dippe)Ru](+) fragment, a detailed analysis of the reaction profiles for the two possible acetylene rearrangement pathways has been performed. Our results show that once the eta(2)-C-H coordinated acetylene intermediate is accessed, the system can easily evolve towards a hydrido-alkynyl intermediate, this process being kinetically favored with respect to the direct 1,2-shift leading to the vinylidene product.

It has been recently shown that lustre decoration of Medieval and Renaissance pottery consists of silver and copper nanoparticles, dispersed within the glassy matrix of the ceramic glaze. Lustre surfaces show peculiar optical effects, such as metallic reflection and iridescence. Here we report the findings of a study on lustred glazes of several shards belonging to Iranian pottery of the 10th and 13th centuries, decorated on both sides. Two different glazes, depending on the side of the sample, have been identified. Different lustre chromatic effects are characterised by the relative presence of silver- and copper-metal nanoparticles dispersed in the glassy matrix.

The nitrenium ions of formula C6NH6 + arising from the reaction between benzene molecules and atomic nitrogen ions, N + (3P), are studied by means of theoretical methods so that their structures and relative stabilities are investigated. All calculations are carried out by using the DFT hybrid functional B3LYP in conjunction with the split valence 6-31G* basis set.

DFT/TDDFT calculations were performed to investigate the structural, electronic and optical properties of the [Ru(dppz)(phen) 2 ] 2+ complex in solution. TDDFT calculations in water show two groups of metal-to-ligand charge transfer (MLCT) transitions at ≈450 and 415 nm whose superposition gives account of the broad absorption band experimentally characterized at 440 nm. Also, a group of almost coincident MLCT transitions partially mixed with dppz intraligand π–π ∗ transitions centered at ≈380 nm is found to give rise to the narrow absorption band experimentally found at 380 nm. Our results provide insight into the hypochromic shifts experimentally characterized upon intercalation of the title complex into DNA.

Lustre is one of the most important decorative techniques of the Medieval and Renaissance pottery of the Mediterranean basin, capable of producing brilliant metallic reflections and iridescence. Following the recent finding that the colour of lustre decorations is mainly determined by copper and silver nanoclusters dispersed in the glaze layer, the local environment of copper and silver atoms has been studied by extended X-ray absorption fine structure (EXAFS) spectroscopy on original samples of gold and red lustre. It has been found that, in gold lustre, whose colour is attributed mainly to the silver nanocluster dispersion, silver is only partially present in the metallic form and copper is almost completely oxidised. In the red lustre, whose colour is attributed mainly to the copper nanocluster dispersion, only a fraction of copper is present in the metallic form. EXAFS measurements on red lustre, carried out in the total electron yield mode to probe only the first 150 nm of the glaze layer, indicated that in some cases lustre nanoclusters may be confined in a very thin layer close to the surface.

Density functional calculations have been performed on nickel Schiff base complexes and nickel porphyrinogen complexes in order to understand the behaviour of these systems in redox processes. In nickel Schiff base complexes CC σ bonds are formed upon reduction, while in nickel porphyrinogen complexes CC σ bonds are formed upon oxidation. In both systems the formation or the cleavage of CC bonds avoids a variation in the oxidation state of the metal. The lack of variation in the oxidation state of the metal is the first step towards the reversibility of the redox process. The complementary behaviour in redox processes of nickel Schiff base complexes and nickel porphyrinogen complexes can be considered in a reversible electrochemical cell with the discharge process based on CC bonds breaking and the recharge based on CC bonds formation.

A theoretical investigation has been performed on the reaction pathways involved in the Pt(PH3)2-catalysed ethylene hydrosilylation by SiH4. We have analysed both mechanisms proposed to explain the formation of the vinyl-silane product, i.e. (i) the Chalk–Harrod mechanism, which consists of Si–H oxidative addition of SiH4 to Pt(PH3)2, ethylene insertion into the Pt–H bond followed by Si–C reductive elimination; and (ii) the modified Chalk–Harrod mechanism, which consists of the same oxidative addition of SiH4 to Pt(PH3)2 followed by ethylene insertion into the Pt–Si bond and by C–H reductive elimination. We characterised all the elementary steps involved in both mechanisms by gradient-corrected DFT methods, and performed ab initio molecular dynamics simulations on the initial oxidative addition step. We compute a maximum energy barrier of 25.1 and 38.2 kcal/mol for the Chalk–Harrod and modified Chalk–Harrod mechanisms, respectively, both in correspondence of the ethylene insertion-isomerisation step. Our DFT approach is found to provide comparable results to those obtained at the MP4SDQ level on the Hartree–Fock optimised geometries, except for the ethylene insertion and isomerisation steps in the modified Chalk–Harrod mechanism, for which we compute lower energy barriers at the DFT level. Our results confirm that the Chalk–Harrod mechanism is kinetically favored over its modified version.

Renaissance ceramic shards from Gubbio and Deruta (Italy) were investigated by scanning and transmission electron microscopy, coupled with energy dispersive spectrometry. The study mainly focuses on the glaze layer, which was applied over the main ceramic body in a second firing process. The glaze contains several inclusions, such as K–feldspar, cassiterite, calcium phosphate and quartz. Most of the shards have blue and yellow decorations, which correspond to the so-called Blue Smalt and Naples Yellow. Overall evidence (glaze bulk-chemistry, the number and type of inclusions, pigment characteristics and the microtextural–microchemical relationships among glaze, inclusions and the main ceramic body) constrains the nature and provenance of raw materials and can be used to estimate firing temperatures in the different processing steps.

Nowadays the relevance of silicon chemistry is increasing due to the applications of Si, in particular in semiconductors, in order to obtain an increasing number of performing materials. A theoretical investigation based on the Density Functional Theory has been done on the reaction paths implied in the Pt-catalysed alkene hydrosilylation, a process through which the selective grafting of organic molecules to silicon can be obtained. We studied the Si-H oxidative addition of SiH4 to a diphosphine molecule, the C2H4 insertion on Pt-H and Pt-SiH3 bonds, the isomerisation of the product of the C2H4 insertion, and the two following Si-C and C-H reductive eliminations. The set of these processes are known as the Chalk- Harrod mechanism and the modied Chalk-Harrod mechanism, respectively. The goal of this work has been to identify the rate determining step of both mechanisms. The dynamics of the oxidative addition step has been simulated.

It has been recently shown that lustre decoration of medieval and renaissance pottery consists of silver and copper nanocrystals, dispersed within the glassy matrix of the ceramic glaze. Lustre surfaces show peculiar optical effects such as metallic reflection and iridescence. In many cases, lustre appears overlapped to colored drawings. Here we report the findings of a study on glazes, pigments and lustre of several shards belonging to Deruta and Gubbio pottery of XVI century. The components of glazes and pigments have been identified. Lustre is confirmed to be characterised by silver and copper metal nanocrystals inhomogeneously dispersed in the glassy matrix of the glaze. In the case of lustre overlapped to colored decorations, we found two contradictory cases. The first consists of a lustre surface successfully applied over a blue smalt geometrical drawing. The second consists of a lustre surface, unsuccessfully applied over a yellow lead-antimonate pigment. The yellow pigment hinders the formation of lustre and removes crystals of tin dioxide, normally present in the glaze as opacifier.

The alkyne to vinylidene isomerization in [(Cp)(PMe3)2Ru(HCCH)]+ and [(Cp)(PMe3)2Ru(HCCMe)]+ has been investigated by density functional calculations. For both systems, the potential energy surface for the two possible isomerization mechanisms, i.e., through a 1,2-hydrogen shift or through an oxidative addition to a hydrido-alkynyl intermediate, has been analyzed by a gradient-corrected DFT approach. The vinylidene complexes have been found more stable than the corresponding alkyne complexes, 13.1 and 10.4 kcal mol-1, respectively, and are the thermodynamically most stable species on the potential energy surfaces of the two systems. The 1,2-hydrogen shift, proceeding via an ?2-(C-H)-coordinated alkyne intermediate, is the energetically most favorable path for both ethyne and propyne isomerizations, with highest energy barriers of 26.8 and 18.6 kcal mol-1, respectively. However, while the higher energy barrier computed for the oxidative addition rules out such a process in the propyne rearrangement (29.0 vs 18.6 kcal mol-1), the barriers for the 1,2-hydrogen shift and for the oxidative addition are almost comparable in the case of the ethyne rearrangement (26.8 vs 31.7 kcal mol-1), so that the oxidative addition process might become competitive. For the inverse vinylidene to propyne rearrangement we calculate an overall activation enthalpy and entropy of 25.5 kcal mol-1 and -3.0 cal K-1 mol-1, respectively, in excellent agreement with the experimental values of 26.8 ± 0.7 kcal mol-1 and -4.9 ± 1.9 cal K-1 mol-1.

The acetylene to vinylidene isomerization in (Cp)(CO)2Mn(HCCH) has been investigated by both static and dynamic density functional calculations. The potential energy surface for the conversion of coordinated acetylene to vinylidene has been analyzed by a gradient-corrected DFT approach. The ?1-vinylidene complex has been found 9.0 kcal mol-1 more stable than the acetylene complex and is the thermodynamically most stable species of this reaction system. An unprecedented ?2-vinylidene species has been characterized and found 38.4 kcal mol-1 higher than the corresponding ?1-isomer. The direct 1,2 hydrogen shift, proceeding via an agostic intermediate, is the energetically most favorable path, with a free energy barrier of 27.3 kcal mol-1. The higher barrier computed for the oxidative addition leading to the corresponding alkynylhydrido complex (35.0 kcal mol-1) rules out the intermediacy of such a species in the investigated process. Dynamics simulations have also been performed on the (Cp)(CO)2Mn(HCCH) complex in order to study the detailed features of the possible acetylene to vinylidene conversion pathways and show that the isomerization effectively takes place through a direct 1,2 hydrogen shift from the agostic intermediate.

The stepwise migratory insertion of methyl isocyanide into the zirconium-carbon bonds in [calix[4](OMe)2(O)2-ZrMe2] has been investigated by means of both static and dynamic density functional calculations. Dynamics simulations have shown that methyl isocyanide insertion takes place via the initial formation of an ?1-iminoacyl species that is suddenly converted into the more stable ?2-isomer. The energy profiles for the two pathways branching from the initially formed ?2-iminoacyl, i.e., (i) the insertion of a second isocyanide molecule into the residual alkyl group leading to a bis-?2-iminoacyl and (ii) the insertion of the residual alkyl group into the iminoacyl moiety leading to an ?2-bound imine, have been characterized. Formation of the bis-?2-iminoacyl species was found to be thermodynamically favored at low temperature (?G =7.2 vs 6.4 kcal mol-1, ?E = -38.5 vs -12.2 kcal mol-1). However, the large entropic contribution to the barrier for this intermolecular process kinetically favors the intramolecular imine formation at room temperature (?G =11.5 vs 6.4 kcal mol-1), providing a rationale for the experimentally characterized temperature selectivity of the overall reaction.

Polycyclic aromatic hydrocarbons (PAHs) have been employed as models to investigate the functionalization of SWNTs sidewalls with transition metal complexes at the NL-DFT level, pointing out the most favorable coordination sites and the electronic properties of the resulting system. Molecular fragments topologically resembling the honeycomb lattice have been chosen, i.e., the pyrene (C16H10) and the coronene (C24H12 )t o respectively investigate an 2 interaction on the CsC bond and an 6 interaction on the hexagonal ring with suitable transition metal complexes. To reproduce the curvature of both (n,0) and (n,n) SWNTs surface, constrained geometry optimizations have been performed on these systems. Both geometrical parameters and bonding energies for the (PH3)2M(C16H10 )( M) Ni, Pt) complexes suggest that 2 bonding of metal fragments to nanotubes is much weaker than to fullerene, making questionable the stability of 2 M(PH3)2 complexes of carbon nanotubes. The 2 metal interaction with (9,0) and (5,5) bent-constrained pyrene models do not show any dependence on the (n,m) chiral vector. The analysis of the Cr(CO)3(C24H12) complexes shows that the (9,0) and (5,5) bent-constrained coronenes, modeling carbon nanotubes, enforce the chromium atom to arrange an 2 or 4 coordination on the hexagonal ring, rather than an 6 coordination, that is more favorable for the planar aromatic systems of benzene and planar coronene, modeling graphene sheets.

A theoretical investigation (DFT) allowed us to single out the most appropriate valence structure for the [M � /Cx � /M?] unit in a variety of homonuclear [{Cp(CO)2M}2(m-Cx )] (M � /Cr, Fe � , x� /3; M� /Mn, x� /3, 5, 7) and heteronuclear [{Cp(CO)2M}(mCx ){Cp(CO)2M?}] (M � /Fe � ,M ?� /Mn, x� /3; M� /Fe, M ?� /Cr, x� /3, 5, 7) model compounds and to foresee it on the basis of the molecular parameters, such as (i) the nature of the metals and their oxidation states; (ii) the d n configurations and the metal coordination numbers; (iii) the chain length. The valence structure of the [M � /Cx � /M?] unit is diagnostic of the metal-to-metal communication. A simple electron-counting scheme has been developed to predict the valence structure, based on the d n configuration of the MLm (M?L ? m ?) fragments and the number of pp electrons of the ‘linear’ Cx unit. # 2002 Elsevier Science B.V. All rights reserved.

A theoretical investigation has been performed on the oxidative addition reaction pathway involved in the platinum-catalysed hydrosilylation of alkenes, analysing the oxidative addition of SiH 4 to a platinum-diphosphine complex, Pt(PH 3 ) 2 . Static and dynamic density functional calculations have been performed in order to investigate the reaction mechanism and dynamics of the oxidative addition reaction in the considered system. Our results indicate that the oxidative addition is both thermodynamically and kinetically favored and takes place via a reactant-like transition state leading directly to the more thermodynamically stable cis -[PtH(PH 3 ) 2 –SiH 3 ] square-planar product.

A density functional investigation on niobium–oxo complexes, as models of an oxo surface, is reported. The structural and electronic properties of niobium-silsesquioxanes, considered as possible models for silica-supported transition-metal catalysts, and niobium-2,2′-thiobisphenolic frameworks are compared. The latter is proposed as model for providing insight into the chemistry of carbon-supported transition-metal catalysts. The main difference between them is that the 2,2′-thiobisphenolic framework is structurally less flexible with respect to the SiOSi one when coordinating additional ligands.

Density functional theory method is applied to investigate the geometry and electronic structure of a molecular system considered as a model of a metal–oxo surface. Vanadyl cation complexed by 2-hydroxyisonaphthalic acid is chosen as a building block, which gives an extended layer-type structure where planar, dimeric units are held together by a platinum or zirconium atom. The resulting two metal-linked two-dimer systems can be used as suitable model systems for polyoxomatrices, where the coordination around both platinum and zirconium atoms is square-planar. The electronic structures point out unpaired electrons which are responsible for magnetic interactions occurring mainly within isolated dimeric units for platinum system and with a more complicated spin distribution between adjacent dimers in the plane for zirconium system.

The migratory insertion of CO into the titanium–methyl bond in Ti(Cp)2(CH3)2 has been investigated by both static and dynamic density functional calculations. CO coordination prior to insertion has been analysed considering both “lateral” and “central” approaches, finding the two pathways kinetically equivalent. The relative stability of the O-outside and O-inside η2-bound acyl complexes has been investigated, finding the O-outside isomer more stable by 4.0 kcal mol−1, with an energy barrier for the conversion from the O-outside isomer of 9.6 kcal mol−1. Dynamic simulations have also been performed on the Ti(Cp)2(CH3)2–CO adduct in order to study the detailed features of the CO migratory insertion and of the η1- → η2-acyl conversion, and show that migratory insertion takes place within 1 ps through detachment of the methyl group upon CO migration, leading to an O-inside η2-acyl complex.

Luster is a decorative metallic film that was applied on the surface of medieval glazed pottery. It can be obtained via the low-temperature (∼650°C), controlled reduction of copper and silver compounds. In this paper, we show that luster is a thin layered film (200–500 nm thick) that contains metallic spherical nanocrystals dispersed in a silicon-rich matrix and has a metal-free outermost glassy layer that is 10–20 nm thick. Silver nanocrystals seem to be separated from those of copper, forming aggregates 5–100 μm in diameter. This composite structure exhibits optical properties that are dependent on both the particle size and the matrix. Luster is indeed the first reproducible nanostructured thin metallic film that was made by humans.

The density functional approach has been used to compare the geometry and the frontier orbitals of the [(COT)Zr] 2+ , [CpZr] 3+ , [Cp 2 Zr] 2+ and [calix[4]–(O) 4 Zr] fragments. The investigation on the [(COT)Zr] 2+ and [Cp 2 Zr] 2+ complexes shows that, in spite of the same number of low-lying empty orbitals available for bonding with additional ligands, the symmetries and the spatial extensions of these orbitals are different, and this has important consequences on their chemical behavior.

Density functional calculations have been performed on all complexes involved in the rearrangements undergone by ethylene on the calix[4]arene tungsten fragment. [{p-But-calix[4](O)4}W(η2-C2H4)], which is the starting reactant of the rearrangement process, has been carefully investigated. In order to reduce the computational effort a simplified model of tungsten calix[4]arene has been considered. Preliminary calculations have shown that four vinylate groups reproduce adequately the geometrical, electronic, and bonding parameters of the calix[4]arene ligand so that we have employed the {(C2H3O)4}W fragment to investigate the ethylene rearrangements. Calculated geometrical and thermodynamical data obtained for ethylene rearrangements agree with the experimental X-ray data and the observed reaction trend.

Density functional calculations were performed on a series of M(OH3)-substituted (M = Ti,V) cyclopolyenes as simple models is metal carbides. We studied the oligomerization of the metal acetylide complexes [MLn]2 (mu-C2) as a possible precursor of these [(OH)3MC]n (n = 4,6) hypothetical species. Special emphasis was placed on the comparison of the main properties of these metal substituted cyclopolyenes with those of the corresponding cyclopolyenes in an attempt to study the effects of the metal substituents on the organic C4 and C6 cyclic moieties. Whereas for the titanium species, the pi system of the polyene moiety is slightly perturbed, the electronic structures and molecular geometries found for the vanadium species suggest a metalla radialene nature for these compounds, with the endocyclic conjugation of carbon-carbon double bonds replaced by an exocyclic arrangement of carbon-metal double bonds.

Complexes that undergo extensive molecular reorganization upon unimolecular metastable dissociation are obtained after ionization of atmospheric gases containing ozone and chlorofluorocarbons [Eq. (1)]. These unusual processes, which require fission of all bonds initially present in the CHX(2) unit (X=Cl, F) and combination of C with one of the O atoms of ozone, were demonstrated and their mechanism rationalized by the joint application of mass spectrometric and theoretical methods.

The chemistry of the title cations was investigated in the gas phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and by theoretical methods. Cl2H+ (1) was obtained from the protonation of Cl2 by gaseous Bronsted acids and the chlorination of HCl by Cl2F+ and XeCl+. The proton affinity (PA) of Cl2 measured by the FT-ICR equilibrium method is 131.4 ± 1 kcal mol-1 at 298 K, which compares with the 132.0 kcal mol-1 value computed at the CCSD(T) level of theory. 1 has the Cl−Cl−H connectivity, as indicated by its 2-fold reactivity as a Bronsted acid and a chlorinating agent, and confirmed by the theoretical results. The FClH+ cation was obtained from the fluorination of HCl by XeF+ and its most stable protomer (2) has the F−Cl−H connectivity, as shown by its behavior as a pure Bronsted acid and by the theoretical results that identify 2 as the global minimum, whereas protomer 3, having the H−F−Cl connectivity, is computed to be less stable by 5.0 kcal mol-1 at the CCSD(T) level of t...

A theoretical investigation (DFT) allowed us to single out the most appropriate valence structure for the [M–Cx–M] unit in a variety of model compounds [{Cp(CO)2M}2(µ-Cx)] (M = Cr, Mn or Fe; x = 2–8) and to foresee it on the basis of the molecular parameters, such as (i) the chain length, (ii) the nature of the metal and its oxidation state, (iii) the dn configuration and the metal co-ordination number. The valence structure of the [M–Cx–M] unit is diagnostic of the metal-to-metal communication. A simple electron-counting scheme has been developed to predict the valence structure, based on the dn configuration of the MLm fragments and the number of pπ electrons of the linear Cx unit.

The electronic structures and the geometries of lithioacetamide, lithioacetylphosphine, lithioacetophenone and lithiomethyl-phenylsulfoxide have been investigated through ab initio calculations, including correlation at the MP2 level. The different coordination modes, η1O, η3 and η1C, of the lithium center have been analyzed. In lithiomethylphenylsulfoxide, there is a clear preference for an η3 coordination of the lithium towards the enolate anion, while in the other systems the η1O and η3 coordinations are almost isoenergetic. The η1C coordination is destabilized in all the systems.

Density functional calculations were performed on a series of Cu(PH(3))-substituted cyclopolyenes as simple models of molecular metal carbides. We studied the oligomerization of the copper acetylide PH(3)CuCtbd1;CCuPH(3) as a possible precursor of these (PH(3)CuC)(n)() (n = 4, 6, 8) hypothetical species. Special emphasis was placed on the comparison of the main properties of these metal-substituted cyclopolyenes with those of the corresponding cyclopolyenes in an attempt to study the effects of metal substituents on the organic C(4), C(6), and C(8) cyclic moieties. We found comparable geometries of the C(n)() units and, for n = 3, a thermodynamically stable species with respect to dissociation toward dinuclear copper acetylides.

Density functional calculations were performed on a series of Cu(PH3)-substituted cyclopolyenes as simple models of molecular metal carbides. We studied the oligomerization of the copper acetylide PH3CuC⋮CCuPH3 as a possible precursor of these (PH3CuC)n (n = 4, 6, 8) hypothetical species. Special emphasis was placed on the comparison of the main properties of these metal-substituted cyclopolyenes with those of the corresponding cyclopolyenes in an attempt to study the effects of metal substituents on the organic C4, C6, and C8 cyclic moieties. We found comparable geometries of the Cn units and, for n = 3, a thermodynamically stable species with respect to dissociation toward dinuclear copper acetylides.

The mode of interaction of bridging ‘C2’ units in dinuclear LmMC2MLm complexes of late transition metals in low oxidation states, for which only an acetylenic structure has been experimentally observed, has been studied. Density functional calculations were carried out on the model complexes [{MCln(CO)m–n}2(µ-C2)] (M = Cr, Mn, Fe or Co; m = 4 or 5; n = 0 or 1) and allowed clarification of why only the acetylenic structure is found and most of the experimentally known compounds show a d7 configuration of the MLm metal fragment.

Triple ionization of molecular systems is investigated theoretically by means of the three‐particle propagator. This enables us to efficiently calculate the very dense triple ionization spectra. To be able to interpret these spectra an atomic three‐hole population analysis is developed which provides information about the charge distribution in the molecular trication. In exemplary applications on CO and a series of fluorides the use of the approach is demonstrated. A large number of triply ionized electronic states are energetically accessible in the available particle impact ionization experiments and it is shown that many of these states contribute to the observations. Triply ionized states are also produced by Auger decay. In particular, shake‐off satellite lines of molecular Auger spectra can be reproduced using the triple ionization energies from the propagator calculation and an estimate of the transition rates based on the three‐hole population analysis. In general a dramatically growing complexit...

Reduction of cyclopentadienyloxochloro complexes such as [R-cpTiCl(μ2-O)]4 (R = H (bd1), Me (bd2) or SiMe3 (bd3))) with a variety of reducing agents (Zn, Na, Mg or LiN3) in tetrahydroforum gave the hexanuclear titanoxanes [(R-cp)6Ti6O6Cl2] (RH (bd4), Me (5) or SiMe3 (6)) with a good yield. Complexes 4–6 are soluble in organic solvents, are diamagnetic and contain four electrons more than are required for the existence of the hexanuclear skeleton. Complex 4 was equally satisfactorily obtained by reducing R-cpTiCl2(μ2-O)] (RH) (7) will Bu3SnH. Reduction of 1 and 2 with Al powder gave [ R-cp 6 Ti 6 O 4 Cl 4 ] ( R  H ( 8 ) or Me ( 9 )) . They contain two unpaired electrons. Reaction of H2O with 4 and 5 gave the diamagnetic titanoxane [R-cp6Ti6 or Me (12)). Complexes 4–6, 8, 9, 11 and 12 all contain an octahedron of titanium atoms faced bridging oxygen and chlorine in the reported ration. Molecular orbital calculations showed that a band of 12 non-bonding orbitals may accomodate electrons in excess of those needed for the existence of the Ti6 skeleton and so the systems can be used in their intact form for storing and releasing electrons. The structures of complexes 4, 6, 9, and 12 were determined by single-crystal X-ray diffraction studies.

The possibility of tuning the electronic properties of graphene by tailoring the morphology at the nanoscale or by chemical functionalization opens interesting perspectives towards the realization of devices for nanoelectronics. Indeed, the integration of the intrinsic high carrier mobilities of graphene with functionalities that are able to react to external stimuli allows in principle the realization of highly efficient nanostructured switches. In this paper, we report a novel approach to the design of reversible switches based on functionalized graphene nanoribbons, operating upon application of an external redox potential, which exhibit unprecedented ON/OFF ratios. The properties of the proposed systems are investigated by electronic structure and transport calculations based on density functional theory and rationalized in terms of valence-bond theory and Clar's sextet theory.

X-Ray absorption spectroscopy investigations combined with ab initio structural simulations of ternary forms (Pb-Sb-Zn and Pb-Sb-Fe) of the pigment Naples yellow (Pb2Sb2O7) allowed to achieve a detailed insight into the incorporation route of Zn and Fe ions inside the pigment lattice. The study of newly synthesised yellow pyroantimonates provided direct evidence that Zn and Fe ions enter the pyrochlore octahedral sites replacing Sb ions. Experimental results were discussed considering, also, the information provided by Raman spectroscopy used in this study as a sensitive probe of the pyrochlore lattice distortions in modified yellow pyroantimonates. Finally, by exploiting the non-invasiveness of the X-ray absorption spectroscopy technique, the same structural arrangement has been unambiguously observed for Zn cations in a modified Naples yellow pigment of a Renaissance ceramic shard.

The bonding modes of the porphyrinogen ligand L4–(H4L = 5,10,15,20,22,24-hexahydroporphyrin) with the metal ion Zr4+ are analysed in detail for the limiting co-ordinations η5,σ,η5,σ and σ,σ,σ,σ. It is concluded that in both co-ordination modes σ bonding is by far the most dominant, mainly due to strong charge donation from the pyrroli nitrogen lone pairs into the empty 4dx2-y2 and 4dxz, with additional effects from donation into the 4dz2 and 5s orbitals. The π bond, resulting from donation from occupied pyrrolyl π orbitals into the metal 4dxy and 4dyz orbitals is significant for η5,σ,η5,σ- but rather weak for σ,σ,σ,σ-co-ordination due to the diminished donation into the 4dxy orbital. The total orbital interaction contribution (the covalent component) is about one third of the ionic component of the bond, the latter being identified as the sum of the Pauli repulsion and the attractive electrostatic interaction between L4– and Zr4+. The ionic contribution is about the same in the two configurations, but the covalent component of the bond decreases by ca. 3 eV for σ:σ:σ:σ co-ordination due mostly to a weaker π-bond interaction. The η5,σ,η5,σ complex is only 2.1 eV more stable, indicating that the interconversion between the two co-ordination modes is a relatively easy process. The most stable co-ordination mode is preserved after interaction of the substrate with the Lewis bases tetrahydrofuran or H–.

High-yield syntheses (90%) of the monocyclopentadienyloxochloro complexes [(TiLCl2)2(µ-O)]2, [(TiLCl)4(µ-O)4]3, [(TiL)4(µ-O)6]4 and [(TiLCl)3(µ-O)3]5 have been achieved by hydrolysis of the precursors [TiLCl3]1[L =η5-C5H5, C5H4Me, C5H4(SiMe3) or C5Me5]. In addition [Ti{η5-C5H4(SiMe3)}Br3]7, [Ti4{η5-C5H4(SiMe3)}4Br4(µ-O)4]8 and [Ti4{η5-C5H4(SiMe3)}4(NCS)4(µ-O)4]9 have been prepared. The structures of the cyclic trimer 5(L =η5-C5Me5) and tetramer 9 have been determined by X-ray analysis: 5, monoclinic, space group P21/c, a= 16.724(2), b= 8.958(1), c= 22.871(2)A, β= 91.73(1)°, Z= 4 and R= 0.065 for 3159 independent observed reflections; 9, monoclinic, space group Cc, a= 24.527(2), b= 12.520(2), c= 18.931(2)A, β= 115.46(2)°, Z= 4 and R= 0.060 for 3088 independent observed reflections. The electronic configurations of complexes 2–5 have been calculated by extended-Huckel molecular orbital calculations.

Ab initio restricted Hartree–Fock and configuration interaction calculations have been carried out on the system Ni(PH3)2+ CH4 in order to study the energetics and the mechanism of the oxidative-addition reaction of CH4 and to model the activation of the C–H bond by zerovalent, co-ordinatively unsaturated transition-metal complexes. Energy-gradient optimizations and transition-state localizations have been performed on reactants and products in various constrained geometries. The results indicate that the oxidative addition of methane to Ni(PH3)2 is endothermic by 7.0 kcal mol–1 and the planar trans product is the most stable, being lower in energy than the cis isomer by 3.4 kcal mol–1.

We present recent developments in the implementation of the density fitting approach for the Coulomb interaction within the four-component formulation of relativistic density functional theory [Belpassi et al., J. Chem. Phys. 124, 124104 (2006)]. In particular, we make use of the Poisson equation to generate suitable auxiliary basis sets and simplify the electron repulsion integrals [Manby and Knowles, Phys. Rev. Lett. 87, 163001 (2001)]. We propose a particularly simple and efficient method for the generation of accurate Poisson auxiliary basis sets, based on already available standard Coulomb fitting sets. Just as is found in the nonrelativistic case, we show that the number of standard auxiliary fitting functions that need to be added to the Poisson-generated functions in order to achieve a fitting accuracy equal or, in some cases, better than that of the standard procedure is remarkably small. The efficiency of the present implementation is demonstrated in a detailed study of the spectroscopic properties and energetics of several gold containing systems, including the Au dimer and the CsAu molecule. The extraction reaction of a H(2)O molecule from a Au(H(2)O)(9) (+) cluster is also calculated as an example of mixed heavy-light-atom molecular systems. The scaling behavior of the algorithm implemented is illustrated for some closed shell gold clusters up to Au(5) (+). The increased sparsity of the Coulomb matrices involved in the Poisson fitting is identified, as are potential computational applications and the use of the Poisson fitting for the relativistic exchange-correlation problem.

Ab initio Hartree-Fock calculations have been performed on a series of zinc complexes containing a coordinated water molecule in order to understand the factors governing the pK a of the coordinated water and the nucleophilicity of the Zn-OH and Zn-OH 2 groups. The systems investigated include Zn(NH 3 ) n OH 2 2+ (n=3-5), Zn(NH 3 ) 2 (OCOH)OH 2 + , Zn(NH 3 ) 2 (SCH 3 )OH 2 + , Zn-(NH 3 )(SCH 3 ) 2 OH 2 , and their deprotonated partners. The order of nucleophilicity for the various compounds is presented, as well as the order of acidity of the aqua complexes

Non-empirical LCAO MO SCF calculations within the Hartree-Fock formalism have been performed on linear and bent conformations of NiCO, for both the states arising from the 3d94s1 and 3d10 electronic configurations of Ni, for ground and core ionized species. Binding and relaxation energies are computed for the nickel atom, the free ligand (CO) and the absorbed species (NiCO). Comparison with available experimental data reveals good agreement for the shifts in core binding energies for the bent, 3d94s1 NiCO system. The main contribution to these binding energy shifts is found to be due to relaxation effects, which may be rationalized in terms of atomic population and bond overlap changes accompanying core ionization.