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SCF wave functions have been computed for the ground state of Fe 2 (CO) 9 , Fe 2 (CO) 6 (C 2 H 2 ) and Mn 2 (CO) 10 complexes. These wave functions were the starting point for theoretical studies of the topology of the charge density of several carbonyl types. An analysis of the Laplacian of ρ shows that a bridging carbonyl shows a distribution of the electron density similar to that of a keto group, whereas in the terminal coordination the CO fragment retains the axial symmetry. The integration of the electron density suggests a large transfer of charge from the metal atom to each bridging CO. According to the back-bonding model, the electron density in a bridging carbon is more concentrated in the coordination plane. The atomic properties of equatorial and axial carbonyls in Mn 2 (CO) 10 present some differences

Addition of a dithiol ligand HS(CH2)nSH (n= 2–4) to [{Rh(µ-OMe)(cod)}2](cod = cycloocta-1,5-diene) afforded dithiolate-bridged complexes [{Rh2[µ-S(CH2)nS](cod)2}x](x= 1 or 2). These diene complexes react with carbon monoxide to give the binuclear tetracarbonyl complexes [Rh2{µ-S(CH2)nS}(CO)4]. The crystal structures of [Rh2{µ-S(CH2)nS}(cod)2](n= 2 or 3) have been determined by X-ray diffraction methods. A study of the electronic structure of the complexes by analysis of the topology of the charge density shows that in spite of a zero formal bond order between the rhodium atoms a direct metal–metal interaction may be characterized.

The Ti8C12 molecule, recently observed as a positive ion by Castleman et al., is presented as a cluster with seven possible conformations which have been characterized as local minima by means of ab initio Hartree–Fock calculations.

Palladium metal-catalyzed boron addition to unsaturated carbon–carbon bonds provides an efficient and convenient route for the preparation of organoboranes, which are versatile intermediates for organic synthesis. Palladium complexes are responsible for the exclusive catalytic performance and eventually allow access to selectively functionalized molecules by catalytic consecutive tandem sequences. The final objective is to find suitable palladium complexes that make it possible to perform a one-pot sequential reaction (B-addition/functionalization) by means of a multifaceted palladium catalyst. Mechanistic insights into the concatenated reactions through B chemistry are required if one wants to understand the experimental results.

The Ti8C12 molecule, recently observed as a positive ion by Castleman et al., is presented as a cluster with seven possible conformations which have been characterized as local minima by means of ab initio Hartree–Fock calculations.

[PMo12O40(VO)2]5- is a highly reduced cluster with eight d metal electrons. The localization−delocalization nature of d metal electrons is one of the points of interest in reduced polyoxoanions. We report DFT calculations which suggest that the oxidation state of the V atoms is +4 and that the other six metal electrons are delocalized between the 12 Mo atoms. The calculations also provide evidence that the anion [PMo12O40(VO)2]5- should be paramagnetic.

Palladium metal-catalyzed boron addition to unsaturated carbon–carbon bonds provides an efficient and convenient route for the preparation of organoboranes, which are versatile intermediates for organic synthesis. Palladium complexes are responsible for the exclusive catalytic performance and eventually allow access to selectively functionalized molecules by catalytic consecutive tandem sequences. The final objective is to find suitable palladium complexes that make it possible to perform a one-pot sequential reaction (B-addition/functionalization) by means of a multifaceted palladium catalyst. Mechanistic insights into the concatenated reactions through B chemistry are required if one wants to understand the experimental results.

Bis(catecholato)diboron was added to cyclic alkenes with internal strain to selectively form bis(boryl)cycloalkanes quantitatively in the presence of [Pd(NHC)pyCl2], [Pd(NHC)pyI2], and [Pt(NHC)pyI2] (NHC=N-heterocyclic carbene, py=pyridyl), which have been synthesized and characterized for the first time in this study.

The Laplacian of the charge density has been computed for the binuclear complex V2(μ-η2S2)2(S2CH)4. The inner-valence shell charge concentration of the vanadium atom exhibits four maxima. The Fermi hole distributions show the localized nature of the lone pairs of the ligand with a low penetration into the metal basin. The 3d electrons of vanadium also have a localized character. The Laplacian of the electron density as well as the Fermi hole distributions give support to the ligand field model for the metal-ligand interactions in organometallic chemistry.

By the deliberate choice of the internal ligands of the porous nanocapsules [{(Mo)Mo(5)}(12){Mo(2)(ligand)}(30)](n-), the respective cavities' shells can be differently sized/functionalized. This allows one to trap the same large number of water molecules, that is, 100 in a capsule cavity with formate ligands having a larger space available, as well as in a cavity containing sulfates and hypophosphites, that is, with less space. Whereas the 100 molecules fill the space completely in the second case in which they are organized in three shells, a four-shell system with underoccupation and broken hydrogen bonds is observed in the other case. This is an unprecedented result in terms of the structurally well defined special forms of "higher and lower density" water molecule assemblies. Precisely, by replacing the larger ligands in the mentioned nanocapsule type by formates, voids in the capsule cavity of (HC(NH(2))(2))(22)[{(HC(NH(2))(2))(20)+(H(2)O)(100)} subset{(Mo)Mo(5)O(21)(H(2)O)(6)}(12){Mo(2)O(4)(HCO(2))}(30)]ca. 200 H(2)O are generated that get filled with water molecules concomitant with an expansion of the three to four shell {H(2)O}(100) cluster. The water shells in both capsules containing different ligands are organized in the form of dodecahedra (partly with underoccupation) and a strongly distorted rhombicosidodecahedron spanned by a {H(2)O}(60)={(H(2)O)(5)}(12) aggregate. The well-defined water shells only emerge if cations cannot enter into the capsules, which is achieved by closing the pores with plugs/guests such as formamidinium cations. The work is based on the syntheses of two new compounds, related single-crystal X-ray diffraction studies, and molecular dynamics simulations, which show remarkably that water molecule shell structuring occurs in the capsules due to the confined conditions even in the case of open pores and at room temperature if cation uptake is prevented.

Converging UV-vis, EPR, rRaman, and DFT calculations highlight the evolution of [Ru(4)(H(2)O)(4)(mu-O)(4)(mu-OH)(2)(gamma-SiW(10)O(36))(2)](10-), 1, to high-valent intermediates. In analogy with the natural enzyme, five different oxidation states, generated from 1, have been found to power the catalytic cycle for water oxidation. A high electrophilic tetra ruthenium (V)-hydroxo species is envisaged as the competent intermediate, undergoing nucleophilic attack by an external water molecule as a key step in the formation of a new O-O bond under catalytic conditions

In zeolites and other rigid solid-state oxides, substrates whose sizes exceed the pore dimensions of the material are rigorously excluded. Now, using a porous 3 nm diameter capsule-like oxomolybdate complex [{Mo(VI)(6)O(21)(H(2)O)(6)}(12){(Mo(V)(2)O(4))(30)(OAc)(21)(H(2)O)(18)}](33-) as a water-soluble analogue of solid-state oxides (e.g., as a soluble analogue of 3 A molecular sieves), we show that carboxylates (RCO(2)(-)) can negotiate passage through flexible Mo(9)O(9) pores in the surface of the capsule and that the rates follow the general trend R = 1 degree >> 2 degrees > 3 degrees >> phenyl (no reaction). Surprisingly, the branched alkanes (R = iso-Pr and tert-Bu) enter the capsule even though they are larger than the crystallographic dimensions of the Mo(9)O(9) pores. Four independent lines of spectroscopic and kinetic evidence demonstrate that these organic guests enter the interior of the capsule through its Mo(9)O(9) apertures and that no irreversible changes in the metal oxide framework are involved. This unexpected phenomenon likely reflects the greater flexibility of molecular versus solid-state structures and represents a sharp departure from traditional models for diffusion through porous solid-state (rigid) oxides.

An improved version of our method for computing QTAIM [J.I. Rodríguez, A.M. Köster, P.W. Ayers, A. Santos-Valle, A. Vela, G. Merino, J. Comput. Chem. (2009), in press, doi:10.1002/jcc.21134] is presented. Vectorization and parallelization of the previous algorithm, together with molecular symmetry, make the present algorithm as much as two orders of magnitude faster than our original method. The present method scales linearly with both system size and the number of processors. The performance of the method is demonstrated by computing the QTAIM atomic properties of a series of carbon nanotubes. Our results show that the CPU time for a QTAIM property calculation is comparable to that of a SCF-single point calculation. The accuracy of the original method is also improved. © 2009 Elsevier B.V. All rights reserved.

6 pags, 6 figs, Protein p53 is a transcription factor crucial for cell cycle and genome integrity. It is able to induce both cell arrest when DNA is damaged and the expression of DNA repair machinery. When the damage is irreversible, it triggers apoptosis. Indeed, the protein, which is a homotetramer, is mutated in most human cancers. For instance, the inherited mutation p53-R337H results in destabilization of the tetramer and, consequently, leads to an organism prone to tumor setup. We describe herein a rational designed molecule capable of holding together the four monomers of the mutated p53-R337H protein, recovering the tetramer integrity as in the wild-type structure. Two ligand molecules, based on a conical calix[4]arene with four cationic guanidiniomethyl groups at the wider edge (upper rim) and hydrophobic loops at the narrower edge (lower rim), fit nicely and cooperatively into the hydrophobic clefts between two of the monomers at each side of the protein and keep the tetrameric structure, like molecular templates, by both ion-pair and hydrophobic interactions. We found a good agreement between the structure of the complex and the nature of the interactions involved by a combination of theory (molecular dynamics) and experiments (circular dichroism, differential scanning calorimetry and 1H saturation transfer difference NMR). © 2008 by The National Academy of Sciences of the USA., S.G. thanks B. Claasen for her help on the STD experiments design and T. Tarrago for biological support. Some of the simulations were run at the Barcelona Supercomputing Center-Mare Nostrum. This work was supported by the Spanish Ministry of Science and Education (MEC) Projects BIO2005-00295, CTQ2005-06909-C02-02/BQU, and CTQ2005-08989-C02/BQU, Consolider Ingenio 2010 Grant CSD2006-0003, and the Institute of Chemical Research of Catalonia (ICIQ) Foundation. S.G., V.M., and E.S. received predoctoral fellowships from Generalitat de Catalunya-Agencia de Gestio d'Ajuts Universitaris i de Recerca, MEC, and the ICIQ Foundation.

We describe herein a detailed study of the inclusion processes of several positively charged organometallic sandwich complexes inside the aromatic cavity of the self-folding octaamide cavitand 1. In all cases, the binding process produces aggregates with a simple 1:1 stoichiometry. The resulting inclusion complexes are not only thermodynamically stable, but also kinetically stable on the (1)H NMR spectroscopy timescale. The binding constants for the inclusion complexes were determined by different titration techniques. We have also investigated the kinetics of the binding process and the motion of the metallocenes included in the aromatic cavity of the host. Using DFT-based calculations, we have evaluated the energies of a diverse range of potential binding geometries for the complexes. We then computed the proton chemical shifts of the included guest in each one of the binding geometries. The agreement between the averaged computed values and the experimentally determined chemical shifts clearly supports the proposed binding geometries that we assigned to the inclusion complexes formed in solution. The combination of experimental and theoretical results has allowed us to elucidate the origins of the distinct features detected in the complexation process of the different guests, as well as their different motions inside the host.

Quantum mechanics/molecular mechanics (QM/MM) methods are a useful tool for the computational study of inorganic systems. They allow a quantitative description of systems larger than those treatable with pure QM methods, in principle with a comparable quality. QM/MM calculations are being currently applied to the research in a variety of topics, including structural effects of ligand bulk, selectivity in homogeneous catalysis and mechanical embedding in heterogeneous catalysis. The QM/MM approach is also useful for the separation of steric and electronic contributions, and as an auxiliary tool for geometry optimization when full QM methods are mandatory. The power of QM/MM methods in inorganic chemistry is illustrated in this Perspective with a summary of recent representative applications.

Dichloromethane solutions of mono- and dinuclear diolefin and carbonyl iridium(I) and dihydridoiridium(III) complexes containing thioether or dithioether ligands, and mixed complexes with PR3 (R = Me, p-MePh, p-ClPh, OMe, OPh) have been used as catalyst precursors in the hydrogenation of olefins at 25 °C and 1 atm of H2. The effect of the different ligands on the catalytic activity has been studied. Theoretical calculations using EHT method have been performed.

The Laplacian of the charge density has been computed for models of the binuclear complexes Rh2(tcl)4 (1) and Rh2(tcl)4CO (2), with tcl=thiocaprolactamate. The inner-valence shell charge concentration (i-VSCC) of the rhodium atom exhibits eight maxima in agreement with the pattern predicted by the crystal field theory assumptions. The rhodium-rhodium bond in complexes 1 and 2 is not characterized by large accumulations of charge between both metals. As previously suggested by electron deformation density analysis, the metal-metal bond does not behave like usual covalent bonds. The analysis of Laplacian charge-density maps suggests a weakening of the rhodium-rhodium bond induced by axial CO coordination.

Novel catalytic activation of the B–B bond by palladium(II)–NHC complexes in presence of a mild base (NaOAc) and an excess of diboron reagent enables chemoselective 1,2-diboration of alkenes, suggesting the heterolytic cleavage of diboron rather than oxidative addition of a B–B bond to the metal.

The Preyssler anion, of general formula [Xn+P5W30O110](15-n)-, is the smallest polyoxometalate (POM) with an internal cavity allowing cation exchange with the solution. The Preyssler anion features a rich chemistry evidenced by its ability to accept electrons at low potentials, to selectively capture various metal cations, and to undergo acid-base reactions. A deep understanding of these topics is herein provided by means of DFT calculations on the title series of compounds. We evaluate the energetics of the release/encapsulation process for several Xn+ cations and identify the effect of the encapsulated ion on the properties of the Preyssler anion. We revisit the relationship between the internal cation charge and the electrochemical behavior of the POM. A linear dependence between the first one-electron reduction energies and the encapsulated Xn+ charge is found, with a slope of 48 mV per unit charge. The protonation also shifts the reduction potential to more positive values, but the effect is much larger. In connection to this, the last proton's pKa = 2 for the Na+ derivative was estimated to be in reasonable agreement with experiment. The electronic structure of lanthanide derivatives is more complex than conventional POM structures. The reduction energy for the CeIV-Preyssler + 1e- --CeIII-Preyssler process was computed to be more exothermic than that of very oxidant species such as S2Mo18O624-.

X-ray structural data for the cubane-type clusters [Mo3CuS4(dmpe)3Cl4](+) and Mo3NiS4(dmpe)3Cl4 (dmpe = 1,2-bis(dimethylphosphino)ethane) with 16 metal electrons have been compared with optimized structural parameters calculated using "ab initio" methodologies. Compound Mo3NiS4(dmpe)3Cl4 crystallizes in the cubic noncentrosymmetric space group P213 with a Mo-Ni distance of 2.647 Angstrom, that is 0.2 Angstrom shorter than the Mo-Cu bond length in the isoelectronic copper cluster. The best agreement between theory and experiments has been obtained using the B3P86 method. In order to validate the B3P86 results, accurate infrared and Raman spectra have been acquired and the vibrational modes associated to the cubane-type Mo3M'S4 (M' = Cu or Ni) unit have been assigned theoretically. The electronic changes taking place when incorporating the M' into the Mo3S4 unit have been analyzed from a theoretical and experimental perspective. The bond dissociation energies between M'-Cl and Mo3S4 fragments show that formation of [Mo3CuS4(dmpe)3Cl4](+) is 135 kcal/mol energetically less favorable than the Ni incorporation. The more robust nature of the Mo3NiS4 fragment has been confirmed by mass spectrometry. The X-ray photoelectron spectroscopy (XPS) spectra of the trimetallic and tetrametallic complexes have been measured and the obtained binding energies compared with the computed electronic populations based on topological approaches of the electron localization function (ELF). The energies and shapes of the Cu 2p and Ni 2p lines indicate formal oxidation states of Cu(I) and Ni(II). However, the reductive addition of nickel into [Mo3S4(dmpe)3Cl3](+) causes a small decrease in the Mo 3d binding energies. This fact prevents an unambiguous assignment of an oxidation state in a conventional way, a circumstance that has been analyzed through the covariance of the electronic populations associated to the C(M') core and V(Mo3Ni) and V(S(2)') valence basins where Mo3NiS4 is a particularly electronically delocalized chemical entity.

The origin of the chemoselectivity of palladium catalysts containing bidentate phosphine ligands toward either methoxycarbonylation of ethene or the copolymerization of ethene and carbon monoxide was investigated using density functional theory based calculations. For a palladium catalyst containing the electron-donating bis(dimethylphosphino)ethane (dmpe) ligand, the rate determining step for chain propagation is shown to be the insertion of ethene into the metal-acyl bond. The high barrier for chain propagation is attributed to the low stability of the ethene intermediate, (dmpe)Pd(ethene)(C(O)CH3). For the competing methanolysis process, the most likely pathway involves the formation of (dmpe)Pd(CH3OH)(C(O)CH3) via dissociative ligand exchange, followed by a solvent mediated proton-transfer/reductive- elimination process. The overall barrier for this process is higher than the barrier for ethene insertion into the palladium-acetyl bond, in line with the experimentally observed preference of this type of catalyst toward the formation of polyketone. Electronic bite angle effects on the rates of ethene insertion and ethanoyl methanolysis were evaluated using four electronically and sterically related ligands (Me)2P(CH2)nP(Me)2 (n = 1-4). Steric effects were studied for larger tert-butyl substituted ligands using a QM/MM methodology. The results show that ethene coordination to the metal center and subsequent insertion into the palladium-ethanoyl bond are disfavored by the addition of steric bulk around the metal center. Key intermediates in the methanolysis mechanism, on the other hand, are stabilized because of electronic effects caused by increasing the bite angle of the diphosphine ligand. The combined effects explain successfully which ligands give polymer and which ones give methyl propionate as the major products of the reaction.

Electronic effects induced by diphosphine bidentate ligands on the regioselectivity of the rhodium-catalyzed hydroformylation of propene were investigated using density functional theory based calculations (B3LYP). To this end, the key hydride migration step was evaluated for HRh(propene)(CO)L2 (L2 = PF3, PF3; PH3, PH3; PMe3, PMe3; PH3, PF3; PH3, PMe3) incorporating either two identical or two electronically distinct phosphorus moieties. The phosphorus moieties span a wide range of ligand basicities. While the electronic properties of the ligands do not influence the regioselectivity of the hydride migration reaction directly, they do govern the amount of back-donation from the metal to the alkene substrate. As a result, important differences in transition-state geometries are obtained for different ligand systems. For electron-withdrawing ligands low activation energies and trigonal-bipyramidal transition-state geometries are observed. Increasing the basicity of the diphosphine ligand leads to higher act...

The chemistry of polyoxoanions has been studied extensively.1-12 A large body of experimental knowledge has been accumulated on these oxide clusters, which can in many instances be reduced, maintaining their integrity with only subtle changes in their structure.7a,12 Trends in the obtention of blue reduced species have been empirically established over time, and the nature of electronic delocalization within the cluster has been discussed. Both hopping and ground-state delocalization mechanisms have been proposed to explain their electronic structure.1 However, there is still a need for a systematic analysis of the effects of reduction on the molecular and electronic structure, stability, and reactivity of these clusters. Ab initio theoretical studies in this field are still scarce and are restricted to some fully oxidized polyanions.13-15 The aim of this note is to present the first theoretical calculations of this type on the redox properties of a Keggin anion. For that, a bicapped, highly reduced Keggin complex [PMo12O40(VO)2],5- 1, whose synthesis and characterization have been reported recently by Hill et al.,16 was chosen. In contrast with the isostructural [V15O42]9- cluster,17 in which all of the vanadium atoms are formally d0 metals, in anion 1 the metal centers contain eight electrons (see Figure 1 for a 3D representation of the anion). In their study Hill suggests, on the basis of valence bond calculations, that two of the electrons are in vanadium centers while the other six electrons are on molybdenum. The results of our density functional calculations (DFT) on the bicapped cluster [PMo12O40(VO)2]5- agree with this., All calculations were carried out on workstations purchased with funds provided by the DGICYT of the Government of Spain and by the CIRIT of the Generalitat de Catalunya (Grants PB95-0639-C02-02 and SGR95-426).

Density functional theory (DFT) calculations are used to investigate dimerization, by acid condensation, between Lindvist and Keggin cluster anions. When specific addendum atoms are present (notably, Ti, V, Nb, and Cr), these clusters dimerize in the presence of acid via the formation of mu-O linkages. These processes may be viewed as molecular models for the formation of metal-oxygen bonds in the active sites of metalloenzymes, or in materials chemistry, and we here provide detailed information concerning these reactions. For this, DFT calculations are used to provide insight into the mechanism(s) associated with dimerization of Lindqvist clusters, W5MO19(n-) (M = W, Mo, V, Nb, and Ti) in acidic media. In full accord with experimental data, our calculations show that dimerization of Nb and Ti derivatives is thermodynamically favored and that this is not the case for dimerization between Mo, V, and W addendum atoms. In addition, dimerization of PW11TiO40(5-) and SiW11NbO40(5-) to give the corresponding dimers was also calculated to be exothermic. Two possible mechanisms, involved in two competing pathways, are evaluated and discussed. Conclusions are presented concerning the role played by the nature of the metal atoms taking part in M-mu-O-M bond formation. A correlation is established between the relative strengths of specific terminal M=O bonds in monomeric precursors and the tendency of these to form M-mu-O-M bonds.

The Keggin anion, PW12O40(3-), is one of the most representative polyoxometalates (POMs). In recent years increasing theoretical work focused on this family of compounds has explained or even predicted some of their properties using quantum mechanics methods. In this report we applied for the first time molecular dynamics (MD) to the title compound to analyze its interactions with water. We used three atomic charge definitions (Mulliken, ChelpG, and formal charges) to carry out MD simulations. The results show that the terminal oxygens of the cluster are invariably most effectively solvated by water because of their prominent position within the framework. On the other hand, bridging oxygens, which are confined in more internal positions, concentrate a smaller portion of the whole solvation. We investigated the hydrogen bonds existing between water and the cluster, confirming that the terminal positions form more contacts with H2O than any other site of the cluster. In the end, the lifetime of such contacts is longer with bridging oxygens, presumably due to their higher atomic charge.

[structures: see text] Fullerenes containing a trimetallic nitride template (TNT) within the cage are a particularly interesting class of endohedral metallofullerenes. Recently two exohedral derivatives of the Sc3N@C80 fullerene have been synthesized: a Diels-Alder and a fulleropyrrolidine cycloadduct. The successful isolation, purification, and structural elucidation of these metallofullerenes derivatives have encouraged us to understand how the chemical reactivity is affected by TNT encapsulation. First of all, we predicted the most reactive exohedral sites, taking into account the double bond character and the pyramidalization angle of the C-C bonds. For this purpose, a full characterization of all different types of C-C bonds of the following fullerenes was carried out: I(h)-C60:1, D3-C68:6140, D3-Sc3N@C68, D(5h)-C70:1, D(3h')-C78:5, D(3h)-Sc3N@C78, I(h)-C80:7 and several isomers of Sc3N@C80. Finally the exohedral reactivity of these TNT endohedral metallofullerenes, via [4 + 2] cycloaddition reactions of 1,3-butadiene, was corroborated by means of DFT calculations.

The origin of stereoinduction by chiral aminophosphane phosphinite (AMPP) ligands in asymmetric hydroformylation was investigated with a theoretical approach. The roles of the stereogenic center at the aminophosphane phosphorus atom (NP*) and of the chirality of the backbone were analyzed by considering three experimentally tested cases: 1) P-stereogenic yielding high ee, 2) P-nonstereogenic yielding low ee, and 3) P-stereogenic yielding low ee. We succeeded in reproducing the experimentally observed trends for the three studied AMPP ligands. Our results indicated that alkene insertion into the rhodium-hydride bond is the selectivity-determining step, and not alkene coordination. Additional calculations on model systems revealed that the different nonbonding weak-type interactions of styrene with the substituents of the NP* stereogenic center in an axial position is responsible for stereodifferentiation. The chirality of the AMPP backbone plays a secondary role. The rationalization of the stereochemical outcome is not straightforward, because two competitive equatorial/axial reaction paths, showing opposite asymmetric induction, must be considered. Construction of stereochemical models and evaluation of stereoinduction for novel ligand systems suggested that two prerequisites are required to improve the performance of AMPP-type ligands in asymmetric hydroformylation: 1) combination of stereorecognition and stereohindrance by substituents at the NP* atom, and 2) more rigid backbones.

The Keggin anion, PW12O403-, is one of the most representative polyoxometalates (POMs). In recent years increasing theoretical work focused on this family of compounds has explained or even predicted some of their properties using quantum mechanics methods. In this report we applied for the first time molecular dynamics (MD) to the title compound to analyze its interactions with water. We used three atomic charge definitions (Mulliken, ChelpG, and formal charges) to carry out MD simulations. The results show that the terminal oxygens of the cluster are invariably most effectively solvated by water because of their prominent position within the framework. On the other hand, bridging oxygens, which are confined in more internal positions, concentrate a smaller portion of the whole solvation. We investigated the hydrogen bonds existing between water and the cluster, confirming that the terminal positions form more contacts with H2O than any other site of the cluster. In the end, the lifetime of such contacts...

Continuing our studies into the effect that N-N' ligands have on CO/styrene copolymerization, we prepared new C(1)-symmetrical pyridine-imidazoline ligands with 4',5'-cis stereochemistry in the imidazoline ring (5) and 4',5'-trans stereochemistry (6-10) and compared them with our previously reported ligands (1-4). Their coordination to neutral methylpalladium(II) (5 a-10 a) and cationic complexes (5 b-10 b), investigated in solution by NMR spectroscopy, indicates that both the electronic and steric properties of the imidazolines determine the stereochemistry of the palladium complexes. The crystal structures of two neutral palladium precursors [Pd(Me)(2-n)Cl(n)(N-N')] (n=1 for 8 a; n=2 for 9 a') show that the Pd-N coordination distances and the geometrical distortions in the imidazoline ring depend on the electronic nature of the substituents in the imidazoline fragment. Density functional calculations performed on selected neutral and cationic palladium complexes compare well with NMR and X-ray data. The calculations also account for the formation of only one or two stereoisomers of the cationic complexes. The performance of the cationic complexes as catalyst precursors in CO/4-tert-butylstyrene copolymerization under mild pressures and temperatures was analyzed in terms of the productivity and degree of stereoregularity of the polyketones obtained. Insertion of CO into the Pd-Me bond, which was monitored by multinuclear NMR spectroscopy, shows that the N ligand influences the stereochemistry of the acyl species formed.

Mononuclear complexes, which contain a dianionic ene-diamido ligand bound to a group 4 metal atom in the formal d 0 configuration, are analyzed by the DFT method to interpret the electronic origin of the folding at the five-membered 2,5-diazapent-3-ene metallacycle moiety. Geometry optimizations were carried out for the following models, TiCl 2 [ o -(Me 3 SiN) 2 C 6 H 4 ], Ti(OPh) 2 (DAD), CpTiCl(DAD), CpTiMe(DAD) and Cp 2 Zr(DAD) (DAD = HNCHCHNH). They show some common electronic features, the nature of the HOMO, in particular. In all cases, the latter results from the donation of a filled π 3 ∗ ene-diamido level into an empty σ metal orbital, this being maximized upon the folding of the metallacycle. Such a geometric rearrangement involves the filled nitrogen p π lobes, while the C C π bond remains essentially uninvolved. The feature is confirmed by the application of the atom in molecules (AIM) theory, that provides no evidence of critical points between the metal center and the pair of two carbon atoms.

In this review we summarise the quantum chemistry studies carried out by several groups over the last ten years on polyoxometalates, or polyoxoanions. This is an immense family of compounds made up of transition metal ions in their highest oxidation state and oxo ligands. The continuous progress of computers in general, and quantum chemistry software in particular, has enabled a number of topics in polyoxometalate chemistry to be studied from the electronic structure of the most representative polyoxometalate, the so-called Keggin anion, to the factors governing the inclusion complexes and the magnetism in reduced complexes.

A review of theoretical progress in the modeling of rhodium diphosphine hydroformylation is presented. Early studies using pure quantum mechanics (QM) methods with phosphines modeled as PH3 were able to provide a general picture of the complicated reaction cycle, but could not tackle more delicate issues as regioselectivity and enantioselectivity. The modeling of real-world phosphines with hybrid quantum mechanics/molecular mechanics (QM/MM) methods, together with the availability of new experimental data, has allowed a more detailed comprehension of the mechanism, as well as an understanding of the role played by steric and electronic effects of the substituents.

Keggin heteropolyanions [XM(12)O(40)](n-) have various isomeric structures, alpha and beta being the most common. Conventionally, the alpha structure appears to be the most stable, but calculations carried out at the DFT level for X = P(V), Si(IV), Al(III), As(V), Ge(IV), and Ga(III) and M = W(VI) and Mo(VI) show that this stability depends on several factors, particularly on the nature of the heteroatom (X) and the total charge of the cluster. In this paper, we apply the clathrate model to the Keggin molecule to carry out a fragment-interaction study to elucidate when and why the traditional relative stability of various isomers can be inverted. The fully oxidized anions that have inverted the traditional stability trend in this series are [AlW(12)O(40)](5-) and [GaW(12)O(40)](5-), both of which contain a third-group heteroatom and an overall charge of -5. beta-isomers are always more easily reduced than alpha-isomers. This experimental observation suggests that reduction favors the stability of beta-isomers and one of the most important results of this study is that the alpha/beta inversion is achieved in most cases after the second reduction. The alpha- and beta-isomers may have different properties because the energy of the LUMO, a symmetry-adapted d(xy)-metal orbital, is different.