Photoelectron spectroscopic study of the hydrated nucleoside anions: Uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1)

The hydrated nucleoside anions, uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1), have been prepared in beams and studied by anion photoelectron spectroscopy in order to investigate the effects of a microhydrated environment on parent nucleoside anions. Vertical detachment energies (VDEs) were measured for all eight anions, and from these, estimates were made for five sequential anion hydration energies. Excellent agreement was found between our measured VDE value for thymidine(-)(H(2)O)(1) and its calculated value in the companion article by S. Kim and H. F. Schaefer III.     

Microsolvation pattern of the hydrated radical anion of uracil: U-(H2O)n (n = 3-5)

The microsolvation patterns of the uracil radical anion in water clusters U-(H2O)n with n ranging from 3 to 5 were investigated by the density functional theory approach. The electron detachment energies (VDE) of the stable anionic complexes with different numbers of hydration water are predicted. The linear dependence of the VDE value of the most stable anionic complexes with respect to the hydration number suggests the importance of the clustered waters in the microsolvation of the radical anion of the nucleobases. The formation of the water clusters is found to be necessary in the most stable conformers of the tri-, tetra-, and pentahydrated radical anion of uracil. The microsolvation pattern with three or more well-separated hydration water molecules in the first hydration layer is less stable than the arrangement with the waters in tight clusters. The charge transfer between the anionic uracil and the hydration water is high. Good agreement between the experimental and the theoretical vertical detachment energy yield in this study further demonstrates the practicability of the B3LYP/DZP++ approach in the study of radical anions of the DNA subunits.     

Microsolvation of N2H): the nature of interactions in N2H+-(H2)n (n = 1-14) complexes

Experimental studies of the consecutive growth of N2H + (H2)n clusters led to the discovery of an unusual bonding pattern for species with n = 2-4. Theoretical studies revealed that the ligands are located within five well-separated solvation shells that are visible in structures, values of successive enthalpies and entropies of clustering reactions, vibrational motions, the distribution of atomic charges, and interaction energy decomposition components. The pattern of consecutive enthalpy changes for the second shell (n = 2-5) is complicated. This pattern shows anomalous behavior, although its interpretation is not univocal. A large part of consecutive enthalpies for the clustering reactions is a contribution due to the rotational and vibrational properties of clusters which are difficult for adequate modeling in large systems. The structures of clusters are rationalized based on interaction energy contributions of a different nature. Geometries of complexes are determined by prevailing covalent forces.     

Excited-state interactions for [Au(CN)(2)(-)](n) and [Ag(CN)(2)(-)](n) oligomers in solution. Formation of luminescent gold-gold bonded excimers and exciplexes.

Solutions of K[Au(CN)(2)] and K[Ag(CN)(2)] in water and methanol exhibit strong photoluminescence. Aqueous solutions of K[Au(CN)(2)] at ambient temperature exhibit luminescence at concentration levels of > or =10(-2) M, while frozen methanol glasses (77 K) exhibit strong luminescence with concentrations as low as 10(-5) M. The corresponding concentration limits for K[Ag(CN)(2)] solutions are 10(-1) M at ambient temperature and 10(-4) M at 77 K. Systematic variations in concentration, solvent, temperature, and excitation wavelength tune the luminescence energy of both K[Au(CN)(2)] and K[Ag(CN)(2)] solutions by >15 x 10(3) cm(-1) in the UV-visible region. The luminescence bands have been individually assigned to *[Au(CN)(2)(-)](n) and *[Ag(CN)(2)(-)](n) excimers and exciplexes that differ in "n" and geometry. The luminescence of Au(I) compounds is related for the first time to Au-Au bonded excimers and exciplexes similar to those reported earlier for Ag(I) compounds. Fully optimized unrestricted open-shell MP2 calculations for the lowest-energy triplet excited state of staggered [Au(CN)(2)(-)](2) show the formation of a Au-Au sigma single bond (2.66 A) in the triplet excimer, compared to a weaker ground-state aurophilic bond (2.96 A). The corresponding frequency calculations revealed Au-Au Raman-active stretching frequencies at 89.8 and 165.7 cm(-1) associated with the ground state and lowest triplet excited state, respectively. The experimental evidence of the exciplex assignment includes the extremely large Stokes shifts and the structureless feature of the luminescence bands, which suggest very distorted excited states. Extended Hückel (EH) calculations for [M(CN)(2)(-)](n) and *[M(CN)(2)(-)](n) models (M = Au, Ag; n = 2, 3) indicate the formation of M-M bonds in the first excited electronic states. From the average EH values for staggered dimers and trimers, the excited-state Au-Au and Ag-Ag bond energies are predicted to be 104 and 112 kJ/mol, respectively. The corresponding bond energies in the ground state are 32 and 25 kJ/mol, respectively.     

Solvation-induced cluster anion core switching from NNO2(-)(N2O)n-1 to O(-)(N2O)n

We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0or=4 (and up to at least n=9) signatures of an O(-) core are predominantly observed. Photofragmentation studies at 355 nm support these results.     

Cyanide-limited complexation of molybdenum(III): synthesis of octahedral [Mo(CN)(6)](3-) and cyano-bridged [Mo(2)(CN)(11)](5-)

Octahedral coordination of molybdenum(III) is achieved by limiting the amount of cyanide available upon complex formation. Reaction of Mo(CF(3)SO(3))(3) with LiCN in DMF affords Li(3)[Mo(CN)(6)] x 6DMF (1), featuring the previously unknown octahedral complex [Mo(CN)(6)](3-). The complex exhibits a room-temperature moment of mu(eff) = 3.80 mu(B), and assignment of its absorption bands leads to the ligand field parameters Delta(o) = 24800 cm(-1) and B = 247 cm(-1). Further restricting the available cyanide in a reaction between Mo(CF(3)SO(3))(3) and (Et(4)N)CN in DMF, followed by recrystallization from DMF/MeOH, yields (Et(4)N)(5)[Mo(2)(CN)(11)] x 2DMF x 2MeOH (2). The dinuclear [Mo(2)(CN)(11)](5-) complex featured therein contains two octahedrally coordinated Mo(III) centers spanned by a bridging cyanide ligand. A fit to the magnetic susceptibility data for 2, gives J = -113 cm(-1) and g = 2.33, representing the strongest antiferromagnetic coupling yet observed through a cyanide bridge. Efforts to incorporate these new complexes in magnetic Prussian blue-type solids are ongoing.     

A study of structural and bonding variations in the homologous series [Mo(2)(CN)(6)(dppm)(2)](n-) (n = 2, 1, 0)

Reaction of Mo(2)Cl(4)(dppm)(2) (dppm = bis(diphenylphosphino)methane) with 6 equiv of [n-Bu(4)N][CN] or [Et(4)N][CN] in dichloromethane yields [n-Bu(4)N](2)[Mo(2)(CN)(6)(dppm)(2)] (1) and [Et(4)N](2)[Mo(2)(CN)(6)(dppm)(2)] (2), respectively. The corresponding one- and two-electron oxidation products [n-Bu(4)N][Mo(2)(CN)(6)(dppm)(2)] (3) and Mo(2)(CN)(6)(dppm)(2) (4)were prepared by reactions of 1 with the oxidant NOBF(4). Single-crystal X-ray structures of 2.2CH(3)CN, 3.2CH(3)CN.2H(2)O, and 4.2CH(3)NO(2) were performed, and the results confirmed that all three complexes contain identical ligand sets with trans dppm ligands bisecting the Mo(2)(mu-CN)(2)(CN)(4) equatorial plane. The binding of the bridging cyanide ligands is affected by the oxidation state of the dimolybdenum core as evidenced by an increase in side-on pi-bonding overlap of the mu-CN in going from 1 to 4. The greater extent of pi-donation into Mo orbitals is accompanied by a lengthening of the Mo-Mo distance (2.736(1) A in Mo(2)(II,II) (2), 2.830(1) A in Mo(2)(II,III) (3), and 2.936(1) A in Mo(2)(III,III) (4)). A computational study of the closed-shell members of this homologous series, [Mo(2)(CN)(6)(dppm)(2)](n)() (n = 2-, 0), indicates that the more pronounced side-on pi-donation evident in the X-ray structure of 4 leads to significant destabilization of the delta orbital and marginal stabilization of the delta() orbitals with respect to nearly degenerate delta and delta orbitals in the parent compound, 2. The loss of delta contributions combined with the reduced orbital overlap due to higher charges on molybdenum centers in oxidized complexes 3 and 4 is responsible for the observed increase in the length of the Mo-Mo bond.     

Di-tert-butyl phosphate as synthon for metal phosphate materials via single-source coordination polymers [M(dtbp)(2)](n) (M = Mn,Cu) and [Cd(dtbp)(2)(H2O)](n) (dtbp-H = (tBuO)(2)P(O)OH)

Reaction of M(OAc)(2).xH(2)O (M = Mn, Cu, or Cd) with di-tert-butyl phosphate (dtbp-H) in a 1:2 molar ratio in methanol followed by slow crystallization of the resultant solid in MeOH/THF medium results in the formation of three new polymeric metal phosphates [M(dtbp)(2)](n)() [M = Mn, 1 (beige); M = Cu, 2 (blue)] and [Cd(dtbp)(2)(H(2)O)](n)(), 3 (colorless)] in good yields. The formation of [Mn(dtbp)(2)](n) (1) proceeds via tetrameric manganese phosphate [Mn(4)(O)(dtbp)(6)] (4), which has been isolated in an analytically pure form. Perfectly air- and moisture-stable compounds 1-4 were characterized with the aid of analytical, thermoanalytical, and spectroscopic techniques. The molecular structures of 1-3 were further established by single-crystal X-ray diffraction studies. Crystal data for 1: C(32)H(72)Mn(2)O(16)P(4), monoclinic, P2(1)/c, a = 19.957(4) A, b = 13.419(1) A, c = 18.083(2) A, beta = 91.25(2) degrees, Z = 4. Crystal data for 2: C(16)H(36)CuO(8)P(2), orthorhombic, Pccn, a = 23.777(2) A, b = 10.074(1) A, c = 10.090(1) A, Z = 4. Crystal data for 3: C(48)H(114)Cd(3)O(27)P(6), triclinic, P1, a = 12.689(3) A, b = 14.364(3) A, c = 22.491(5) A, alpha = 84.54(3) degrees, beta = 79.43(3) degrees, gamma = 70.03(3) degrees, Z = 2. The diffraction studies reveal three different structural forms for the three compounds investigated, each possessing a one-dimensional coordination polymeric structure. While alternating triple and single dtbp bridges are found between the adjacent Mn(2+) ions in 1, uniform double dtbp bridges across the adjacent Cu(2+) ions are present in 2. The cadmium ions in the structure of 3 are pentacoordinated. Thermal analysis (TGA and DSC) indicates that compounds 1-3 convert to the corresponding crystalline metaphosphate materials M(PO(3))(2), in each case at temperatures below 500 degrees C. Similarly, the thermal decomposition of 4 results in the formation of Mn(PO(3))(3) and Mn(2)P(2)O(7). The final materials obtained by independent thermal decomposition of bulk samples have been characterized using IR spectroscopic, powder diffraction, and N(2) adsorption studies.     

Theoretical Modeling of Water Exchange on [Pd(H(2)O)(4)](2+), [Pt(H(2)O)(4)](2+), and trans-[PtCl(2)(H(2)O)(2)

Density functional theory is applied to modeling the exchange in aqueous solution of H(2)O on [Pd(H(2)O)(4)](2+), [Pt(H(2)O)(4)](2+), and trans-[PtCl(2)(H(2)O)(2)]. Optimized structures for the starting molecules are reported together with trigonal bipyramidal (tbp) systems relevant to an associative mechanism. While a rigorous tbp geometry cannot by symmetry be the actual transition state, it appears that the energy differences between model tbp structures and the actual transition states are small. Ground state geometries calculated via the local density approximation (LDA) for [Pd(H(2)O)(4)](2+) and relativistically corrected LDA for the Pt complexes are in good agreement with available experimental data. Nonlocal gradient corrections to the LDA lead to relatively inferior structures. The computed structures for analogous Pd and Pt species are very similar. The equatorial M-OH(2) bonds of all the LDA-optimized tbp structures are predicted to expand by 0.25-0.30 ?, while the axial bonds change little relative to the planar precursors. This bond stretching in the transition state counteracts the decrease in partial molar volume caused by coordination of the entering water molecule and can explain qualitatively the small and closely similar volumes of activation observed. The relatively higher activation enthalpies of the Pt species can be traced to the relativistic correction of the total energies while the absolute DeltaH() values for exchange on [Pd(H(2)O)(4)](2+) and [Pt(H(2)O)(4)](2+) are reproduced using relativistically corrected LDA energies and a simple Born model for hydration. The validity of the latter is confirmed via some simple atomistic molecular mechanics estimates of the relative hydration enthalpies of [Pd(H(2)O)(4)](2+) and [Pd(H(2)O)(5)](2+). The computed DeltaH() values are 57, 92, and 103 kJ/mol compared to experimental values of 50(2), 90(2), and 100(2) kJ/mol for [Pd(H(2)O)(4)](2+), [Pt(H(2)O)(4)](2+), and trans-[PtCl(2)(H(2)O)(2)], respectively. The calculated activation enthalpy for a hypothetical dissociative water exchange at [Pd(H(2)O)(4)](2+) is 199 kJ/mol. A qualitative analysis of the modeling procedure, the relative hydration enthalpies, and the zero-point and finite temperature corrections yields an estimated uncertainty for the theoretical activation enthalpies of about 15 kJ/mol.     

A theoretical study of the NMR spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-) (n = 0-3) and [(NC)(5)Pt-Tl-Pt(CN)(5)](3-): a lesson on environmental effects

The molecular geometries and the nuclear spin-spin coupling constants of the complexes [(NC)(5)Pt-Tl(CN)(n)](n-), n = 0-3, and the related system [(NC)(5)Pt-Tl-Pt(CN)(5)](3-) are studied. These complexes have received considerable interest since the first characterization of the n = 1 system by Glaser and co-workers in 1995 [J. Am. Chem. Soc. 1995, 117, 7550-7551]. For instance, these systems exhibit outstanding NMR properties, such as extremely large Pt-Tl spin-spin coupling constants. For the present work, all nuclear spin-spin coupling constants J(Pt-Tl), J(Pt-C), and J(Tl-C) have been computed by means of a two-component relativistic density functional approach. It is demonstrated by the application of increasingly accurate computational models that both the huge J(Pt-Tl) for the complex (NC)(5)Pt-Tl and the whole experimental trend among the series are entirely due to solvent effects. An approximate inclusion of the bulk solvent effects by means of a continuum model, in addition to the direct coordination, proves to be crucial. Similarly drastic effects are reported for the coupling constants between the heavy atoms and the carbon nuclei. A computational model employing the statistical average of orbital-dependent model potentials (SAOP) in addition to the solvent effects allows to accurately reproduce the experimental coupling constants within reasonable limits.     

Fe(bipy)(CN)(4)](-) as a versatile building block for the design of heterometallic systems: synthesis, crystal structure, and magnetic properties of PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O, [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O, and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O [bipy = 2,2'-Bipyridine; M = Mn and Zn

The new cyano complexes of formulas PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O (1), [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O with M = Mn (2) and Zn (3), and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O (4) [bipy = 2,2'-bipyridine and PPh(4) = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 is made up of mononuclear [Fe(bipy)(CN)(4)](-) anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral centrosymmetric [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)(4)](-) entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H(2)O)(4) units [M = Mn(II) (2) and Zn(II) (3)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)(4)](-) units in trans positions build a distorted octahedron surrounding Mn(II) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)(4)](-) complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe-M distances are 5.126(1) and 5.018(1) A in 2 and 3, respectively. 4 exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)(4)](-) units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1-3, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron-zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) A at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) A at Fe(2). The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10 A) being very weak. More interestingly, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.     

Assignment of the electronic spectra of [Mo(CN)(8)](4)(-) and [W(CN)(8)](4)(-) by Ab initio calculations

CASPT2 calculations are performed on the dodecahedral and square antiprismatic isomers of the [Mo(CN)(8)](4)(-) and [W(CN)(8)](4)(-) complexes. The high-energy experimental bands above 40000 cm(-)(1) are assigned to MLCT transitions. The experimental observed trend of the extinction coefficients for the molybdenum and tungsten complex is reproduced by our CASSCF oscillator strengths. All bands below 40000 cm(-)(1) can be ascribed to ligand-field transitions, although small contributions from forbidden MLCT transitions cannot be excluded. In order to account for all experimental bands in the electronic spectrum of these octacyanocomplexes, a dynamic equilibrium in solution between the two isomeric forms must be hypothesized. Spin-orbit coupling effects are found to be more important for the square antiprismatic isomers; in particular, large singlet-triplet mixings are calculated for this isomer of [W(CN)(8)](4)(-). Ligand-field and Racah parameters as well as spin-orbit coupling constants are determined on the basis of the calculated transition energies. The obtained values for these parameters support the recently proposed model for exchange interactions in magnetic clusters and networks containing pentavalent octocyanometalates of molybdenum and tungsten.     

Solvent coordination in gas-phase [Mn.(H(2)O)(n)](2+) and [Mn.(ROH)(n)](2+) complexes: theory and experiment

An experimental gas-phase study of the intensities and fragmentation patterns of [Mn.(H(2)O)(n)](2+) and [Mn.(ROH)(n)](2+) complexes shows the combinations [Mn.(H(2)O)(4)](2+) and [Mn.(ROH)(4)](2+) to be stable. Evidence in complexes involving the alcohols methanol, ethanol, 1-propanol, and 2-propanol favors preferential fragmentation to [Mn.(ROH)(4)](2+), whereas the fragmentation data for water is less clear. Supporting density functional calculations show that both [Mn.(H(2)O)(4)](2+) and [Mn.(MeOH)(4)](2+) adopt stable tetrahedral configurations, similar to those proposed for biochemical systems where solvent availability and coordination is restricted. Calculated incremental binding energies show a gradual decline on going from one to six solvent molecules, with a step occurring between four and five molecules. The addition of further solvent molecules to the stable [Mn.(MeOH)(4)](2+) unit shows a preference for [Mn.(MeOH)(4)(MeOH)(1,2)](2+) structures, where the extra molecules occupy hydrogen-bonded sites in the form of a secondary solvation shell. Very similar behavior is seen on the part of water. As part of an analysis of the experimental data, the calculations have explored the influence different spins states of Mn(2+) have on solvent geometry. It is concluded that the experimental observations are best reproduced when the central Mn(2+) ion is in the high-spin (6)S ground state. The results are also considered in terms of the biochemical activity of Mn(2+) where the ion is capable of isomorphous substitution with Zn(2+), which itself exhibits a preference for tetrahedral coordination.     

Heterobimetallic coordination polymers incorporating [M(CN)(2)](-) (M = Cu,Ag) and [Ag(2)(CN)(3)](-) units: increasing structural dimensionality via M-M' and M...NC interactions

A series of new heterometallic coordination polymers has been prepared from the reaction of metal-ligand cations and KAg(CN)(2) units. Many of these contain silver-silver (argentophilic) interactions, analogous to gold-gold interactions, which serve to increase supramolecular structural dimensionality. Compared to [Au(CN)(2)](-) analogues, these polymers display new trends specific to [Ag(CN)(2)](-), including the formation of [Ag(2)(CN)(3)](-) and the presence of Ag...N interactions. [Cu(en)(2)][Ag(2)(CN)(3)][Ag(CN)(2)] (1, en = ethylenediamine) forms 1-D chains of alternating [Ag(CN)(2)](-) and [Ag(2)(CN)(3)](-) units via argentophilic interactions of 3.102(1) A. These chains are connected into a 2-D array by strong cyano(N)-Ag interactions of 2.572(3) A. [Cu(dien)Ag(CN)(2)](2)[Ag(2)(CN)(3)][Ag(CN)(2)] (2, dien = diethylenetriamine) forms a 1-D chain of alternating [Cu(dien)](2+) and [Ag(CN)(2)](-) ions with the Cu(II) atoms connected in an apical/equatorial fashion. These chains are cross-linked by [Ag(2)(CN)(3)](-) units via argentophilic interactions of 3.1718(8) A and held weakly in a 3-D array by argentophilic interactions of 3.2889(5) A between the [Ag(CN)(2)](-) in the 2-D array and the remaining free [Ag(CN)(2)](-). [Ni(en)][Ni(CN)(4)].2.5H(2)O (4) was identified as a byproduct in the reaction to prepare the previously reported [Ni(en)(2)Ag(2)(CN)(3)][Ag(CN)(2)] (3). In [Ni(tren)Ag(CN)(2)][Ag(CN)(2)] (5, tren = tris(2-aminoethyl)amine), [Ni(tren)](2+) cations are linked in a cis fashion by [Ag(CN)(2)](-) anions to form a 1-D chain similar to the [Au(CN)(2)](-) analogue. [Cu(en)Cu(CN)(2)Ag(CN)(2)] (6) is a trimetallic polymer consisting of interpenetrating (6,3) nets stabilized by d(10)-d(10) interactions between Cu(I)-Ag(I) (3.1000(4) A). Weak antiferromagnetic coupling has been observed in 2, and a slightly stronger exchange has been observed in 6. The Ni(II) complexes, 4 and 5, display weak antiferromagnetic interactions as indicated by their relatively larger D values compared to that of 3. Magnetic measurements on isostructural [Ni(tren)M(CN)(2)][M(CN)(2)] (M = Ag, Au) show that Ag(I) is a more efficient mediator of magnetic exchange as compared to Au(I). The formation of [Ni(CN)(4)](2)(-), [Ag(2)(CN)(3)](-), and [Cu(CN)(2)](-) are all attributed to secondary reactions of the dissociation products of the labile KAg(CN)(2).     

Solution and solid-state structure of the anion [Ag(2)[closo-CB(11)H(12)](4)](2-)

Addition of the carbene 1,3-dimesitylimidazol-2-ylidene (IMes) to a toluene solution of Ag[closo-CB(11)H(12)] results in the formation of the complex [(IMes)(2)Ag](2)[Ag(2)[closo-CB(11)H(12)](4)], the anionic component of which contains two silver(I) centers bridged by two carboranes in addition to one terminally bound carborane on each metal, in the solid-state. Comparison of the observed (11)B[(1)H] NMR chemical shifts of [(IMes)(2)Ag](2)[Ag(2)[closo-CB(11)H(12)](4)] or Ag[closo-CB(11)H(12)] with [NBu(4)][closo-CB(11)H(12)] in CD(2)Cl(2) demonstrates that the silver ion interacts significantly with the cage in solution. Theoretical investigations using the ab initio/GIAO/NMR method of [closo-CB(11)H(12)](-) and Na[closo-CB(11)H(12)] as model geometries for the silver salts support experimental evidence for these Ag...[BH] interactions in solution.     

The mechanism for water exchange in [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-), as studied by quantum chemical methods.

The mechanisms for the exchange of water between [UO(2)(H(2)O)(5)](2+), [UO(2)(oxalate)(2)(H(2)O)](2)(-)(,) and water solvent along dissociative (D), associative (A) and interchange (I) pathways have been investigated with quantum chemical methods. The choice of exchange mechanism is based on the computed activation energy and the geometry of the identified transition states and intermediates. These quantities were calculated both in the gas phase and with a polarizable continuum model for the solvent. There is a significant and predictable difference between the activation energy of the gas phase and solvent models: the energy barrier for the D-mechanism increases in the solvent as compared to the gas phase, while it decreases for the A- and I-mechanisms. The calculated activation energy, Delta U(++), for the water exchange in [UO(2)(H(2)O)(5)](2+) is 74, 19, and 21 kJ/mol, respectively, for the D-, A-, and I-mechanisms in the solvent, as compared to the experimental value Delta H(++) = 26 +/- 1 kJ/mol. This indicates that the D-mechanism for this system can be ruled out. The energy barrier between the intermediates and the transition states is small, indicating a lifetime for the intermediate approximately 10(-10) s, making it very difficult to distinguish between the A- and I-mechanisms experimentally. There is no direct experimental information on the rate and mechanism of water exchange in [UO(2)(oxalate)(2)(H(2)O)](2-) containing two bidentate oxalate ions. The activation energy and the geometry of transition states and intermediates along the D-, A-, and I-pathways were calculated both in the gas phase and in a water solvent model, using a single-point MP2 calculation with the gas phase geometry. The activation energy, Delta U(++), in the solvent for the D-, A-, and I-mechanisms is 56, 12, and 53 kJ/mol, respectively. This indicates that the water exchange follows an associative reaction mechanism. The geometry of the A- and I-transition states for both [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-) indicates that the entering/leaving water molecules are located outside the plane formed by the spectator ligands.