Structure of solvated mercury(II) halides in liquid ammonia, triethyl phosphite and tri-n-butylphosphine solution

Liquid ammonia, trialkyl phosphites, and especially trialkylphosphines, are very powerful electron-pair donor solvents with soft bonding character. The solvent molecules act as strongly coordinating ligands towards mercury(ii), interacting strongly enough to displace halide ligands. In liquid ammonia mercury(ii) chloride solutions separate into two liquid phases; the upper contains tetraamminemercury(ii) complexes, [Hg(NH(3))(4)](2+), and chloride ions in low concentration, while the lower is a dense highly concentrated solution of [Hg(NH(3))(4)](2+) entities, ca. 1.4 mol dm(-3), probably ion-paired by hydrogen bonds to the chloride ions. Mercury(ii) bromide also dissociates to ionic complexes in liquid ammonia and forms a homogeneous solution for which (199)Hg NMR indicates weak bromide association with mercury(ii). When dissolving mercury(ii) iodide in liquid ammonia and triethyl phosphite solvated molecular complexes form in the solutions. The Raman nu(I-Hg-I) symmetric stretching frequency is 132 cm(-1) for the pseudo-tetrahedral [HgI(2)(NH(3))(2)] complex formed in liquid ammonia, corresponding to D(S) = 56 on the donor strength scale. For the Hg(ClO(4))(2)/NH(4)I system in liquid ammonia a (199)Hg NMR study showed [HgI(4)](2-) to be the dominating mercury(ii) complex for mole ratios n(I(-)) : n(Hg(2+)) > or = 6. A large angle X-ray scattering (LAXS) study of mercury(ii) iodide in triethyl phosphite solution showed a [HgI(2)(P(OC(4)H(9))(3))(2)] complex with the Hg-I and Hg-P bond distances 2.750(3) and 2.457(4) A, respectively, in near tetrahedral configuration. Trialkylphosphines generally form very strong bonds to mercury(ii), dissociating all mercury(ii) halides. Mercury(ii) chloride and bromide form solid solvated mercury(ii) halide salts when treated with tri-n-butylphosphine, because of the low permittivity of the solvent. A LAXS study of a melt of mercury(ii) iodide in tri-n-butylphosphine at 330 K resulted in the Hg-I and Hg-P distances 2.851(3) and 2.468(4) A, respectively. The absence of a distinct I-I distance indicates flexible coordination geometry with weak and non-directional mercury(ii) iodide association within the tri-n-butylphosphine solvated complex.     

Coordination chemistry of mercury(II) in liquid and aqueous ammonia solution and the crystal structure of tetraamminemercury(II) perchlorate

The ammonia solvated mercury(II) ion has been structurally characterized in solution by means of EXAFS, (199)Hg NMR, and Raman spectroscopy and in solid solvates by combining results from X-ray single crystal and powder diffraction, thermogravimetry, differential scanning calorimetry, EXAFS, and Raman spectroscopy. Crystalline tetraamminemercury(II) perchlorate, [Hg(NH3)4](ClO4)2, precipitates from both liquid ammonia and aqueous ammonia solution, containing tetraamminemercury(II) complexes. The orthorhombic space group ( Pnma) imposes C s symmetry on the tetraamminemercury(II) complexes, which is lost at a phase transition at about 220 K. The Hg-N bond distances are 2.175(14), 2.255(16), and 2 x 2.277(9) A, with a wide N-Hg-N angle between the two shortest Hg-N bonds, 122.1(7) degrees , at ambient temperature. A similar distorted tetrahedral coordination geometry is maintained in liquid ammonia and aqueous ammonia solutions with the mean Hg-N bond distances 2.225(12) and 2.226(6) A, respectively. When heated to 400 K the solid tetraamminemercury(II) perchlorate decomposes to diamminemercury(II) perchlorate, [Hg(NH3)2](ClO4)2, with the mean Hg-N bond distance 2.055(6) A in a linear N-Hg-N unit. The mercury atoms in the latter compound form a tetrahedral network, connected by perchlorate oxygen atoms, with the closest Hg...Hg distance being 3.420(3) A. The preferential solvation and coordination changes of the mercury(II) ion in aqueous ammonia, by varying the total NH 3:Hg(II) mole ratio from 0 to 130, were followed by (199)Hg NMR. Solid [Hg(NH 3)4](ClO4)2 precipitates while [Hg(H2O)6](2+) ions remain in solution at mole ratios below 3-4, while at high mole ratios, [Hg(NH3)4](2+) complexes dominate in solution. The principal bands in the vibrational spectrum of the [Hg(NH3)4](2+) complex have been assigned.     

Coordination chemistry study of hydrated and solvated lead(II) ions in solution and solid state

The coordination chemistry of lead(II) in the oxygen donor solvents water, dimethylsulfoxide (dmso, Me(2)SO), N,N-dimethylformamide (dmf), N,N-dimethylacetamide (dma), N,N'-dimethylpropyleneurea (dmpu), and 1,1,3,3-tetramethylurea (tmu), as well as in the sulfur donor solvent N,N-dimethylthioformamide (dmtf), has been investigated by extended X-ray absorption fine structure (EXAFS) and/or large angle X-ray scattering (LAXS) in solution, and by single crystal X-ray diffraction and/or EXAFS of solid hydrates and solvates. Lead(II) may either form hemidirected complexes with large bond distance distribution and an apparent gap for excess electron density, or holodirected ones with a symmetric coordination sphere with normal bond distance distribution, depending on the strength of antibonding lead 6s/ligand np molecular orbital interactions and ligand-ligand interactions. The crystallographic data show that the solid lead(II) perchlorate and trifluoromethanesulfonate hydrate structures are hemidirected, while the solid lead(II) solvates of dma and dmpu have regular octahedral configuration with holodirected geometry and mean Pb-O bond distances in the range 2.50-2.52 ?. EXAFS data on the hydrated lead(II) ion in aqueous solution show broad bond distance distribution and a lack of inner-core multiple scattering contributions strongly indicating a hemidirected structure. The Pb-O bond distances found both by EXAFS and LAXS, 2.54(1) ?, point to a six-coordinate hydrated lead(II) ion in hemidirected fashion with an unevenly distributed electron density. The results obtained for the dmso solvated lead(II) ion in solution are ambiguous, but for the most part support a six-coordinate hemidirected complex. The mean Pb-O bond distances determined in dmf and dma solution by LAXS, 2.55(1) and 2.48(1) ?, respectively, indicate that in both solvate complexes lead(II) binds six solvent molecules with the former complex being hemidirected whereas the latter is holodirected. The dmpu and tmu solvated lead(II) ions have a regular holodirected octahedral configuration, as expected given their space-demanding characteristics and ligand-ligand intermolecular interactions. The dmtf solvated lead(II) ion in solution is most likely five-coordinate in a hemidirected configuration, with a mean Pb-S bond distance of 2.908(4) ?. New and improved ionic radii for the lead(II) ion in 4-8-coordination in hemi and holodirected configurations are proposed using crystallographic data.     

Binuclear Pt-Tl bonded complex with square pyramidal coordination around Pt: a combined multinuclear NMR, EXAFS, UV-Vis, and DFT/TDDFT study in dimethylsulfoxide solution

The structure and bonding of a new Pt-Tl bonded complex formed in dimethylsulfoxide (dmso), (CN)(4)Pt-Tl(dmso)(5)(+), have been studied by multinuclear NMR and UV-vis spectroscopies, and EXAFS measurements in combination with density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations. This complex is formed following the equilibrium reaction Pt(CN)(4)(2-) + Tl(dmso)(6)(3+) ? (CN)(4)Pt-Tl(dmso)(5)(+) + dmso. The stability constant of the Pt-Tl bonded species, as determined using (13)C NMR spectroscopy, amounts to log K = 2.9 ± 0.2. The (NC)(4)Pt-Tl(dmso)(5)(+) species constitutes the first example of a Pt-Tl bonded cyanide complex in which the sixth coordination position around Pt (in trans with respect to the Tl atom) is not occupied. The spectral parameters confirm the formation of the metal-metal bond, but differ substantially from those measured earlier in aqueous solution for complexes (CN)(5)Pt-Tl(CN)(n)(H(2)O)(x)(n-) (n = 0-3). The (205) Tl NMR chemical shift, δ = 75 ppm, is at extraordinary high field, while spin-spin coupling constant, (1)J(Pt-Tl) = 93 kHz, is the largest measured to date for a Pt-Tl bond in the absence of supporting bridging ligands. The absorption spectrum is dominated by two strong absorption bands in the UV region that are assigned to MMCT (Pt → Tl) and LMCT (dmso → Tl) bands, respectively, on the basis of MO and TDDFT calculations. The solution of the complex has a bright yellow color as a result of a shoulder present on the low energy side of the band at 355 nm. The geometry of the (CN)(4)Pt-Tl core can be elucidated from NMR data, but the particular stoichiometry and structure involving the dmso ligands are established by using Tl and Pt L(III)-edge EXAFS measurements. The Pt-Tl bond distance is 2.67(1) ?, the Tl-O bond distance is 2.282(6) ?, and the Pt-C-N entity is linear with Pt-C and Pt···N distances amounting to 1.969(6) and 3.096(6) ?, respectively. Geometry optimizations on the (CN)(4)Pt-Tl(dmso)(5)(+) system by using DFT calculations (B3LYP model) provide bond distances in excellent agreement with the EXAFS data. The four cyanide ligands are located in a square around the Pt atom, while the Tl atom is coordinated in a distorted octahedral fashion with the metal being located 0.40 ? above the equatorial plane described by four oxygen atoms of dmso ligands. The four equatorial Tl-O bonds and the four cyano ligands around the Pt atom are arranged in an alternate geometry. The coordination environment around Pt may be considered as being square pyramidal, where the apical position is occupied by the Tl atom. The optimized geometry of (CN)(4)Pt-Tl(dmso)(5)(+) is asymmetrical (C(1) point group). This low symmetry might be responsible for the unusually large NMR linewidths observed due to intramolecular chemical exchange processes. The nature of the Pt-Tl bond has been studied by MO analysis. The metal-metal bond formation in (CN)(4)Pt-Tl(dmso)(5)(+) can be simply interpreted as the result of a Pt(5d(z(2)))(2) → Tl(6s)(0) donation. This bonding scheme may rationalize the smaller thermodynamic stability of this adduct compared to the related complexes with (CN)(5)Pt-Tl entity, where the linear C-Pt-Tl unit constitutes a very stable bonding system.     

From the surface to volume: concepts for the next generation of optical-holographic data-storage materials

Optical data storage has had a major impact on daily life since its introduction to the market in 1982. Compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs (BDs) are universal data-storage formats with the advantage that the reading and writing of the digital data does not require contact and is therefore wear-free. These formats allow convenient and fast data access, high transfer rates, and electricity-free data storage with low overall archiving costs. The driving force for development in this area is the constant need for increased data-storage capacity and transfer rate. The use of holographic principles for optical data storage is an elegant way to increase the storage capacity and the transfer rate, because by this technique the data can be stored in the volume of the storage material and, moreover, it can be optically processed in parallel. This Review describes the fundamental requirements for holographic data-storage materials and compares the general concepts for the materials used. An overview of the performance of current read-write devices shows how far holographic data storage has already been developed.     

Single-molecule chemistry and physics explored by low-temperature scanning probe microscopy

It is well known that scanning probe techniques such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) routinely offer atomic scale information on the geometric and the electronic structure of solids. Recent developments in STM and especially in non-contact AFM have allowed imaging and spectroscopy of individual molecules on surfaces with unprecedented spatial resolution, which makes it possible to study chemistry and physics at the single molecule level. In this feature article, we first review the physical concepts underlying image contrast in STM and AFM. We then focus on the key experimental considerations and use selected examples to demonstrate the capabilities of modern day low-temperature scanning probe microscopy in providing chemical insight at the single molecule level.     

Formation of a heteronuclear hydrolysis complex in the Th(IV)-Fe(III) system

The speciation in the mixed Th(IV)-Fe(III) system has been studied in aqueous solution in the pH range of 2.0-4.8. In the individual systems iron(III) and thorium(IV) hydrolyze easily and hydrolysis products precipitate at approximately pH ≥ 2.0 and 4.0, respectively, at the metal concentrations used in this study, 0.02-0.05 mol dm(-3). In the mixed Th(IV)-Fe(III) system precipitation of ferrihydrite takes place after months of storage at low pH values, 2.0 (six-line ferrihydrite) and 2.3 (two-line ferrihydrite), as identified by X-ray powder diffraction. In the pH range 2.9-4.5 no precipitation was observed after 24 months. Two thorium(IV)-iron(III) solutions with pH = 2.9, C(Th) = 0.02 and 0.05 mol dm(-3) and C(Fe) = 0.02 mol dm(-3), were studied by extended X-ray absorption fine structure, EXAFS, using the Fe K and Th L(3) edges, and a third solution with pH = 2.9 and C(Th) = C(Fe) = 0.40 mol dm(-3) by large angle X-ray scattering, LAXS, to determine the structure of the predominating species. A heteronuclear hydrolysis complex with the composition [Th(2)Fe(2)(μ(2)-OH)(8)(H(2)O)(12)](6+) is proposed to form in solution, with Th···Th, Th···Fe and Fe···Fe distances of 3.94(2) and 3.96(2), 3.41(3) and 3.43(2), 3.04(2) and 3.02(4) ?, as determined by EXAFS and LAXS, respectively.     

On the structure and dynamics of the hydrated sulfite ion in aqueous solution--an ab initio QMCF MD simulation and large angle X-ray scattering study

Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism has been applied in conjunction to experimental large angle X-ray scattering to study the structure and dynamics of the hydrated sulfite ion in aqueous solution. The results show that there is a considerable effect of the lone electron-pair on sulfur concerning structure and dynamics in comparison with the sulfate ion with higher oxidation number and symmetry of the hydration shell. The S-O bond distance in the hydrated sulfite ion has been determined to 1.53(1) ? by both methods. The hydrogen bonds between the three water molecules bound to each sulfite oxygen are only slightly stronger than those in bulk water. The sulfite ion can therefore be regarded as a weak structure maker. The water exchange rate is somewhat slower for the sulfite ion than for the sulfate ion, τ(0.5) = 3.2 and 2.6 ps, respectively. An even more striking observation in the angular radial distribution (ARD) functions is that the for sulfite ion the water exchange takes place in close vicinity of the lone electron-pair directed at its sides, while in principle no water exchange did take place of the water molecules hydrogen bound to sulfite oxygens during the simulation time. This is also confirmed when detailed pathway analysis is conducted. The simulation showed that the water molecules hydrogen bound to the sulfite oxygens can move inside the hydration shell to the area outside the lone electron-pair and there be exchanged. On the other hand, for the hydrated sulfate ion in aqueous solution one can clearly see from the ARD that the distribution of exchange events is symmetrical around the entire hydration sphere.