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Direct Coordination (direct + coordination)
Selected AbstractsRegulating the mobility of Cd, Cu and Pb in an acid soil with amendments of phosphogypsum, sugar foam, and phosphoric rockEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2006F. Garrido Summary When acid soil has been contaminated by metals as a result of industrial discharges, accidental spills, or acid mine drainage it may be desirable to retain the metals in the soil rather than allow them to leach away. We have investigated the potential of phosphogypsum (PG), sugar foam (SF), and phosphoric rock (PR) to regulate the availability and mobility of Pb, Cd and Cu. We have also identified changes in attenuation during incubation for 1 year and the effect of aging on metal speciation in amended soils. We studied miscible displacement in columns of undisturbed soil previously treated with solutions of the amendments and soluble metals and, subsequently, single and sequential chemical metal extractions. All amendments increased the soil's metal retention capacity. This, in turn, increased the amount of metal extractable by diethylenetriaminepentaacetic acid (DTPA). However, over time the amounts of DTPA-extractable metal decreased, particularly for Cu and Pb. Both Cu and Cd were held preferentially within the acetic acid-extractable fraction (operationally defined exchangeable fraction , EX fraction), whereas Pb was associated mainly with the hydroxylammonium-extractable fraction (operationally defined bound to Fe and Al hydroxides , OX fraction). Both Pb and Cu in the oxide and organic fractions increased in the PG- and SF-treated soils. In general, the distribution of metal did not change in the PR-treated columns after the incubation. Finally, scanning electron microscopy in back-scattered electron mode (SEM,BSE) showed the formation of Al-hydroxy polymers which provides the soils with additional cation sorption capacity. In the PG- and PR-treated columns, P and S were associated with these formations. The three metals were associated with the Al polymers, probably through direct coordination or the formation of ternary complexes with the inorganic ligands phosphate and sulphate. [source] Structures and stability of lithium monosilicide clusters SiLin (n = 4,16): What is the maximum number, magic number, and core number for lithium coordination to silicon?JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2008Ning He Abstract In the coordination, hypervalent and cluster chemistry, three important characteristic properties are the maximum coordination number, magic number, and core coordination number. Yet, few studies have considered these three numbers at the same time for an MLn cluster with n larger than 8. In this article, we systematically studied the three properties of SiLin (n = 4,16) clusters at the B3LYP/6-31G(d), B3LYP/6-311++G(2d), and CCSD(T)/6-311++G(3df)//B3LYP/6-311++G(2d) (for energy only) levels. Various isomeric forms with different symmetries were calculated. For each SiLin (n = 4,9), silicon cohesive energy (cE) from SiLin , Si + Lin reaction, vertical ionization potential (vIP), and vertical electron affinity (vEA) were obtained for the lowest-energy isomer. We found that the maximum Li-coordination number of Si is 9, which is the largest number among the known MLin clusters. All cE, vIP, and vEA values predicted that 6 is the magic Li-coordination number of Si. For small SiLin (n , 6) clusters, Li atoms favor direct coordination to Si, whereas for larger SiLin (n , 7) clusters, there is a core cluster that is surrounded by excessive Li atoms. The core Li-coordination number is 6 for SiLin (n = 7,8), 7 for SiLin (n = 9,10), 8 for SiLin (n = 11,15) and 9 for SiLin (n , 16). Through the calculations, we verified the relationship between the structure and stability of SiLin with the maximum coordination number, magic number, and core coordination number. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Theoretical studies on farnesyl transferase: Evidence for thioether product coordination to the active-site zinc sphereJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2007Sérgio Filipe Sousa Abstract Farnesyltransferase (FTase), an interesting zinc metaloenzyme, has been the subject of great attention in anticancer research over the last decade. However, despite the major accomplishments in the field, some very pungent questions on the farnesylation mechanism still persist. In this study, the authors have analyzed a mechanistic paradox that arises from the existence of several contradicting and inconclusive experimental evidence regarding the existence of direct coordination between the active-site zinc cation and the thioether from the farnesylated peptide product, which include UV,vis spectroscopy data on a Co2+ -substituted FTase, two X-ray crystallographic structures of the FTase-product complex, and extended X-ray absorption fine structure results. Using high-level theoretical calculations on two models of different sizes, and QM/MM calculations on the full enzyme, the authors have shown that the farnesylated product is Zn coordinated, and that a subsequent step where this Zn bond is broken is coherent with the available kinetic results. Furthermore, an explanation for the contradicting experimental evidence is suggested. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] Coordination behaviour and two-dimensional-network formation in poly[[,-aqua-diaqua(,5 -propane-1,3-diyldinitrilotetraacetato)dilithium(I)cobalt(II)] dihydrate]: the first example of an MII,1,3-pdta complex with a monovalent metal counter-ionACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2008Urszula Rychlewska The title compound, {[CoLi2(C11H14N2O8)(H2O)3]·2H2O}n, constitutes the first example of a salt of the [MII(1,3-pdta)]2, complex (1,3-pdta is propane-1,3-diyldinitrilotetraacetate) with a monopositive cation as counter-ion. Insertion of the Li+ cation could only be achieved through application of the ion-exchange column technique which, however, appeared unsuccessful with other alkali metals and the ammonium cation. The structure contains two tetrahedrally coordinated Li+ cations, an octahedral [Co(1,3-pdta)]2, anion and five water molecules, two of which are uncoordinated, and is built of two-dimensional layers extending parallel to the (010) lattice plane, the constituents of which are connected by the coordinate bonds. O,Hwater...O hydrogen bonds operate both within and between these layers. The crystal investigated belongs to the enantiomeric space group P21 with only one (,) of two possible optical isomers of the [Co(1,3-pdta)]2, complex. A possible cause of enantiomer separation during crystallization might be the rigidification and polarization of the [M(1,3-pdta)]2, core, resulting from direct coordination of Li+ cations to three out of four carboxylate groups constituting the 1,3-pdta ligand. The structure of (I) differs considerably from those of the other [MII(1,3-pdta)]2, complexes, in which the charge compensation is realized by means of divalent hexaaqua complex cations. This finding demonstrates a significant structure-determining role of the counter-ions. [source] Formation of Superoxo Species by Interaction of O2 with Na Atoms Deposited on MgO Powders: A Combined Continuous-Wave EPR (CW-EPR), Hyperfine Sublevel Correlation (HYSCORE) and DFT StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2010Francesco Napoli Dr. Abstract The formation of O2, radical anions by contact of O2 molecules with a Na pre-covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O2. The typical EPR signal of isolated Na atoms disappears when the reaction with O2 takes place and new paramagnetic species are observed, which are attributed to different surface-stabilised O2, radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O2,Na+ species to be determined, demonstrating the direct coordination of the O2, adsorbate to surface Na+ cations. DFT calculations enable the structural details of the formed species to be determined. Matrix-isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface-stabilised Na+O2, complexes. [source] (Pentamethylcyclopentadienyl)ruthenium ,-Complexes of Metalloporphyrins: Platforms with Novel Photo- and Electrochemical PropertiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2008Luciano Cuesta Dr. Unusually accepting: A series of metalloporphyrin,ruthenocene complexes, synthesized by direct coordination of a [Ru(Cp*)]+ fragment to the ,-electron "face" of several metallooctaethylporphyrins (see graphic), undergo efficient photoinduced electron transfer from the ruthenocene unit (acting as the electron donor) to the singlet excited state of the metalloporphyrin unit (acting as the electron acceptor) to give a charge-separated state. [source] Reaction Mechanism of Porphyrin Metallation Studied by Theoretical MethodsCHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2005Yong Shen Dr. Abstract We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ into a porphyrin ring with density functional calculations with large basis set and including solvation, zero-point and thermal effects. We have followed the reaction from the outer-sphere complex, in which the metal is coordinated with six water molecules and the porphyrin is doubly protonated, until the metal ion is inserted into the deprotonated porphyrin ring with only one water ligand remaining. This reaction involves the stepwise displacement of five water molecules and the removal of two protons from the porphyrin ring. In addition, a step seems to be necessary in which a porphyrin pyrrolenine nitrogen atom changes its interaction from a hydrogen bond to a metal-bound solvent molecule to a direct coordination to the metal ion. If the protons are taken up by a neutral imidazole molecule, the deprotonation reactions are exothermic with minimal barriers. However, with a water molecule as an acceptor, they are endothermic. The ligand exchange reactions were approximately thermoneutral (±20 kJ,mol,1, with one exception) with barriers of up to 72 kJ,mol,1 for Mg and 51 kJ,mol,1 for Fe. For Mg, the highest barrier was found for the formation of the first bond to the porphyrin ring. For Fe, a higher barrier was found for the formation of the second bond to the porphyrin ring, but this barrier is probably lower in solution. No evidence was found for an initial pre-equilibrium between a planar and a distorted porphyrin ring. Instead, the porphyrin becomes more and more distorted as the number of metal,porphyrin bonds increase (by up to 191 kJ,mol,1). This strain is released when the porphyrin becomes deprotonated and the metal moves into the ring plane. Implications of these findings for the chelatase enzymes are discussed. [source] | ||||||||||