Showing papers on "Spectrochemical series published in 1995"

Co-ordination of nitriles to fe(cn)5(oh2)3- and use of the acetonitrile complex for the determination of equilibrium constants

Abstract: Solid Na3[Fe(CN)5(NH3)], which hydrolyses rapidly in solution to the unstable complex [Fe(CN)5(OH2)]3–1, when dissolved in aqueous acetonitrile gave a solution of the much more stable complex [Fe(CN)5(MeCN)]3– with log K 2.6 (25 °C, I= 0.1 mol dm–3). It was shown that the use of solutions of the acetonitrile complex instead of 1 provides a general method for determining equilibrium constants for ligand substitution (directly of MeCN, hence indirectly of H2O in 1) under equilibrium conditions and over a wider range of pH from 3 to 13. Values of log K have also been determined for other nitriles such as PhCN (3.5), NCCH2NH3+(3.3), NCCH2CO2–(2.5) and NCNH2(2.3), also for NH3(4.2), NH2CH2CN (3.7) and pyridine (3.7). The data suggest that, in the absence of complicating factors, the values of log K for nitriles show little dependence on their basicity. Comparison of present and published data on the wavelength of the first d–d transition in these and analogous complexes shows that the position of a nitrogen-containing base in the spectrochemical series depends primarily on the state of hybridisation of the donor N atom and increase in the order H2O (443) < sp3 N (all 400 ± 4) < sp2 N (all 383 ± 3) < sp N (all 375 ± 2) < CN–(325 nm).

Template Synthesis of New Nickel(II) Comlexes of 14-Membered Pentaaza Macrocyclic Ligands: Effects of C-Alkyl and N-Hydroxyalkyl Pendant Arms on the Solution Behaviors of the Complexes

Abstract: New square planar nickel(Ⅱ) complexes with various 1-alkyl (4a-4c) and 1-hydroxyalkyl (4d-4f) derivatives of the 14-membered pentaaza macrocycle 8-ethyl-8-nitro-1,3,6,10,13-pentaazacyclotetradecane have been synthesized by two-step metal template condensation reactions of ethylenediamine, nitroethane, formaldehyde, and appropriate primary amines. The nitro group and/or hydroxyl group of 4a-4f are not directly involved in the coordination. The nickel(Ⅱ) complexes exist in coordinating solvents such as MeCN, Me2SO, and H2O as equilibrium mixtures of the square planar [Ni(L)]2+(L=4a-4f) and octahedral species [Ni(L)S2]2+(S=solvent molecule). Although the ligand field strength and redox potentials of the complexes are not affected by the nature of the substituents, the formation of octahedral species for 4d-4f in MeCN is strongly restricted by the hydroxyl group. Synthesis, characterization, and solution behaviors of the nickel(Ⅱ) complexes are described.

Trivalent metallocene chemistry of some uranium, titanium, and zirconium complexes

Abstract: Dicyclopentadienyluranium halide dimers have been prepared and their solution behavior examined. These molecules exist as dimers in solution, and the halide ligands undergo rapid site exchange on the NMR timescale above 50 C. Analogous dicyclopentadienyluranium hydroxide dimers have also been prepared; they oxidatively eliminate hydrogen to give the corresponding oxide dimers. Mechanism of this reaction is consistent with {alpha}migration of one of the hydroxide hydrogen atoms to a uranium center followed by elimination of hydrogen. Ground state of [(Me{sub 3}Si){sub 2}C{sub 5}H{sub 3}]{sub 3}M M = Nd, U and their base adducts has been examined by variable temperature magnetic susceptibility and EPR spectroscopy. The ground state is found to be {sup 4}I{sub 9/2} with a crystal field state consisting largely of J{sub z} = 1/2 lowest, in agreement with previous studies on tris-cyclopentadienylneodymium complexes. The zirconium metallocene Cp{sub 3}Zr has been prepared, characterized crystallographically, and its reactivity studied. Its chemical behavior is controlled by presence of an electron in the non-bonding, d{sub z}2 orbital which prevents formation of base adducts Of Cp{sub 3}Zr, but allows Cp{sub 3}Zr to abstract atoms from other molecules. Electonic and EPR spectra of Cp*{sub 2}TiX complexes, where Cp* is Me{sub 5}C{sub 5} and X is a monodentate, anionic ligand such as halide, have been studied. A {pi}-bonding spectrochemical series is developed, and trends in {pi}-bonding ability are found similar to those in other inorganic complexes. The {beta}-agostic interactions in Cp*{sub 2}TiN(Me)Ph have been examined using variable temperature EPR spectroscopy, and the enthalpy/entropy of the interaction determined. In Cp*{sub 2}TiEt, enthalpy of the {beta}-agostic interaction is {minus}1.9 kcal/mol. The titanocene anion, Cp*{sub 2}TiLi(TMEDA) (TMEDA is N,N,N`,N`-tetramethylethylenediamine), has been prepared and its structure determined.