Terms Of Ligand Field Research Articles

Unravelling the Role of the Pentafluoroorthotellurate Group as a Ligand in Nickel Chemistry.

The pentafluoroorthotellurate group (teflate, OTeF5 ) is able to form species, for which only the fluoride analogues are known. Despite nickel fluorides being widely investigated, nickel teflates have remained elusive for decades. By reaction of [NiCl4 ]2- and neat ClOTeF5 , we have synthesized the homoleptic [Ni(OTeF5 )4 ]2- anion, which presents a distorted tetrahedral structure, unlike the polymeric [NiF4 ]2- . This high-spin complex has allowed the study of the electronic properties of the teflate group, which can be classified as a weak/medium-field ligand, and therefore behaves as the fluoride analogue also in ligand-field terms. The teflate ligands in [NEt4 ]2 [Ni(OTeF5 )4 ] are easily substituted, as shown by the formation of [Ni(NCMe)6 ][OTeF5 ]2 by dissolving it in acetonitrile. Nevertheless, careful reactions with other conventional ligands have enabled the crystallization of nickel teflate complexes with different coordination geometries, i.e. [NEt4 ]2 [trans-Ni(OEt2 )2 (OTeF5 )4 ] or [NEt4 ][Ni(bpyMe2 )(OTeF5 )3 ].

Modulation of Ligand-Field Parameters by Heme Ruffling in Cytochromes c Revealed by EPR Spectroscopy

Electron paramagnetic resonance (EPR) spectra of variants of Hydrogenobacter thermophilus cytochrome c(552) (Ht c-552) and Pseudomonas aeruginosa cytochrome c(551) (Pa c-551) are analyzed to determine the effect of heme ruffling on ligand-field parameters. Mutations introduced at positions 13 and 22 in Ht c-552 were previously demonstrated to influence hydrogen bonding in the proximal heme pocket and to tune reduction potential (E(m)) over a range of 80 mV [Michel, L. V.; Ye, T.; Bowman, S. E. J.; Levin, B. D.; Hahn, M. A.; Russell, B. S.; Elliott, S. J.; Bren, K. L. Biochemistry 2007, 46, 11753-11760]. These mutations are shown here to also increase heme ruffling as E(m) decreases. The primary effect on electronic structure of increasing heme ruffling is found to be a decrease in the axial ligand-field term Δ/λ, which is proposed to arise from an increase in the energy of the d(xy) orbital. Mutations at position 7, previously demonstrated to influence heme ruffling in Pa c-551 and Ht c-552, are utilized to test this correlation between molecular and electronic structure. In conclusion, the structure of the proximal heme pocket of cytochromes c is shown to play a role in determining heme conformation and electronic structure.

Role of Orbital Degeneracy in the Single Molecule Magnet Behavior of a Mononuclear High-Spin Fe(II) Complex

To explain the single-molecule magnet behavior of the mononuclear complex [(tpaMes)Fe](-) we have developed a model that takes into account the trigonal ligand field splitting of the atomic (5)D term of the Fe(II) ion, and the spin-orbital splitting and mixing of the ligand field terms. The ground ligand field term is shown to be the orbital doublet (5)E possessing an unquenched orbital angular momentum. We demonstrate that the splitting of this term cannot be described by the conventional zero-field splitting Hamiltonian proving thus the irrelevance of the spin-Hamiltonian formalism in the present case. The first-order orbital angular momentum is shown to lead to the strong magnetic anisotropy with the trigonal axis being the easy axis of the magnetization.

Fe L-Edge XAS Studies of K4[Fe(CN)6] and K3[Fe(CN)6]: A Direct Probe of Back-Bonding

Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand pi orbitals due to metal-to-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of pi back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.

Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets

Many molecular magnetic clusters, such as single-molecule magnets, are characterized by spin ground states with defined total spin S exhibiting zero-field-splittings. In this work, the spectroscopic intensities of the transitions within the ground-state multiplet are analyzed. In particular, the effects of a mixing with higher-lying spin multiplets, which is produced by anisotropic interactions and is neglected in the standard single-spin description, are investigated systematically for the two experimental techniques of inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR), with emphasis on the former technique. The spectroscopic transition intensities are calculated analytically by constructing corresponding effective spin operators perturbationally up to second order and consequently using irreducible tensor operator techniques. Three main effects of spin mixing are observed. Firstly, a pronounced dependence of the INS intensities on the momentum transfer Q, with a typical oscillatory behavior, emerges in first order, signaling the many-spin nature of the wave functions in exchange-coupled clusters. Secondly, as compared to the results of a first-order calculation, the intensities of the transitions within the spin multiplet are affected differently by spin mixing. This allows one, thirdly, to differentiate the higher-order contributions to the cluster magnetic anisotropy which come from the single-ion ligand-field terms and spin mixing, respectively. The analytical results are illustrated by means of the three examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid molecule, and the single-molecule magnet Mn12.

Structure and Bonding of the Vanadium(III) Hexa-Aqua Cation. 1. Experimental Characterization and Ligand-Field Analysis

Spectroscopic and crystallographic data are presented for salts containing the [V(OH(2))(6)](3+) cation, providing a rigorous test of the ability of the angular overlap model (AOM) to inter-relate the electronic and molecular structure of integer-spin complexes. High-field multifrequency EPR provides a very precise definition of the ground-state spin-Hamiltonian parameters, while single-crystal absorption measurements enable the energies of excited ligand-field states to be identified. The EPR study of vanadium(III) as an impurity in guanidinium gallium sulfate is particularly instructive, with fine-structure observed attributable to crystallographically distinct [V(OH(2))(6)](3+) cations, hyperfine coupling, and ferroelectric domains. The electronic structure of the complex depends strongly on the mode of coordination of the water molecules to the vanadium(III) cation, as revealed by single-crystal neutron and X-ray diffraction measurements, and is also sensitive to the isotopic abundance. It is shown that the AOM gives a very good account of the change in the electronic structure, as a function of geometric coordinates of the [V(OH(2))(6)](3+) cation. However, the ligand-field analysis is inconsistent with the profiles of electronic transitions between ligand-field terms.

A general approach to the electronic spin relaxation of Gd(III) complexes in solutions. Monte Carlo simulations beyond the Redfield limit

The time correlation functions of the electronic spin components of a metal ion without orbital degeneracy in solution are computed. The approach is based on the numerical solution of the time-dependent Schrödinger equation for a stochastic perturbing Hamiltonian which is simulated by a Monte Carlo algorithm using discrete time steps. The perturbing Hamiltonian is quite general, including the superposition of both the static mean crystal field contribution in the molecular frame and the usual transient ligand field term. The Hamiltonian of the static crystal field can involve the terms of all orders, which are invariant under the local group of the average geometry of the complex. In the laboratory frame, the random rotation of the complex is the only source of modulation of this Hamiltonian, whereas an additional Ornstein–Uhlenbeck process is needed to describe the time fluctuations of the Hamiltonian of the transient crystal field. A numerical procedure for computing the electronic paramagnetic resonance (EPR) spectra is proposed and discussed. For the [Gd(H2O)8]3+ octa-aqua ion and the [Gd(DOTA)(H2O)]− complex [DOTA=1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclo dodecane] in water, the predictions of the Redfield relaxation theory are compared with those of the Monte Carlo approach. The Redfield approximation is shown to be accurate for all temperatures and for electronic resonance frequencies at and above X-band, justifying the previous interpretations of EPR spectra. At lower frequencies the transverse and longitudinal relaxation functions derived from the Redfield approximation display significantly faster decays than the corresponding simulated functions. The practical interest of this simulation approach is underlined.

Spectroscopic and dielectric behavior of pure and nickel-doped polyvinyl butyral films

Pure and Ni (2%, 4%, 6%, 8%, and 10%)-doped polyvinyl butyral (PVB) films were prepared by a casting method. Infrared spectra revealed a strong interaction between Ni and two different groups in the polymer chain depending on Ni concentration.Optical absorption measurements identified the structure and assignments of energy bands, which were derived in terms of ligand field theory.The variation of capacitance and dielectric loss (tanδ) with temperature have been interpreted in terms of interacting of the dopant with different groups in the polymer chain as emphasized from the infrared spectra.

High pressure effect on optical properties of cerium ion in fluoroaluminate glass

High pressure effects on optical properties of Ce 3+ doped ternary fluoroaluminate (18BaF 2·37CaF 2·45AlF 3) glass have been investigated in terms of ligand field around cerium ion. Five bands due to the 4f–5d transitions were observed in ultraviolet absorption spectra for the glass. A photoluminescence band was observed at around 300 nm under excitation at each band. The photoluminescence band shifted toward smaller energies by densifying the glass using high pressures and temperatures. An energy shift of the band was also found in in situ photoluminescence measurement under high pressure using a sapphire anvil cell. However, the shift observed under high pressure was large compared with that of the densified glass. We deduced that the large peak shift under high pressure is mainly due to a local elastic contraction around cerium ion.

Crystal field analysis of the 3d N ions at low symmetry sites including the ‘‘imaginary’’ terms

The crystal field (CF) package applicable to orthorhombic or higher symmetry has been extended to enable calculations of energy levels and ligand field states for 3dN ions in crystals at arbitrary symmetry sites by taking into account the contributions of the ‘‘imaginary’’ CF terms. The full Hamiltonian matrix, including the electrostatic terms, Trees correction, the spin–orbit interaction, and the ligand field terms, is diagonalized within the whole 3dN configuration. Several techniques based on the transformation properties of the crystal field Hamiltonian have been devised to test the package for internal consistency. The package is especially suitable for analysis of the low symmetry effects for transition metal ions at sites with the tetragonal and trigonal symmetry involving the imaginary CF terms as well as monoclinic and triclinic symmetry. It can be useful in analyzing the electron paramagnetic resonance spectra of 3dN ions and in correlating the ligand field parameters with the spin Hamiltonian parameters.

Influence of Axial Coordination on the g-Factor Anisotropy of the Tetraimine Macrocyclic Cobalt(II) Complex [Co(C10H20N8)Cl(H2O)]Cl.cntdot.H2O

The single-crystal EPR and powder laser EPR spectra of the tetraimine Co(II) complex [Co(C10H20N8)-Cl(H2O)]Cl.H2O have been measured. It is shown that the axial coordination partially quenches the anisotropy of the phase-coupling ligand field term.

Darstellung und Redoxverhalten einer Serie von Cp*/aqua/tripod-KompIexen des Co, Rh und Ru

The following series of complexes [MLL']n+ was prepared: M = Co, Rh, Ru, a) L, L' = η5-C5Me5, b) L, L ' = η5 - C5Me5, H2O, c) L, L' = η5 - C5Me5, tripod (CpCo(P(O)(OEt)2)3 -), d) L, L' = tripod. Redox transitions o f the complexes were investigated by cyclic voltammetry. The results are discussed in terms of ligand field and ligand charge stabilization of the respective electron configurations.

Partial and ligand field terms

In this article, the derivation of partial terms and their application to yield ligand field terms are described.

On the cation distribution of spinels

We present a detailed theoretical study of the relative energetics of normal and inverse spinel structures using both perfect lattice and point defect simulation techniques. We consider both electrostatic and short-range energies as well as ligand field terms. A crucial feature of our approach is the inclusion of the dependence of short-range potentials on coordination number. Inclusion of this latter effect allows us to make correct predictions of the cation distribution in a wide range of 2,3 spinel structured oxides. Keywords: Computer simulations, spinels, cation distribution.

The noncorrelation of Dq and E1/2 in metal complexes. The importance of the spherical ligand field term

There are several studies in which the redox potentials vary significantly (1-2 V) in a series of closely related complexes containing the same metal ion. Within these series Dq may or may not change by an appreciable amount, thereby casting considerable doubt on the concept that changes in Dq are responsible for changes in E/sub 1/2/. Other bonding parameters such as the repulsive spherical ligand field term, V/sub s/, are potentially much more important and are expected to be sensitive to subtle changes in the ligands. 2 tables.

Electronic spectra of some tetragonal complexes of rhodium(III), iridium(III) and platinum(IV) [1

Solution electronic spectral data are reported for 30 tetragonal complexes of the types trans-M(en) 2X n+ 2, M(NH 3) 5X n+ (M = Rh(III), Ir(III), or Pt(IV); X = Cl, Br, or I); trans-M(NH 3) 4X n+ 2 (M = Rh(III) or Pt(IV); X = Cl, Br, or I); trans-PtL 4X 2− 2 (L = CN − or NO − 2, X = Cl or Br); and Pt(CN) 5X 2− (X = Cl, Br, or I). These data are interpreted in terms of ligand field (LF) excited states in C 4v or D 4h symmetry and ligand to metal charge transfer (LMCT) excited states involving the halide ligands. Intensity patterns among the LMCT transitions of intermediate intensity are rationalized by including halide spin-orbit coupling in excited configurations. Trends in LF and LMCT excited state energies are discussed.

Zero-field splitting and the low-temperature magnetic susceptibility of tris (pyrrolidine-I-carbodithioato) iron (III)

It is demonstrated that the existing discrepancy in the interpretation of low-temperature magnetic-susceptibility data on high-spin tris (pyrrolidine-I-carbodithioato) iron (III) disappears when high-order magnetic-field effects and fourth-order ligand-field terms are considered.

Approximate Molecular Orbital Calculations for the Transition-Metal Carbonyls, Ni(CO)4, Co(CO)4−, Fe(CO)4=, Fe(CO)5, and Cr(CO)6

The nature of the bonding in five transition-metal carbonyls is investigated by means of self-consistent charge molecular orbital calculations. The inclusion of ligand field terms is shown to have a critical influence on the results obtained. The results are compared with available ionization data for the transition-metal carbonyls.

Low-temperature optical absorption of nickel fluosilicate crystals

Abstract The vibronic absorption spectrum of the Ni++ ion in an electric field of trigonal (nearly cubic) symmetry at temperatures down to 4°K is reported and analysed in terms of ligand field and vibronic theory. The positions, widths, shapes and strengths of the bands are satisfactorily explained, together with their associated fine structures. In particular, the complex structure of the red band is well explained in detail as a consequence of the mutual perturbation of the 1E and 3T4 levels, which strongly modifies the vibronic coupling and results in two fairly narrow bands, each showing resolved vibrational structure, superposed on a broad band. A Trees correction term, αL(L+1), helps the agreement between observed and calculated electronic levels. The following parameters are found to give good agreement: Δ = 9100 cm−1, B=955 cm−1, C=3750 cm−1, α=80 cm−1, ζ = 600 cm−1.