Michail Atanasov

Fluxionality and stereochemistry of 5-coordinate Cu2+ complexes. The potential energy surface and spectroscopic implications

Abstract Cu2+ ions in five-coordination (Cl−1, NH3, NCS−, etc.) generally stabilize an elongated square pyramid, which is slightly preferred to a compressed trigonal bipyramid. This stereochemical behaviour can be understood by considering the vibronic interaction between the A′1 ground state and the first excited E′ state via the e′ vibrations in D3h symmetry (pseudo-Jahn-Teller coupling), in combination with an E′ ⊗ e′ interaction (Jahn-Teller coupling in the excited state), leading into the lower C2v (C4v) and Cs symmetries. The adiabatic ground state potential surface is calculated in a semi-empirical model on the basis of available spectroscopic data. The minima at the points, which characterize the elongated C2v (C4v) geometry, are rather flat and can be shifted to any other point of the potential surface by steric ligand and/or geometric packing influences. The CuCl53− square pyramids in [Co (NH3)6] CuCl5 undergo a pseudorotation to (dynamically averaged) trigonal bipyramids at 280 K, with the Cl− ligands along the threefold axis remaining fixed in space. In contrast the nuclear displacements along the e′ coordinates occur in an unrestricted manner above 285 K in case of the Cu(NH3)52+ square pyramids in [Cu(NH3)5]Br2 (“Berry rotation”). The wavefunctions, g-tensor components and d-d transition energies have been derived as functions of the e′ distortion coordinates and were explicitly calculated for the CuCl53− model case.

SECOND-SPHERE LIGAND FIELD EFFECTS ON OXYGEN LIGATOR ATOMS AND EXPERIMENTAL EVIDENCE : THE TRANSITION METAL-OXYGEN BOND IN OXIDIC SOLIDS

Abstract The properties of the M–O bond in oxidic solids, where M is a low-valent 3d transition metal ion such as Ni II , Cu II , Co II or Cr III , are very variable depending on the structure and constitution of the respective compound, and are studied by optical and EPR spectroscopy. Specifically, high-valent cations in the cationic coordination of the oxygen ligator atoms beside M determine the bonding behaviour of oxygen towards the transition metal ions. Applying a newly developed extension of the angular overlap model (AOM) to fit the d–d spectra and the EPR g and hyperfine tensor components (Cu 2+ ), the bonding parameters e σ and e π for a great variety of structures and solids are evaluated. In these calculations a 2s–2p hybridisation on oxygen is anticipated, where the number and the geometrical arrangement of the high-valent cations (Nb V , Sb V ; W VI , Te VI , etc.) determine the kind of hybridisation (sp, sp 2 , sp 3 ) and the directional properties of the hybrid orbitals. A wide range of AOM parameters— e σ ranging from ∼3 to ∼5×10 3 cm −1 in the case of Ni 2+ —is obtained, the most striking changes being observed, if a d 0 configurated cation (Nb V , W VI ) is substituted by a cation with d 10 configuration (Sb V , Te VI ). Particularly interesting are anisotropies in the π -interactions in the case of sp 2 hybridised oxygen atoms, which lead to large symmetry splittings of the d–d transitions. The e σ and e π energies, which constitute the ligand field parameter Δ, turn out to be rather complex bonding parameters. They can be classified according to the local symmetry of the M–O bond in the respective crystal structure and correspond to the σ - and π -overlap capacities resulting from the modeling of the oxygen bonding properties by strongly contrapolarising cations in the oxygen coordination sphere. Additional information, specifically about the degree of covalence within the transition metal–oxygen bond, comes from the nephelauxetic ratio β and the mixing coefficient α of Cu II in the ground state MO—both parameters deduced from the experiment. The obtained results can be nicely interpreted by defining effective electronegativities of oxygen towards the transition metal. Thus, high-valent and small cations as S VI and P V are strongly contrapolarising and generate highly ionic oxygen atoms comparable to the fluoride ions. The collected data are finally used to define oxygen ligand fields with the property to stablise the higher oxidation states of the late 3d metal ions (Cu III , Ni III , Ni IV ).

Angular overlap model parameterisation of Anderson's superexchange theory. I, A quantification of Goodenough-Kanamori rules

Abstract A parameterisation of Andersons exchange formulas on the basis of an extension of the angular overlap model (AOM) is proposed. Transfer integrals are expressed in terms of the metal-metal and metal-ligand bonding parameters, which can be estimated from independent spectroscopic studies, or calculated using solid state expressions. Analytical expressions for the transfer integrals between various d-orbitals, appropriate for cubic crystal lattices, comprising octahedra sharing common vertices and common edges serve as a quantification of the Goodenough-Kanamori rules. On the basis of the present parameterisation we give an explanation of the “exchange integral versus bond-distance” dependence. Some potential applications of the model are briefly discussed.

The electronic spectrum of bis(N,N-diethyldithiocarbamato) Cu(II)

Abstract The electronic spectrum of Cu(II) doped in a Zn(dtc)2 matrix (dtc-diethyldithiocarbamate anion) has been studied theoretically using the angular overlap model (AOM) and crystal field theory (CFT). The effect of the ligand bite angle (α), polar angle (θ) and spin-orbit coupling on the electronic spectrum has been traced. The contributions of these factors have been found to be small, yet important in producing the transition energies and polarizations of the observed spectrum. Spectral AOM parameters, derived from the analysis of the charge transfer bands have been used to interpret the photoelectron spectra and the “d-d” transitions. Two sets of CFT parameters (called σ- and π-sets) were needed in the CFT treatment to reproduce the AOM treatment and this explains why previous CFT treatments were unsuccessful.

Polarizable continuum model for lithium interface transitions between a liquid electrolyte and an intercalation electrode

Abstract A model is described and used to calculate the energy of lithium when crossing the interface between a liquid electrolyte and an electrode surface. The model is based on a classical treatment of the solute–solvent interactions in terms of a polarizable continuum model (PCM), and of the lithium crystal interactions by electrostatic (Madelung) energy calculations including short-range closed shell repulsion. In addition, the coupling of Li + with the charge compensating electron on a neighbour Mn 4+ site is taken into account. A first application of the model to the Li–Mn 2 O 4 (spinel) system shows that diffusion of Li from the surface to bulk of the electrode requires an activation energy, which is higher than the one for bulk diffusion. Surface charges, as deduced from electronegativity calculations are strongly reduced compared to the bulk ones. As a result, a barrier is also formed for the solvent to the surface Li + diffusion. On the basis of the calculated energy profiles we conclude that for Li/Mn 2 O 4 bulk diffusion is considerably faster than lattice incorporation. Our results are in qualitative agreement with fits of equivalent circuits to alternating current impedance measurements for Li + intercalation in cubic and layered TiS 2 and NiO 2 and potential jump kinetic experiments on Mn 2 O 4 :Li.

Electronic spectra of pseudotetrahedralbis(bidentate)copper(II) complexes: Spectral assignment and effect of ligand asymmetry☆

Abstract The electronic spectra (dd transitions) of the following pseudotetrahedral complexes, Cu(LL) 2 2+ , L-L = 2, 2′-dipyridylamine (HDPA);3,3′-dimethyl-2,2′-dipyridylamine (HMPA);2-pyridyl-2′-pyrimidylamine (HPPA);2-pyridyl-2′-thiazolylamine (HPTA) were interpreted by the Angular Overlap Model (AOM) in D 2 symmetry for symmetrically bonded ligands and C 2 for asymmetrically bonded ligands. AOM expressions were found in which the bite angle (α), the dihedral angle (2ω) and the ligand pitch angle (ψ) were retained as variables. Such an approach permitted the use of experimental α, 2ω and ψ values in the spectral fit procedure so that the extracted AOM parameters were free from geometry contributions. Neglect of the real ψ values produces a large distortion as to the real π-bonding picture in the studied complexes. The geometry-free AOM parameters were further substantiated by using them in calculating the non-coincidence between the molecular symmetry axes and the g-tensor axes, as well as the charge transfer transitions. Contributions totransition energies by sd and pd mixing in D 2 symmetry were found to be negligible.

Intensity of d-d bands in spectra of planar chelate complexes I. Electronic spectrum of nickel(II) diethyldithiophosphate

Abstract The oscillator strengths for the d-d transitions in planar M(L-L) 2 complexes of D 2h symmetry (L-L is a bidentate ligand) have been calculated within the framework of the vibronic mechanism and the Angular Overlap Model. The general expressions obtained have been used to interpret the intensities and polarisations of the d-d bands in the bis (diethyldithiophosphato) Ni(II) spectrum.