Photoelectron spectroscopy of metal β-diketonato complexes

The electronic structures of yttrium(III), gadolinium(III) and ytterbium(III) tris-2,2,6,6-tetramethyl-3,5-heptanedione (tmhd) complexes have been investigated by HeI and HeII photoelectron spectroscopy (UPS), UDFT and OVGF calculations. We discuss metal-ligand bonding in the series of metal β-diketonato complexes on the basis of empirical arguments. The photoionization cross-sections and orbital energies of metal atoms must both be taken into account in order to rationalize changes in relative band intensities of the HeI/HeII spectra.     

Photoelectron spectroscopy of transition metal-sulfur cluster anions

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The M_nS _m ^? cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (n _max) is generally expressed as n _max =2m ? 1 in each series of the number of S atoms (m). For Fe?S and Mn?S, it was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag?S, the EAs of Ag₁S_m are small and around 1 eV, whereas those of Ag_nS_m (_n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag₂S unit is preferentially formed with increasing the number of Ag atoms. For Fe?S and Mn?S, the PES spectra of Fe_nS _m ^? /Mn_nS _m ^? show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to Fe_nS _n ^? /Mn_nS _n ^? has little effect on the electronic property of Fe_nS_n/Mn_nS_n. The PES spectra imply that the Fe_nS_n cluster is the structural framework of these clusters, as similarly as the determined structure of the Fe_nS_n cluster in nitrogenase enzyme.     

Relativistic and electron correlation effects on thed-d spectrum of transition metal fluorides

Summary The electronic spectra of the transition metal complexes CoF ₆ ^2– , RhF ₆ ^2– and IrF ₆ ^2– that occur in the solids Cs₂MeF₆ are calculated. Hartree-Fock and Dirac-Fock calculations followed by non-relativistic and relativistic CI calculations respectively are used to study the influence of relativity and electron correlation. The calculated transitions are found to agree fairly well with experiment, the largest discrepancies arising from the neglect of differential dynamical electron correlation effects.     

NMR spectroscopy on heavy nuclei of transition metal complexes. The role of polarization effect

Chemical shifts (δ) and spin-spin coupling constants (J) in the NMR spectra on heavy nuclei M (⁵¹V, ⁵⁵Mn, ⁵⁷Fe, ⁹⁵Mo, ¹⁰³Rh, ¹⁸⁷Os, ¹⁹⁵Pt) for 27 series of transition metal complexes have been analyzed. In general case in the absence of steric factors the values of δ and J depend on three effects of substituents X: inductive, resonance, and polarization. The latter effect increases with the decrease in the distance between M and X and with growing charge on atom M. The contribution of the polarization effect varies from 0 to 80% depending on the type of the series.     

Photoinduced electron transfer in supramolecular assemblies of transition metal complexes

Photoinduced electron transfer in supramolecular assemblies consisting of π-donor dialkoxyarene-functionalized photosensitizers and bipyridinium electron acceptors is examined. The photosensitizers include Ru(II)-tris-bipyridine complexetethered by multi-branch one-shell and two-shell dialkoxybenzene π-donor sites or a Zn(II)-porphyrin capped by a dialkoxybenzene receptor site. The photosensitizer/electron-acceptor supramolecular complexes behave as non-covalent diads and polyads. Effective internal electron transfer quenching within the supramolecular assemblies proceeds. A quantitative model that accounts for the photoinduced electron transfer in the systems is formulated.     

Divalent transition metal complexes

4-(4-ethoxy-phenylhydrazono)-1-phenyl-3-methyl-1H-pyrazolin-5(4H)-one (5a) (H-EMPhP) as ligand and its Cu(II), Co(II) and Ni(II) complexes 4(a-c) were synthesized and characterized by their thermal and spectral properties. The azocoupling product (H-EMPhP), able of azo-hydrazone tautomerism 5(a-d), act as a bidentate ligand involving in coordination the azogroup nitrogen of its common anion (7) and the oxygen atom that is bound to the pyrazole ring of the mentioned anion (7).     

Studies on transition metal murexide complexes

Iron, cobalt, nickel, copper, zinc, cadmium and mercury murexide complexes have been prepared and characterized. The thermal properties of these complexes are studied deeply by DTA technique where their thermal peaks are explained. The multi-stages thermal decomposition mechanism is proposed. The thermodynamic parameters of the decomposition steps are calculated. The entropy change values for all complexes are of the same magnitude and all transition states of the thermal reactions are more ordered than the reactants. The thermal reactions proceed in complicated mechanisms. The fractions appeared in the calculated orders of the thermal reactions confirmed that these reactions proceeded via complex mechanisms.     

Polyoxometalate tri-supported transition metal complexes containing mixed-valent transition metal ions

Three compounds, [AsMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·4H₂O (1), [PMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·3H₂O (2) and [PMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·3.5H₂O (3), have been synthesized under hydrothermal conditions and characterized by IR, UV–vis, XRD, TG, elemental analysis, and X-ray diffraction analysis. Single-crystal X-ray structure analysis reveals that 1 and 2 are isostructural and isomorphous, whereas 2 and 3 are polymorphs. Polymorphs of 1 have not been synthesized yet. The mixed-valent transition metal ion in 1–3 has been further confirmed by TG analyses. Catalytic properties of 1 and 2 have also been studied.     

Scaling the Coulomb interaction in calculations of electron spectra of transition metal complexes

A simple technique of scaling two-electron integrals in ab initio calculations of the electronically excited states of transition metal complexes is proposed. This technique uses the fact that one-center two-electron integrals depend linearly on the scaling factor when Slater type functions are subjected to scaling transformation. This leads to a linear dependence of the d—d transition energy on the “scale” of Coulomb interaction, which allows one to affect the calculation result by varying the Slater exponential. To test the technique, ab initio configuration interaction and full active space calculations of the low excited states of the CrF ₆ ^3- , MnF ₆ ^2- , and VF ₆ ^3- complexes are performed. For transition elements, a basis of Slater type effective functions chosen from the optical spectra of the atoms and ions of transition elements is used. It is shown that in the STO-6G basis with effective exponentials, experimental transitions are reproduced with an accuracy of about 2000 cm^-1 even with the use of small active space determined by the orbitals localized on the central atom of the complex.     

Molecular orbital calculations on transition metal complexes. Part IX

Potential energy curves for the ground and some low energy excited states of a number of complexes with a 3d ⁵ electronic configuration have been computed from INDO type SCF MO calculations. The results agree extremely well with the known ground states of the complex ions MnF ₆ ^4? , FeF ₆ ^3? , CoF ₆ ^2? , and Fe(CN) ₆ ^3? , in particular the crossover from high to low spin being obtained for changes in both central metal ion oxidation state and ligand. The calculated contraction in metal ligand distance on passing from the high spin to the low spin state is ～ 0.05 Å for each complex in very good agreement with the value indicated by pressure dependent magnetic measurements. Computed electronic transition energies involving bothd-d type and charge-transfer excitations compare favourably with observed spectroscopic values.     

Bimacrocyclic NHC transition metal complexes

The preparation of seven concave NHC metal complexes derived from bimacrocyclic imidazolinium salt 1 is reported. The silver complex 2, obtained in 86% yield by reacting 1 with silver(I) oxide, was used to give copper complex 3, rhodium complex 5 and iridium complex 6 by transmetalation in good yields. Palladium complex 4 was obtained by reaction of the azolium salt 1 with palladium dichloride in 3-chloropyridine. The rhodium and iridium dicarbonyl complexes 7 and 8 were prepared via ligand exchange from the COD complexes 5 and 6. Silver complex 2, copper complex 3 and palladium complex 4 were characterized by single-crystal X-ray analysis. Silver complex 2 and copper complex 3 were tested in the cyclopropanation of styrene and indene with EDA (ethyl diazoacetate), where good results were obtained with 3, while low conversion and catalyst decomposition was observed with 2.     

Molecular orbital calculations on transition metal complexes

INDO SCF MO calculations are reported for the complexes (C₅H₅)M(C₇H₇) (M = Ti, V and Cr), and for the corresponding V and Cr cations. The results correctly predict¹ A ₁ ground states for the V⁺ and the neutral Ti and Cr species, and for the neutral V and Cr⁺ complexes confirm the² A ₁ ground levels. The formally metald-levels followed theH ^core sequencee ₂<a ₁<e ₁, and the most important interactions were those between the metale ₂ level and the ligand C₇H₇ -orbitals, and between the metale ₁ level and the ligand C₅H₅ -orbitals. Calculations also satisfactorily reproduced other experimental quantities, and the results indicate that thee ₂ ligand interaction becomes more important, and thee ₁ ligand interaction less important, with increasing size of the ligand ring.     

Bridging vinylalkylidene transition metal complexes

Vinylalkylidene transition metal complexes have been extensively used as ‘multitalent tools’ in organic synthesis, covering a broad field of applications. The vinylalkylidene ligands can be monodentate; alternatively they can adopt a bridging coordination mode in complexes with two adjacent metal atoms. As for other unsaturated organic ligands which can bond in both mono- and di-nuclear modes, the bridging coordination can give rise to new and different chemical properties from those found when the ligand is bound to a single metal centre. Likewise, the synthetic routes to bridging vinylalkylidene complexes offer a broader range of possibilities compared to those used to make mononuclear vinylalkylidenes. In spite of the fact that bridging vinylalkylidene complexes have been known for about 40 years, their synthetic potential as C₃ activated fragments has so far been under-exploited. Comparison with other C₃ bridged ligands (allenyls and allyls) indicates that vinylalkylidene ligands are reactive and versatile species. This review article gives an overview of the chemistry of bridging vinylalkylidene complexes to focus attention on their potential as synthetic tools.     

Fluorenone hydrazine-S-benzyldithiocarbazate transition metal complexes

Summary New Cu^II, Co^II, Ni^II, Cd^II, Zn^II, Hg^II, Pd^II and UO ₂ ^II complexes of the Schiff base ligand (FBz) formed by condensation of fluorenone withS-benzyldithiocarbazate have been prepared and characterized by elemental analysis, magnetic and spectroscopic measurements. The Cu(FBz)₂(Cl)₂ complex is paramagnetic. The Ni(FBz-H)₂ complex is diamagnetic, four-coordinate and square planar. The Co^II ion is oxidized in the presence of the Schiff base with the concomitant formation of Co^III complex of empirical formulae Co(FBz)Cl₃OH₂. The ligand was tested as a corrosion inhibitor for copper. Inhibition efficiency was calculated and the limiting concentration of FBz to give maximum efficiency was 10^–3 mol dm^–3 at 25°C. The polarographic reduction of FBz was investigated in Britton-Robinson buffer solutions of pH 3–10. The polarograms at dme indicated that the depolarizer is reduced through two two-electron irreversible diffusion-controlled waves. The mechanistic pathway of the electrode reaction is commensurate with this result.     

New vic-dioxime transition metal complexes

New vic-dioxime ligands and their Cu^II, Co^III, Ni^II and VO^IV complexes have been prepared and characterized by elemental analyses, i.r. and u.v. spectra, magnetic moments and molar conductance data. The Co^III complexes are diamagnetic. The ¹H- and ¹³C-n.m.r. and g.c./m.s. spectra of the vic-dioxime ligands and their Co^III complexes were recorded. The compounds are all non-electrolytes.     

Conformations of the transition metal complexes

A historical overview is given on the structural and conformational studies of tris(diamine)metal system. Studies on various coordination compounds revealed that energy minimization calculations can predict the detailed geometries of the complexes. The computed geometry of a coordination compound agrees with that observed in the crystal structure to within several standard deviations. Differences in thermodynamic properties between different conformers are well reproduced. Equilibrium distribution of conformers can be reasonably accounted for on the basis of the minimized strain energies.     

Nitrosophenol complexes of transition metal salts

Complexes derived from 4-substituted-2-nitrosophenols, 3-Me-2-nitrosophenol, nitrosophyrogallol, nitrososalicylic acid and nitrosogallic acid with cobalt(II), nickel(II) and copper(II) have been prepared and characterized by elemental analysis, electronic and vibrational spectra, together with magnetic susceptibility measurements. The e.s.r. spectra of the copper(II) complexes were investigated, and detailed thermal properties of selected complexes are discussed.     

Solvent effects on piezochromism of transition metal complexes

We report solvent effects on the piezochromic behaviour of wavelengths of maximum absorption for metal-to-ligand charge-transfer (m.l.c.t.) bands of several molybdenum–diimine–carbonyl and titanium or vanadium–cyclopentadienyl or ethylmaltolate–thiocyanate complexes. These and previously published data are collated in an attempt to establish a pattern for solvent effects on piezochromism of m.l.c.t. bands. Strategies for the design of compounds and complexes for use in establishing a clear pattern for such effects are outlined.     

Multilayered transition metal complexes of cyclophanes

The reaction of arene-ruthenium complexes ([RuCl₂(ν⁶-arene)]₂) with [2.2]paracyclophane in the presence of AgBF₄ provides double- and triple-layered arene-ruthenium complexes of [2.2]paracyclophane in excellent yield.