Vibrational center-ligand couplings in transition metal complexes

The mode-tracking principle [J. Chem. Phys. 2003, 118, 1634] for the direct quantum chemical calculation of preselected, characteristic molecular vibrations makes vibrational analyses of very large molecules feasible. This is demonstrated here for the [(Ph(3)PAu)(6)C](2+) complex, in which 18 phenyl groups in the ligand sphere are explicitly taken into account. We are aiming at the motion of the endohedral carbon atom, which is in an extraordinary bonding situation because it is surrounded by an octahedral core of gold atoms in this cluster. Secondary effects of the full ligand sphere on the vibrations of the [Au(6)C] core embedded in [(R(3)PAu)(6)C](2+) clusters are investigated. For this purpose, local vibrations of the octahedral core are generated, and their long-range couplings with the phosphine ligand sphere become visible in the mode-tracking iterations. The exact normal modes of these characteristic vibrations of the cluster are then obtained after convergence of the mode-tracking refinement. This protocol allows us to assess the coupling of the outer ligand sphere with the inner core of the cluster in terms of changes of the vibrational frequencies and of the collective motions of the atomic nuclei. The vibrational frequencies of the octahedral [Au(6)C] core split due to symmetry breaking in the C(1)-symmetric [(Ph(3)PAu)(6)C](2+) cluster. Our study demonstrates how effects of the periphery of a large molecule on local vibrations can be quantified. Furthermore, we predict the first set of characteristic vibrational frequencies obtained with first-principles methods for this gold cluster, whose vibrational spectra have not yet been recorded experimentally.     

About the calculation of exchange coupling constants in polynuclear transition metal complexes

The application of theoretical methods based on the density functional theory with hybrid functionals provides good estimates of the exchange coupling constants for polynuclear transition metal complexes. The accuracy is similar to that previously obtained for dinuclear compounds. We present test calculations on simple model systems based on H. He and CH(2). He units to compare with Hartree-Fock and multiconfigurational results. Calculations for complete, nonmodeled polynuclear transition metal complexes yield coupling constants in very good agreement with available experimental data.     

Accurate magnetic exchange couplings in transition-metal complexes from constrained density-functional theory

We demonstrate an accurate method for extracting Heisenberg exchange-coupling constants (J) from density-functional theory (DFT) calculations. We note that the true uncoupled low-spin state of a given molecule should be identified with the ground state of the system subject to a constraint on the spin density of the atoms. Using an efficient optimization strategy for constrained DFT we obtain these states directly, leading to a simple, physically motivated formula for J. Our method only depends on state energies and their associated electron densities and assigns no unphysical meaning to the Kohn-Sham determinant or individual orbitals. We study several bimetallic transition-metal complexes and find that the constrained DFT approach is competitive with, if not better than, the best broken symmetry DFT results. The success of constrained DFT in these cases appears to result from a balanced elimination of self-interaction error and static correlation from the simulation.     

Broken symmetry approach to calculation of exchange coupling constants for homobinuclear and heterobinuclear transition metal complexes

The application of broken symmetry density functional calculations to homobinuclear and heterobinuclear transition metal complexes produces good estimates of the exchange coupling constants as compared to experimental data. The accuracy of different hybrid density functional theory methods was tested. A discussion is presented of the different methodological approaches that apply when a broken symmetry wave function is used with either Hartree–Fock or density functional calculations. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1391–1400, 1999     

Approaching the Hartree-Fock limit for organotransition metal complexes

In theoretical studies of the electronic structure of organometallic complexes, the choice of basis set is critical, much more so than for analogous studies of molecules containing only H, C, N, and O. This problem is discussed in the light of structural predictions for the transition metal hydrides MH, MH₂, and MH₄, for the fluorides MF₂ and MF₃, and for Ni(CO)₄, Ni(C₂H₄)₃, (CO)₃NiCH₂, and Ni(C₄H₄)₂.     

Performance of conventional and range-separated hybrid density functionals in calculations of electronic circular dichroism spectra of transition metal complexes

A number of density functionals, including 'pure' (nonhybrid) functionals, global hybrids, and range-separated hybrids, were used to calculate the electronic circular dichroism (CD) spectra of 10 tris-bidentate transition metal complexes. The results are compared to one another and to experimental CD spectra, in an effort to illustrate the shortcomings of particular approximations in time-dependent density functional theory (TDDFT). The use of an origin invariant formalism to calculate magnetic transition dipole moments with the help of gauge-including atomic orbitals (GIAOs) is also investigated. With valence basis sets of moderate flexibility, good agreement between GIAO results and rotatory strengths calculated from the dipole-velocity representation is obtained for selected test cases. Empirically broadened vertical CD spectra calculated with the global hybrid functionals B3LYP and PBE0 are found to agree overall the best with experimental CD spectra.     

Metal exchange in organomercury complexes; a facile route to cyclometallated transition metal complexes

Cyclometallated derivatives of 2-phenylpyridine (HL) are readily obtained by transmetallation reactions of 2-(2′-pyridyl)phenylmercury(II) chloride, [Hg(L)Cl], with labile transition metal compounds. The products of these reactions are cyclometallated, containing metalcarbon bonds. The yields are high, and comparable with or better than those obtained from direct reactions with 2-phenylpyridine. The products are easily isolated, and are unequivocally metallated. The metal exchange reaction may be used to prepare cyclometallated complexes which are not available by direct reaction with 2-phenylpyridine. The use of the mercury(II) complex enables the use of kinetically inert chloro complexes in the transmetallation.     

Spin exchange between transition metal complexes and nitroxyl radicals in nonaqueous media

New complexes of diaza- and tetraaza-containing crown ethers, viz., 1,10-diaza-18-crown-6 (1), 1,4,8,12-tetraazacyclopentadecane (2), 1,4,8,11-tetraazacyclotetradecane (3), and 1,4,8,11-tetraazacyclotetradecane 1,4,8,11-tetrachloride tetraacetic acid tetrahydrate (4), with the divalent copper and nickel ions and the Cl^–, Br^–, ClO₄ ^–, NO₃ ^–, and AcO^– counterions were synthesized. The exchange interactions of these compounds and paramagnetic copper and nickel salts with the TEMPO radical in MeOH—CHCl₃ binary mixtures of different compositions were studied. The plots of the linewidths of the hyperfine coupling components of TEMPO vs. concentration of the ions and temperature show that the frequency of diffusion collisions is the rate-limiting step for spin exchange (strong exchange regime). A strong dependence of the exchange rate constant (k _ex) on the crown ether and counterion structure was found. The isotropic hyperfine coupling constants (a _Cu) and g factors (g _i ) were measured for the Cu^II complexes with the crown ethers. In the case of the crown ether complexes 1—3 with CuCl₂, the a _Cu constant decreases linearly with an increase in g _i = g _i – 2.0023 in the series 3 < 2 < 1, whereas k _ex increases linearly in the same series with a decrease in the contact HFC on the Cu^II nucleus (K) and a decrease in covalence of bonding. For the complexes of 2 with Cu^II and different axial ligands (counterions), k _ex increases in the series Cl^– < ClO₄ ^– AcO^– Br^–; < NO₃ ^–. In the case of the complexes of 2 with NiCl₂, k _ex increases in the series 1 < 4 < 3 2. For the Cu^II and Ni^II salts with the Cl^–, ClO₄ ^–, and NO₃ ^– anions, the k _ex values are almost independent of the anion nature. The correlation of the k _ex values with the electron-spin parameters of the complexes is discussed.     

The far-ultraviolet spectra of transition metal complexes

The intense bands which are found beyond the d-d bands in the electronic absorption spectra of transition metal complexes are usually assigned to intra-ligand or charge transfer transitions. However, Rydberg transitions originating with either the mainly 3d or ligand orbitals are also expected to contribute in this part of the spectrum. To explore this the vapor phase electronic absorption spectra of the tri-hexafluoroacetyl-acetonate complexes of Al, Sc, V, Cr, Fe and Mn have been recorded up to about 80000 cm^-1. In order to locate the Rydberg bands, quantum chemical calculations were carried out using the multiple scattering Xα MO method. Among the 4p and 4f type Rydberg bands there are several which are spin, Laporte, symmetry and angular momentum allowed and are expected to contribute strongly to the intensity observed in the ultraviolet and far-ultraviolet parts of the spectrum. The corresponding Rydberg states can mix, however, with valence-shell states of the same symmetry.The far-ultraviolet spectra of three sandwich compounds: bicyclopentadienyl Fe, Co and Ni were also determined. Due to the very low ionization potentials of these compunds, Rydberg transitions can contribute to the observed bands at rather low frequencies.     

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.     

Antisymmetric exchange in polynuclear metal complexes

The irreducible tensor operator approach is utilized as a working tool in the spin Hamiltonian formalism for polynuclear systems. The matrix elements for the dinuclear and trinuclear systems are presented that involve the isotropic exchange, spin-Zeeman term, and the antisymmetric exchange. These basics allow an extensive modeling of the energy levels and magnetic functions (temperature dependence of the magnetic susceptibility and field dependence of the magnetization) for individual Cartesian components and their average for homospin diads and triads. Experimental data on the antisymmetric exchange are reviewed. The tabulations involve data from different sources: the electron paramagnetic resonance, the magnetic susceptibility and magnetization measurements.     

The optical rotatory dispersion of transition metal complexes

Using a simple, first order molecular orbital theory the rotatory dispersion is estimated for inorganic complexes which are essentially octahedral. The results restituate the conclusions reached by Moffitt in 1956.     

The stereochemistry of the reactions of organo transition metal complexes

The stereochemistry of homogeneously catalysed hydrogenation, carbonylation and decarbonylation is discussed, as well as the stereochemistry of nucleophilic and electrophilic addition to co-ordinated alkynes and dienes. Examples are presented of formation and lysis of carbon-transition metal bonds together with data relating to the steric course of these processes.     

Indo calculation of electronic spectra for transition metal complexes in the extended approximation of singly excited configurations

Semiempirical INDO-E/S calculations of [RuX₆]^q (X=NH₃, q=+2, +3; X=CN⁻, q=−4, −3) complexes are performed to demonstrate that the MO relaxation in the electronically excited state can be taken into account by introducing certain double excitations into the configuration interaction matrix; the principles of selection of the excitations are discussed. The calculation results are compared with the experimental electronic absorption spectra. St. Petersburg State University. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 2, pp. 195–205, March–April, 1996. Translated by I. Izvekova     

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.     

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.