Metal D-orbitals Research Articles

Small cluster models of the surface electronic structure and bonding properties of titanium carbide, vanadium carbide, and titanium nitride.

Density functional theory (DFT) calculations on stoichiometric, high-symmetry clusters have been performed to model the (100) and (111) surface electronic structure and bonding properties of titanium carbide (TiC), vanadium carbide (VC), and titanium nitride (TiN). The interactions of ideal surface sites on these clusters with three adsorbates, carbon monoxide, ammonia, and the oxygen atom, have been pursued theoretically to compare with experimental studies. New experimental results using valence band photoemission of the interaction of O(2) with TiC and VC are presented, and comparisons to previously published experimental studies of CO and NH(3) chemistry are provided. In general, we find that the electronic structure of the bare clusters is entirely consistent with published valence band photoemission work and with straightforward molecular orbital theory. Specifically, V(9)C(9) and Ti(9)N(9) clusters used to model the nonpolar (100) surface possess nine electrons in virtually pure metal 3d orbitals, while Ti(9)C(9) has no occupation of similar orbitals. The covalent mixing of the valence bonding levels for both VC and TiC is very high, containing virtually 50% carbon and 50% metal character. As expected, the predicted mixing for the Ti(9)N(9) cluster is somewhat less. The Ti(8)C(8) and Ti(13)C(13) clusters used to model the TiC(111) surface accurately predict the presence of Ti 3d-based surface states in the region of the highest occupied levels. The bonding of the adsorbate species depends critically on the unique electronic structure features present in the three different materials. CO bonds more strongly with the V(9)C(9) and Ti(9)N(9) clusters than with Ti(9)C(9) as the added metal electron density enables an important pi-back-bonding interaction, as has been observed experimentally. NH(3) bonding with Ti(9)N(9) is predicted to be somewhat enhanced relative to VC and TiC due to greater Coulombic interactions on the nitride. Finally, the interaction with oxygen is predicted to be stronger with the carbon atom of Ti(9)C(9) and with the metal atom for both V(9)C(9) and Ti(9)N(9). In sum, these results are consistent with labeling TiC(100) as effectively having a d(0) electron configuration, while VC- and TiN(100) can be considered to be d(1) species to explain surface chemical properties.

On the electronic structures of gaseous transition metal halide complexes, FeX4− and MX3− (M=Mn, Fe, Co, Ni, X=Cl, Br), using photoelectron spectroscopy and density functional calculations

We report a photoelectron spectroscopy (PES) and theoretical study on a series of transition metal halide complexes: FeX4− and MX3− (M=Mn, Fe, Co, Ni, X=Cl, Br). PES spectra were obtained at two photon energies (193 and 157 nm), revealing the complicated electronic structures of these metal complexes and their variation with the ligand-field geometry and metal center substitution. Density functional calculations were carried out to obtain information about the structures, energetics, and molecular orbitals of the metal complexes and used to interpret the PES spectra. For the tetrahedrally coordinated ferric complexes (FeX4−), the PES data directly confirm the “inverted level scheme” electronic structure, where the Fe 3d electrons lie below those of the ligands due to a strong spin-polarization of the Fe 3d levels. For the three-coordinate complexes (MX3−), the calculations also revealed strong spin polarizations, but the molecular orbital diagrams present a “mixed level scheme,” in which the ligand orbitals and the Fe 3d majority spin orbitals are spaced closely in the same energy regions. This “mixed level scheme” is due to the larger splitting of the 3d orbitals in the stronger D3h ligand field and the smaller spin polarizations of the divalent metal centers. The calculations show that the metal 3d orbitals are stabilized gradually relative to the ligand orbitals from Mn to Ni in the tri-halide complexes consistent with the PES spectral patterns.

Evolution of local electronic structure in alabandite and niningerite solid solutions [(Mn,Fe)S, (Mg,Mn)S, (Mg,Fe)S] using sulfurK- andL-edge XANES spectroscopy

Synchrotron radiation S K- and L-edge X-ray absorption near-edge structure (XANES) spectra are reported for the cubic, rocksalt (B1) structure sulfides niningerite (MgS), alabandite (MnS), and oldhamite (CaS), and for their solid solutions (Mn,Fe)S and (Mg,Mn)S, and S L-edge XANES spectra are reported also for (Mg,Fe)S solid solutions. Pre-edge features at the S K-edge are attributed to transition of S 1s electrons to the lowest available unoccupied S 3p σ* antibonding states hybridized with metal 3d(e g ) states, and at the S L-edge to transition of S 2p electrons to unoccupied S 3s σ*, 4s σ*, and 3d antibonding states hybridized with metal 3d(e g ) states, and to a lesser extent 3d(t 2g ) states. The S K-edge XANES spectra for the solid solutions show a progressive participation of 3d orbitals in metal-S bonding with increase in substitution by Fe in (Mn,Fe)S and (Mg,Fe)S and Mn in (Mg,Mn)S through progressive increase in the area of the pre-edge feature. However, the pre-peak area does not increase linearly in each solid solution series showing that a real change in bulk electronic properties has occurred. Increase in pre-peak area reflects an increase in overall attainability of metal 3d states for hybridization with S 3p σ* antibonding states as proportionally more metal 3d orbitals become available. The S L-edge XANES spectra show progressive evolution of pre-edge features at the L 3 - and L 2 -edges (a 1 and a 2 , respectively). Only a 2 is present in the S L-edge XANES spectrum of FeS (troilite), and with progressive decrease in Fe content in (Mn,Fe)S and (Mg,Fe)S solid solutions, a 1 first appears, then becomes dominant. Since a 1 is attributed to transition of S 2p 3/2 electrons to S 3s σ* states hybridized with metal 3d(e g ) and 3d(t 2g ) states, this appears to represent an increased contribution from metal-S π-bonding. The results show that the size and position of the pre-edge features to the S K- and L-edges are controlled more by the DOS of hybridized 3d(e g β ) and 3d(t 2g β ) states and nearest-neighbor coordination of the metal atoms than by the precise coordination of S and the extended structure of the sulfide. The full multiple scattering approach has been applied to the calculation of the S K-edge XANES spectra of MgS, MnS, and CaS. Results are consistent with experimental XANES spectra, especially for the pre-edge features, which are often neglected in such calculations.

Structural and optical characterization of topaz implanted with Fe and Co

Natural single crystalline colourless topaz (Al 2SiO 4(F, OH) 2) samples were implanted at room temperature with Fe and Co ions with fluences in the range of 5×10 16–2×10 17 ions/cm 2. Following implantation, the implanted region becomes amorphous in all cases studied and significant changes occur in the absorption spectra. Irradiated samples were subjected to thermal annealings up to 900 °C in steps of 100 °C. At 700 °C almost all F is lost from the implanted region and the implanted ions diffused into the bulk. In contrast, when virgin samples undergo the same thermal treatments, the fluorine loss is not observed. At higher annealing temperatures the implanted ions segregate to the surface. After annealing at 700 °C the samples implanted with the higher concentration of Co do not present colour changes but develop absorption bands centred at 450 and 740 nm. On the contrary, high dose Fe implanted samples submitted to similar annealing protocol show absorption at 280, 350 and 455 nm, becoming yellow. We propose that the absorption bands are related with the crystal field splitting of the transition metal 3d orbitals, probably in a new compound.

Synthesis And Spectroscopic Studies Of New Bis(N-1-hydroxy-2,6-Di-Fe/Tf-Butylphenyl- Salicylideneaminato)Cobalt(Ii) Complexes And Their Oxidation With PbO2

A series of new bis[N-(1-OH-2,6-di-tert-butylphenyl)salicylaldiminato]cobalt(II) complexes possessing one or two HO- and CH3O - substituents on the salicylaldehyde moiety were prepared, and their spectroscopic properties as well as their oxidation with PbO2 were examined. ESR data indicate that oxidation of the complexes produces stable phenoxyl radicals. All phenoxyl radicals have similar g-values and hyperfine coupling constants, which are influenced very little by the substituents and by coordination. The experimental observations indicate that the Co(L·x)2 radicals are ligand-localized and that there is no notable contribution from the metal d-orbitals to the values of the g-factors. Satellite peaks from the 13C nuclei in the aromatic ring were also observed. The ESR spectra of some of the complexes showed partial conversion of primary radicals to secondary paramagnetic species exhibiting interaction with 59Co(I = 7/2) and 14N nuclei

An exploratory study on effect of the isomorphic replacement of Ti4+ ions by various metal ions on the light absorption character of TiO2

The electronic states of anatase and rutile TiO2 crystals and of TiO2 crystals with Ti4+ ions isomorphically replaced by various metal ions (V3+, V4+, V5+, Cr3+, Mn3+, Fe3+, Co3+, Ni2+, Ni3+, and Rh3+) were calculated using the DV-Xα method, which is a density functional method.It was found that the band gap changed in parallel to the radius of the replacing ion, except for a few cases. This tendency agrees with an experimental result in the literature that the light absorption band of anatase shifts to a longer wavelength region, when metal ions are injected. Calculations showed that the replacement of Ti4+ by Ni3+ and V5+ gave the largest decreases in the band gap, suggesting that these metal ions are the most efficient replacing ions for TiO2.The isomorphic replacement resulted in the formation of new energy levels of the metal 3d orbitals in the forbidden band of TiO2. These levels seemed to interact with the 2p orbitals of oxide ions in TiO2, resulting in a decreased band gap. An increase in the degree of covalent bonds between the replacing ions and the oxide ions probably contributes to the decrease in the band gap.

Resonant X-ray emission spectra of K2Ni(CN)4 x H2O at the Ni K-edge.

Resonant X-ray emission spectra were measured at the Ni K-edge for a planar low-spin nickel complex K2Ni(CN)4 x H2O. In the Ni Kbeta emission spectra, a resonant X-ray Raman scattering was observed in the pre-edge region, showing linear energy dispersion of the emitted photon with the incident photon energy. No energy loss features corresponding to the ligand-to-metal charge-transfer (LMCT) was identified. The LMCT feature is characteristic of strongly correlated systems such as NiO; therefore, the LMCT effect proves to be significantly suppressed. This is consistent considering strong covalent character between low-lying pi* ligand and metal 3d (occupied) orbitals in low-spin nickel complexes.

Sulfur K-edge XANES study of local electronic structure in ternary monosulfide solid solution [(Fe, Co, Ni) 0.923 S

Synchrotron radiation S K-edge XANES spectra and unit-cell parameters are used to investigate the local electronic structure of non-stoichiometric binary and ternary Fe-Co-Ni monosulfide solid solution (mss; M0.923S, M = Fe, Co, Ni) quenched from 800 °C and low pressure. The prominent absorption edge feature of the XANES spectra represents transition of S 1s core level electrons to unoccupied S 3p σ* antibonding orbitals hybridized with empty metal 3d(eg) orbitals. There is a progressive increase in area of the edge peak from Fe0.923S to Ni0.923S and Co0.923S, which correlates with progressive decrease in c and a parameters for the NiAs-type subcell and increase in metallic character, and reflects increase in the number and availability of empty eg β orbitals and covalence of metal-S bonds. More generally, the area of the edge peak exhibits an inverse linear correlation with a, c and unit-cell volume of binary and ternary mss. This inverse linear correlation is attributed to progressive increase in covalence and M-S-M bonding interaction in the c-axis direction, through metal-S [M 3d(eg) - S 3p (or 3d)] π bonding. However, the area of the edge peak does not correlate very well with the average number of 3d electrons per metal atom in these solid solutions, showing that the absorption of synchrotron radiation reflects the local electronic structure of individual absorber atoms (i.e. the SM6 cluster), and is not a group (crystal energy band) effect.

ELNES investigations of the oxygen K-edge in spinels

The results of a systematic study of the oxygen K-edge electron energy-loss spectroscopy (ELNES) from a series of aluminium- and chromium-containing spinels are presented. Extra fine structure in the region up to 10 eV above the edge onset is observed for the chromium-containing compounds and is assigned to transitions to states created by mixing of oxygen 2p and metal 3d orbitals. The experimental data has been simulated using the multiple scattering code, FEFF8. Good agreement was obtained in the case of magnesium aluminate, but relatively poor agreement was obtained in the case of the chromites. The possible fingerprints in the oxygen K-edge ELNES corresponding to a high degree of inversion in the spinel structure and to a tetragonal distortion of the cubic structure are discussed.

Adsorption and electrooxidation of carbon monoxide on polycrystalline platinum at pH 0.3–13

Over the whole pH range 0.3–13 electrooxidation of dissolved CO on polycrystalline Pt was found to depend dramatically on the CO dosing potential, occurring at about 0.6 or 0.9 V for CO dosing potentials lower or higher, respectively, than about 0.4 V versus the reversible hydrogen electrode. Only in the former case a small anodic peak at about 0.5 V appeared in stripping voltammograms of chemisorbed CO, suggesting that dissolved CO electrooxidation at 0.6 V takes place on a small fraction of Pt sites that are free from CO. These findings are in agreement with literature reports for pH 0.3 and 14. The IR bands of both bridge and linear CO disappeared at about the same potential for each pH value, showing that at least the main peak of chemisorbed CO corresponded to both linear and bridge CO. As for the small CO stripping peak, it corresponded mostly, if not exclusively, to bridge CO. The ratio of bridge to linearly chemisorbed CO increased with increasing pH, and the stretching frequency of both bridge and linear CO decreased with increasing pH. Both trends were attributed to the exclusion by chemisorbed CO of water and its ions from the inner part of the electrochemical double layer, which would render the point of zero charge of the CO-covered metal independent of pH. At all pH values the intensity of the band of linear CO increased with increasing potential up to 0.4 V, while that of bridge CO decreased. This behaviour is in agreement with literature reports for Pt(100), Pt(111) and Rh(111), and is attributed to a decreasing back-donation from the metal d-orbitals to the antibonding 2π* MO of chemisorbed CO, which would favour a decrease of the coordination degree of CO.

SYNTHESIS AND SPECTROSCOPIC STUDIES OF NEW BIS(N-1-HYDROXY-2, 6-DI-TERT-BUTYLPHENYL-SALICYLIDENEAMINATO)COBALT(II) COMPLEXES AND THEIR OXIDATION WITH PbO2

A series of new bis(N-(1-OH-2,6-di-tert-butylphenyl)salicyladiminato)cobalt(II) complexes possessing one or two HO- and CH3O- substituents on the salicyladehyde moiety were prepared, and their spectroscopic properties as well as oxidation with PbO2 were examined. ESR data indicate that oxidation of the complexes produces stable phenoxyl radicals. All phenoxyl radicals have similar g-values and hyperfine coupling constants, which were less influenced by the substituents and coordination. The experimental observations indicate that some Co(L x )2 radical ligand complexes are ligand-localized and there is not much contribution from the metal d-orbitals to the values of the g-factors. We were also able to observe satellite peaks from the 13C nuclei in the aromatic ring. ESR spectra of some complexes during the time showed the partial conversion of primary radicals to secondary paramagnetic species exhibiting interaction of the radical center with 59Co(I = 7/2) and 14N nuclei.

Synthesis, structure and magnetic properties of tetrakis-mu-carboxylato-bis(dodecylnicotinato)dicopper(II) complexes; crystal and molecular structure of the decyl carboxylate derivative.

Dodecylnicotinate bis-adducts of binuclear copper carboxylates, of the general formula Cu2(O2CC(n-1)H(2n-1))4(C5H4NCOOC12H25)2, were synthesized for n = 10, 12, 14, 16, 18 and 20, and their crystal structure, thermal behavior and magnetic properties studied. The molecular structure of the decyl derivative has been determined from single-crystal X-ray diffraction data. The dimer is centrosymmetric with the CuII ions in a square-pyramidal coordination with four O-alkyl O atoms [average d(Cu-O) 1.960 (6) A] in the basal plane and the nicotine N atom at apical positions [d(Cu-N) 2.183 (3) A]. The copper ions, 2.615 (1) A apart, are bridged by four O-alkyl carboxylate groups. Both the n = 20 and n = 18 homologues exhibit lamellar phases, which can be related to the supramolecular arrangement found in the n = 10 derivative. The magnetic behavior of the decyl and octadecyl dimers was studied in the 2-300 K temperature range. They exhibit a strong intramolecular antiferromagnetic interaction (Cu-Cu superexchange coupling constant J = -347 cm(-1) for the decyl derivative), which can be attributed to a large overlap of the metal 3d orbitals and the oxygen lone pair orbitals of the linking carboxylate groups.

Theoretical Density Functional Analysis of Maleic Anhydride Chemisorption on Pd(111), Re(0001), and Bimetallic PdML/Re(0001) and PdML/Mo(110) Pseudomorphic Overlayers

Nonlocal density functional theory (DFT) calculations were performed to analyze the di-σ, π, and atop chemisorption modes of maleic anhydride on the close-packed Pd(111), Re(0001), and pseudomorphic monolayers of Pd on Re(0001) [PdML/Re(0001)] and Pd on Mo(110) [PdML/Mo(110)] surfaces. The DFT-computed binding energies for maleic anhydride in the atop, π, and di-σ modes on Pd(111) are −28, −34, and −83 kJ/mol, respectively. The calculated adsorption energy and vibrational frequencies for di-σ adsorption are in good agreement with the UHV-TPD and HREELS data of Xu and Goodman (Langmuir 1996, 12, 1807−1816). The atop, π, and di-σ adsorptions of maleic anhydride on Re(0001) are significantly stronger than on Pd(111), with binding energies of −38, −210, and −200 kJ/mol, respectively. The π adsorption mode of maleic anhydride on Re(0001), in particular, is much stronger than that on Pd(111), due to additional interactions of the carbonyl groups of maleic anhydride with the Re(0001) surface. Adsorption energies for maleic anhydride on the bimetallic Pd/Re(0001) and Pd/Mo(110) surfaces are 10−20 kJ/mol weaker than on monometallic Pd(111). This decrease in the adsorption energy is explained on the basis of formal chemisorption theory. Using the analysis of Hammer and Nørskov (Nature 1995, 376, 238, and In Chemisorption and Reactivity on Supported Clusters and Thin Films; Lambert, R. M., Pacchioni, G., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997; pp 285−351), the di-σ adsorption energies have been correlated to the location of the d band center for the clean metal surface. Calculations indicate that the strong Pd−Re and Pd−Mo interactions in the bimetallic surfaces shift the d band center for the surface Pd layer away from the Fermi level. This weakens the interaction of the olefinic group of maleic anhydride with the bimetallic PdML/Re(0001) and PdML/Mo(110) surfaces. The only exception is that for π-bound maleic anhydride on the PdML/Mo(110) surface, where the larger Pd−Pd spacing, relative to Pd(111), allows for better overlap of the carbonyl group π-orbitals with the metal d-orbitals. The π-mode binding energy on the PdML/Mo(110) surface is, therefore, stronger than the corresponding binding energy on Pd(111). The results are consistent with the experimental observations of Xu and Goodman (Langmuir 1996, 12, 1807−1816).

Photodetachment of free hexahalogenometallate doubly charged anions in the gas phase: [ML6]2−, (M=Re, Os, Ir, Pt; L=Cl and Br)

We report the first observation and photodetachment photoelectron spectroscopic study of a series of hexahalogenometallates dianions MCl62− (M=Re, Os, Ir, and Pt) and MBr62− (M=Re, Ir, and Pt) in the gas phase. All of these species were found to be stable as free gaseous doubly charged anions. Photoelectron spectra of all the dianions were obtained at several detachment photon energies. The photon-energy-dependent spectra clearly revealed the dianion nature of these species and allowed the repulsive Coulomb barriers to be estimated. The binding energies of the second excess electron in MCl62− (M=Re, Os, Ir, Pt) were determined to be 0.46 (5), 0.46 (5), 0.82 (5), and 1.58 (5) eV, respectively, and those in MBr62− (M=Re, Ir, Pt) to be 0.76 (6), 0.96 (6), and 1.52 (6) eV, respectively. A wealth of electronic structure information about these metal complexes were obtained and low-lying and highly-excited electronic states of the corresponding singly charged anions were observed. Detachment from metal d orbitals or ligand orbitals were observed and could be clearly distinguished; detachments from the metal d-orbitals all occur at low binding energies whereas those from the ligand-dominated orbitals all take place at rather high binding energies. We also found a remarkable correlation between electron affinities measured in vacuo and the redox potentials obtained in the solution phase of these species.

X-ray structures of trans-dichloro(4-methylpyrimidine) ( η1-phenyl)bis(pyridine)rhodium(III) hydrate and trans-dichloro(phenyl)tris(triphenylstibine)rhodium(III) ethylacetate solvate. Molecular orbital analysis of trans-dichloro (4-methylpyrimidine)( η1-phenyl)bis(pyridine)rhodium(III)

The crystal and molecular structure of trans-dichloro(4-methylpyrimidine)( η 1 -phenyl)cis-bis(pyridine)rhodium(III) water solvate, 1x 0.17H 2 O, and trans-dichloro( η 1 -phenyl)tris(triphenylstibine)rhodium(III) ethylacetate solvate, 2x CH 3 CO 2 CH 2 CH 3 have been studied via X-ray diffraction from a single crystal at room temperature. The final refinement converged to R1 conventional index of 0.0350 and 0.0361 for the structural analysis of 1x 0.17H 2 O [space group R(-3) and 2x CH 3 CO 2 CH 2 CH 3 P(-1) , respectively. The 4-methylpyrimidine ligand (Pym) is only weakly bound to Rh in 1, as shown by the long Rh-N distance (2.251(4) Å), compared to the Rh N(pyridine) lengths average, 2.066(4) . N-C bond distances involving the N donor average 1.329(6) and 1.345(6) for Pym and pyridine (Py) ligands, respectively. The C N C bond angle on the donor is 114.1(5)° for Pym and average 117.4(4) ° for Py. The structure of the complex molecule of 2x CH 3 CO 2 CH 2 CH 3 has some differences when compared to that of the corresponding acetone solvate previously studied in this laboratory (Cini, R., Giorgi, G. and Pasquini, L., Inor`. Chim. Acta, 1992, 196, 7). The two structures differ mainly by the orientation of the phenyl donor with respect to the Cl-Rh-Cl axis (which is more eclipsed for the ethylacetate solvate) and by the conformation of the SbPh 3 ligands. Density functional calculations at the B3LYP/LANL2DZ level with full geometry optimization were carried out on the free Pym molecule and on some Sc(N 1 Pym) 3 +and Sc(N 2 Pym) 3 + model molecules. The effect of metal coordination consists mainly in enlarging the (Sc)N-C bond distances up to 0.150Å, whereas the C-N(Sc)-C bond angle decreases of 1.9°. Significant changes on other bond lengths and angles relevant to ring atoms of Pym occur upon metal coordination to the nitrogen atom. The metal coordination to N(2) is less favorable than to N(1) of 7.5 kcal for 1:1 species of Sc 3 + Extended Hückel calculations showed that HOMO consists mostly of metal-d orbitals with some character of chloride and phenyl and pyrimidine ligands, whereas LUMO is composed of phenyl, pyridine and pyrimidine orbitals. The method well reproduces the Rh-N and Rh-C bonding distances and gives Rh-C dissociation energy 2.38 and 5.48 times that for the Rh-N(Py) and Rh-N(Pym) bonds, respectively.

Stability of thetrans-Bis(H···Si) Structure in the Complex RuH2(PCy3)2(κ-η2-H···SiMe2-o-C6H4-SiMe2···H), Studied by Density Functional Theory

The complex RuH2(PCy3)2(trans-κ-η2-H···SiMe2-o-C6H4-SiMe2···H) exhibits a number of intriguing structural features, in particular the apparent trans configuration of the two H···Si units, the nearly collinear axial trans-H−Ru−H bonds, and the adoption of a near-C2v symmetry. Theoretical studies indicate that, in the experimental structure, the drastic distortion of the two H···Si units avoids the competition for σ*-back-donation while strengthening both the axial Ru−H and the Ru−Si bonds through a favorable orbital interaction with the metal d-orbitals. Such distortion is enhanced by the presence of chelation which forces a small Si−Ru−Si angle. The H···Si interaction distance is 1.842 A, and the density Laplacian plot (−∇2ρ) reveals that the extent of H···Si interaction is quite weak, with little sign of competition for the metal-to-ligand back-donation. Comparisons with the unchelated model complex RuH2(PH3)2(H···SiH3)2 show that the chelation causes a weaker H···Si interaction, with an H···Si distance ...

Density functional study of first row transition metal dihalides

In contrast to former ligand field predictions, the standard energetic order of the metal 3d orbitals, δ<π<σ, is not reproduced here. Throughout, the 3dσ molecular orbital (MO) level is found rather low lying because of σ-donating ligand induced 3d-4s hybridization in these linear dicoordinated compounds, while the 3dπ is rather high due to the pronounced π-donor character of the halogen ligands at the short distances of the digonal halides. This results in an unexpected electron distribution for the ground state and in an unusual order of electronic states. The calculations are in general consistent with recent experimental findings. The approach yields semiquantitatively correct geometric parameters and vibrational frequencies, and qualitatively correct trends for the dissociation energies. Those properties show a secondary periodicity, similar to the one known for octahedral high-spin complexes, though for different electronic reasons. Unknown structures, vibrational frequencies, and dissociation energies are predicted.

Fluorescence quenching of liquid scintillation counters by metal ions

The fluorescence quenching of four secondary liquid scintillation counters—POPOP, PBBO, α-NPO and BBO—by transition metal ions has been carried out in 40% methanol–water mixture at room temperature. The quenching is found to be appreciable and purely dynamic. The quenching rate constants were determined using the Stern–Volmer equation. The mechanism of quenching by a series of transition metal ions on these scintillation counters was studied. A reasonable linear fit of the logarithm of the bimolecular quenching constant (log k q ) with halfwave reduction potential ( E 1/2) of the various metal ions was obtained. A similar correlation between the bimolecular quenching rate constant (log k q ) and the oxidation potential of the scintillation counters was also observed. These results indicate the mechanism of quenching to be electron transfer in nature, mediated by the formation of a nonemissive exciplex and the direction of electron transfer is ascertained to be from the excited fluorophore to the metal d-orbitals, The values of Δ G ↕ 0 (the activation energy for the electron transfer process at Δ G=0) and the free energy at which the quenching is purely diffusion limited have been approximately estimated.

CoS2 and NiS2 studied by means of near infra red and visible spectroscopies

The CoS2 and NiS2 pyrites were investigated by means of near infra red and visible spectroscopies. The high resolution of these spectroscopies in the energy range of 1–6 eV allows us to locate and evaluate experimentally both the position and the energetic expansion of the metal-3d orbitals in agreement with the theoretical proposals.

Framework Bonding and Coordination Sphere Rearrangement in the M(2)X(2) Cores of Synthetic Analogues of Oxyhemocyanin and Related Cu and Pt Complexes.

The electronic structures of oxo- and peroxo-bridged binuclear copper compounds analogous to the active site of oxyhemocyanin are analyzed in terms of their framework electron counts with the help of density functional and extended Hückel calculations. Through-ring bonding in the Cu(2)O(2) framework is discussed by means of a topological analysis of the electron density for the model compounds [(NH(3))(3)Cu(&mgr;-eta(2):eta(2)-O(2))Cu(NH(3))(3)](2+), [(NH(3))(3)Cu(&mgr;-O)(2)Cu(NH(3))(3)](2+), and [(PH(3))(2)Cu(&mgr;-H)(2)Cu(PH(3))(2)]. The existence of isomeric peroxo- and bis(oxo)-bridged Cu complexes can be rationalized in light of the framework electron counting rules by taking into account that two electrons can be localized in the metal 3d orbitals in the former but delocalized through framework bonding molecular orbitals in the latter. An analysis of the theoretical and experimental structural data indicates that a reorganization of the Cu coordination sphere that can be affected by the nature of the terminal ligands is important for the relative stability of the two isomeric forms. In particular, the peroxo-bridged structure is favored by tridentate ligands, whereas the oxo-bridged isomer is favored by bidentate ones. The stability of the two isomers is also compared for analogous complexes with different metal or bridging atoms for which only one isomeric form is known.