Groups 5−7 transition metal oxides (V2O5, Nb2O5, Ta2O5, CrO3, MoO3, WO3, Re2O7) were anchored on a SiO2 support via incipient wetness impregnation and calcination. The molecular and electronic structures of the dehydrated supported metal oxides and the SiO2 support were determined by combined in situ Raman, IR, and UV−vis spectroscopy under dehydrated conditions. In situ Raman characterization reveals that the supported metal oxides are only present as surface species below the maximum dispersion limit (where crystalline metal oxide nanoparticles are absent). In situ IR analysis shows that the surface metal oxides anchor to the SiO2 support at Si−OH and adjacent Si−O−Si sites. The corresponding in situ UV−vis diffuse reflectance spectroscopy indicates that the dehydrated surface metal oxide species are present as isolated structures. Isotopic D2O−H2O exchange demonstrates that the dehydrated surface MOx species possess the MO oxo functionality but no MOH bonds. The number of MO oxo bonds was found to be r...

In Situ Spectroscopic Investigation of the Molecular and Electronic Structures of SiO2 Supported Surface Metal Oxides

Publisher Summary This chapter discusses the development of metal carbonyl chemistry. Solutions of octacarbonyldicobalt in hydrocarbon solvents can be prepared by a high–pressure reaction between a cobalt compound, such as cobalt carbonate, and a mixture of carbon monoxide and hydrogen. Solutions of octacarbonyldicobalt in petroleum solvents react readily at room temperature with a large variety of alkynes to give complexes of general formula Co2(CO)6(RC2R´). Octacarbonyldicobalt decomposes readily at about 50°–60°C to give dodecacarbonyltetracobalt, and this thermal degradation reaction provides a convenient synthesis of Co4(CO)12 on a small scale. The rate of exchange of carbon monoxide with several other CO2(CO)6(RC2R´) complexes in hexane is investigated. The mercury carbonylcobalt compound Hg[Co(CO)4]2 is obtained by the treatment of the anion [Co(CO)4]– with mercury salts, such as Hg(CN)2 and by the treatment of Co2(CO)8 with Na/Hg. Hg[Co(CO)4]2 is obtained as orange air-stable crystals of m.p. 81°–82°C. The infrared spectrum of the solutions of the compound in CS2 show vco at 2028 and 1967 cm–l, and the nuclear magnetic resonance (NMR) spectrum of the CS2 solution shows the cyclopentadienyl proton resonance as a sharp singlet at δ 5.00. The unsymmetrical isomer 2,4–bis(pentafluorophenyl)–3,5–diphenylcyclopentadienone–n–cyclopentadienylcobalt is the only product isolated from the reaction of (π–C5H5)CO(CO)2 and C6H5C=CC6F5. The involvement of metalocyclopentadiene intermediates in the cyclotrimerization of alkynes has been established for some metal systems.

Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt

The first application of the projector augmented wave method to calculate electric-field gradients is presented. The projector augmented wave method is an all-electron electronic structure method that provides an accurate description of the wave function near the nucleus, and thus is well suited to the prediction of hyperfine parameters. Electric-field gradients have been evaluated for a variety of molecules and crystals containing main-group and transition-metal elements. Our results compare well with experiment and previous calculations based on the linear augmented plane-wave method.

Electric-field-gradient calculations using the projector augmented wave method

Segregation isosteres for carbon at the (100) surface of nickel

Discrete variational X α calculations are reported on the neutral carbonyl complexes Ni(CO)4, Fe(CO)5 and Cr(CO)6, as well as on the ions Co(CO)4 -, Fe(CO)4 -, Mn(CO)5 -, Mn(CO)6 + and V(CO)6 -. Valence electron ionization potentials are predicted very well by transition-state calculations. The description of complex binding in Ni(CO)4 according to these calculations differs markedly from the one obtained in MSXα and Hartree-Fock calculations, which contradicted each other. The relative magnitude of σ-donation and π-back-donation is found to be roughly equal in the three neutral complexes (0·2–0·3 and 0·5 e respectively). Large changes occur in the ionic complexes, notably in the π-back-donation. These changes are correlated with the observed shifts in C-O stretching frequencies and force constants.

The electronic structure of transition metal carbonyl complexes

The Raman spectra of the ions Co(CO)4—, Fe(CO)42—, and HFe(CO)4— have been studied in aqueous solutions of the corresponding sodium salts. The spectra of Co(CO)4— and Fe(CO)42—, excited by the helium radiations 5875.6 and 6678.1 A are in full agreement with the expected Td symmetry of these ions; all observed frequencies were assigned to fundamental vibrational modes. A normal coordinate treatment was carried out under the assumption of a simplified valence force field model making use of only two interaction constants. The values of the fundamental frequencies of the tetrahedral ions, calculated by means of the potential constants of this model, were found to be in satisfactory agreement with the observed Raman shifts.The Raman spectrum of HFe(CO)4—, put in correlation with that of Fe(CO)42—, points strongly to a C3v symmetry of the former ion.

Raman Spectra of Metal Carbonyl Compounds. II. Raman Spectra, Structure, and Force Constants of Cobalt and Iron Tetracarbonylate Anions

Raman spectrum and normal co-ordinate analysis of chromyl chloride

The complete Raman spectrum of Cd[Co(CO)4]2 and a partial one of Hg[Co(CO)4]2, excited by helium radiation in the red and near infrared spectral range, are reported. A salient feature of the observed spectra is the appearance of an intense, strongly polarized shift at 152 (161) cm—1, not found in the spectra of mononuclear metal carbonyls, that must be assigned to the symmetric Cd(Hg)–Co stretching mode. The results exclude the presence of one or more bridging CO groups in the compounds referred to and point strongly to both the existence of a three‐fold principal molecular axis containing the atoms O–C–Co–Cd(Hg)–Co–C–O, and to the staggered D3d configuration of the entire molecule.

Raman Spectra of Metal Carbonyl Compounds. IV. Raman Spectra and Structure of Cadmium and Mercury Cobalt Carbonyl

The Raman spectrum of Ni(CO)4, excited by He‐lines 58756 and 66781 A, has been restudied A new frequency assignment is proposed, based upon measurements of intensities and the state of polarization of the observed Raman shifts, and taking into account the results previously obtained on the Raman spectra of Fe(CO)5, Fe(CO)4 2—, and Co(CO)4 — Assuming a simplified quadratic valence force function containing seven potential constants, a normal coordinate analysis has been carried out The force constants as calculated by means of this model satisfactorily reproduce the observed frequencies The numerical values of the constants, as compared with those of the isoelectronic anions Co(CO)4 — and Fe(CO)4 2—, provide direct evidence for partial multiple M—C bonding and conjugation between M—C and C–O bonds in metal carbonyl compounds and lead to approximate evaluations of bond numbers

K. Kawai

Papers

Raman Spectra of Metal Carbonyl Compounds. II. Raman Spectra, Structure, and Force Constants of Cobalt and Iron Tetracarbonylate Anions

Raman spectrum and normal co-ordinate analysis of chromyl chloride

Raman Spectra of Metal Carbonyl Compounds. IV. Raman Spectra and Structure of Cadmium and Mercury Cobalt Carbonyl

Raman Spectra of Metal Carbonyl Compounds. III. Raman Spectrum, Vibrational Analysis, and Bond Structure of Nickel Tetracarbonyl