Electronic, Infrared, Raman and Resonance Raman Spectra of some Hexahalodi(aquo)dimolybdate(II) Anions, Mo₂X₆(H₂O)₂²⁻, with Quadruple Molybdenum‐Molybdenum Bonds

Abstract

The electronic (800-400 nm), infrared, ordinary Raman (400-ZO cm-') and resonance Raman spectra (3000-20 cm-') of several hexahalodi (aquo)dimolybdate(II)anions, Mo,&(H:!O)?-, with X = C1, Br, I, containing quadruple metalmetal bonds were investigated. The electronic spectra of the solid compounds at various temperatures (25, 100 and 300 K) show intense and sometimes structured bands in the visible region (510-580 nm) assigned to the expected S -$ S* transitions; energy values of the O + 0 transition, of the excited-state metal-metal stretching vibrational frequency and of the damping factors were thus determinedl. From the infrared and Raman spectra, the skeletal stretching modes in these binuclear anions have been localized, and from resonance Raman results the ground-state harmonic frequencies, anharmonic constants, force constants and dissociation energies of the Mo-Mo bonds have been estimated. Finally, within the Franck-Condon model €or resonance Raman scattering, theoretical and experimental excitation profiles of the metal-metal fundamental and its first overtone have been compared in order to obtain accur!te values of the displacement parameters (& = 2.78-2.53) and of the shift in equilibrium positions (Ari = 0.13-0.12 A);these magnitudes are consistent with other observations and these data represent one of the first quantitative estimates in such systems.

The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics

“…The creation of the new band at 340−350 cm −1 could be attributed to the stretching mode of Mo−Mo bonds, formed in the distorted lattice of the oxygen-deficient oxides, as reported previously. 57,58 However, in hydrogen-rich environment (curve d), the reduction mechanism proceeds probably, apart from the formation of vacancies upon removal of bridging oxygens, also through hydrogen incorporation in the terminal oxygens. 69,74,75 Note that the Mo oxide deposited in FG with the simultaneous injection of H 2 pulses (curve e) was highly reduced, probably through a two-step mechanism: First, hydrogenation and sequential removal of neighbor hydroxyl groups in the form of water, followed by the formation of highly reduced species (e.g., MoO 2 ), as reported previously in the literature.…”

Molecular Structures, Vibrational Spectroscopy, and Normal-Mode Analysis of M2(C⋮CR)4(PMe3)4 Dimetallatetraynes. Observation of Strongly Mixed Metal−Metal and Metal−Ligand Vibrational Modes

“…This second point deserves elaboration. The resonance-Raman intensities yield information about the 1 (δ→δ*) excited-state distortions along the vibrational coordinates of the resonance-enhanced modes. ,, If the molecule distorts along several noncoupled vibrational modes in the excited state, then isotopic effects upon the frequencies of each of these modes should be distinct according to their particular nuclear motions, while effects upon relative intensities of their bands should be small: of the order of the percentage change in frequency . If, on the other hand, the distortion is along a single internal coordinate (such as the metal−metal bond) that is distributed among a number of vibrational modes, then the bands for each of these modes will be proportionately resonance enhanced.…”

“…4 A solution of [NBu n 4]F‚x(D2O), prepared as above, was added to a stirred, room-temperature suspension of Mo2(CtCSiMe3)4(PMe3)4 (7.45 g, 8.42 mmol) in DME (50 mL). The reaction mixture was warmed to 45 °C and monitored periodically by electronic-absorption spectroscopy (Mo2-(CtCSiMe3)4(PMe3)4, λmax ) 692 nm; Mo2(CtCD)4(PMe3)4, λmax ) 660 nm). The initial royal-blue color of the reaction mixture darkened slightly as the reaction proceeded.…”

Physical, Spectroscopic and Theoretical Results

“…This has been done for several Mo 2 (O 2 CR) 4 2-ions (X = Cl, Br) the derived values are 0.12-0.13 Å. 265 A combined study of resonance Raman and electronic absorption spectra of Mo 2 X 4 (PMe 3 ) 4 molecules has also led to a value of 0.10 Å for the Mo-Mo bond length increase in the singlet state of the 2 4 * confi guration. 266 A related but more sophisticated approach which employs both a Franck-Condon analysis of the 2 * absorption band and the intensities of resonance Raman overtones for the 'M-M vibration is called the sum-over-states method.…”