Theo L. Snoeck

Structural and spectroscopic properties of [(CO)5MM′ (CO)3(R-DAB)] (M,M′=Mn,Re; R-DAB=1,4-diaza-1,3-butadiene complexes. X-ray structure of [(CO)5ReMn(CO)3-(i-Pr-DAB)] and infrared and resonance Raman spectra of [(CO)5MM′(CO)3(R-DAB)]

Abstract The X-ray structure of the title compound has been determined by the heavy-atom method and refined by means of block-diagonal least-squares calculations from 2116 independent reflections. The crystals are monoclinic, space group P 2 Vc with unit cell dimensions a = 18.494(3), b = 7.502(2), c = 16.52(2) A, β = 113.760(6)° and Z = 4. The final R value was 0.037. The complex has an octahedral geometry and the CO ligands of the Re(CO) 3 and Mn(CO) 3 (i-Pr-DAB) moieties are in staggered positions. The Mn-Re bond length is 3.012(2) A and the MnN bond lengths are 1.994(9) and 2.003(9) A. The infrared and resonance Raman (RR) spectra obtained by excitation into the lowest electronic absorption band, are reported. A tentative assignment for the CO-stretching vibrations is presented based on their solvent and temperature dependence. The RR spectra of (CO) 3 MM′(CO) 3 (i-Pr-DAB) (M,M′=Mn, Re) taken from N 2 -matrices at 10 K are very weak, which means that the equilibrium conformation of these complexes hardly changes upon going from the ground state to the excited state. These RR spectra are discussed considering the electronic structure of these complexes.

Reinvestigation of the visible absorption bands of the 2,2′-bipyrimidine complexes W(CO)4(bpym) and (μ-bpym)[M(CO)4]2 (M=Mo, W) with resonance Raman spectroscopy; the emission spectrum of (μ-bpym)[Mo(CO)4]2

Abstract The character of the two lowest energy transitions of W(CO) 4 (bpym) and (μ-bpym)[M(CO) 4 ] 2 (M=Mo, W) were established with resonance Raman spectroscopy. According to these spectra the two bands belong to MLCT transitions to different π* orbitals of the bpym ligand. Contrary to expectations it is not the first (lowest energy) but the second and more intense electronic transition which, according to the resonance Raman spectra, is directed to the lowest lying π* orbital (b 2u *, LUMO) of these complexes. This interpretation explains the different band intensities and the untypically low g values of the ESR signals of corresponding anion radicals. Excitation of (μ-bpym)[Mo(CO) 4 ] 2 in CH 2 Cl 2 at 400 nm produced a weak emission with an onset at 700 nm. According to the excitation spectrum, this emission originates from the lowest MLCT-excited state of the complex.

Ground and electronically excited states of Cr(CO)4(bipyridine): energy factored force field analysis of CO stretching vibrations and resonance Raman study

Abstract IR spectra of Cr(CO) 4 (bipyridine) and its 13 CO-containing isotopomers were used to calculate all the stretching and interaction CO force constants and the normal coordinates of the CO stretching vibrations. Resonance Raman (rR) spectra of Cr(CO) 4 (bpy) were measured and compared with the Fourier transform Raman spectra. The most resonance enhanced Raman bands belongs to the ring-deformation vibrations of the bpy ligand and the A 1 ν(CO) vibration at 2004 cm −1 . The rR spectral pattern confirms a localized Cr → bpy metal-to-ligand charge transfer (MLCT) character of the electronic transition responsible for the visible absorption band. It is shown that the MLCT excitation also affects the bonding within the Cr(CO) 4 molety. Of the two A 1 ν(CO) vibrations, only the one at higher frequency (A 1 2 , 2004 cm −1 ) gives rise to a resonance enhanced Raman band. Analysis of this effect, based on the energy factored force field (EFFF) calculated normal coordinates of both symmetric ν(CO) vibrations, shows that the MLCT excitation affects the CO bonds in both the axial and equatorial CO ligands, the influence on the axial ligands being larger. The Raman band due to the A 1 1 symmetric ν(CO)_vibration is not resonance enhanced because of an out-of-phase coupling between the symmetric vibrations of the axial and equatorial pairs of CO ligands. Raman bands due to CrC stretching and CrCO bending vibrations, apparently coupled with the vibrations of the Cr(bpy) moiety, were identified by the 13 CO isotope effect and found to be resonance enhanced.

The vibrational spectra of gaseous and liquid tetraethoxysilane

Abstract The IR and Raman spectra of gaseous and liquid tetraethoxysilane, Si(OC2H5)4, have been recorded and a complete assignment has been made. A structure for gaseous tetraethoxysilane has been proposed based upon S4 symmetry as in tetramethoxysilane and tetramethoxymethane. Thermodynamic functions of gaseous tetraethoxysilane have been calculated from the moment of inertia, derived from this structure, and the assigned vibrational frequencies.

Resonance raman study of (η2-TCNE)M(CO)5 (M = Cr, W; TCNE = tetracyanoethene). Evidence for an olefin-complex → metallacyclopropane transition upon low-energy metal-to-ligand charge-transfer excitation

Abstract A solid state resonance Raman investigation of the non-hydrogen-containing title complexes was performed with long-wavelength laser excitation (514.5-611 nm). The spectra confirmed the η 2 -coordination of the π-acceptor olefin tetracyanoethene (TCNE) and showed that the absorption bands near 700 nm arise from transitions between the metal and TCNE. The metal-to-ligand charge-transfer (MLCT) character of these transitions is much more pronounced for the tungsten complex. These observations confirm the higher degree of metal ( d )/ligand(π ∗ ) orbital mixing in the chromium complex and also provide evidence for a structural change from that of an alkene π-complex towards a metallacyclopropane arrangement upon MLCT excitation

Valence localization in [M(triphos)(3,5-di-tert-butyl-catecholate)]+ ions (M = Co, Rh or Ir) probed by resonance Raman spectroscopy☆

The nature of the electronic transitions responsible for the intense absorption band of the pentacoordinate [M(triphos)(DBCat)]+ ions, M = Rh, or Ir or Ir, triphos = MeC(CH2PPh2)3, DBCat = 3,5-di-tert-butyl-catecholate, has been investigated by resonance Raman (rR) spectroscopy. The extent of the valence delocalization within the [M(DBCat)]+ fragment, related to the DBCat → M π-donation, has been estimated from the relative intensities of the high-wavenumber Raman peaks attributed to the intra-catecholate ν(C-O), λ(C-O) and ν(C-C) vibrations and the low-wavenumber peaks attributed to vibrations coupled with the ν(M-O) symmetric stretch and/or deformation vibrations of the M(OOCCO) chelate ring. While the rR spectra of the Co complex show only the low-wavenumber peaks, both the high and low-wavenumber peaks are apparent in the spectra of the Rh and Ir species; the ratio of the intensities of the high- to the low-wavenumber peaks being greatest for the Ir species. The rR spectroscopic patterns indicate that the valence delocalization increases substantially in the order Ir < Rh ⪡ Co, the Co-catecholate bonding being completely delocalized. The metal dependence of the spectroscopic, electrochemical and chemical properties of [M(triphos)(DBCat)]+ is closely related to the extent of the valence delocalization in each ion.

Metal-to-ligand charge-transfer excited-state photochemistry of [Cr(CO)5(C4H4N2)] and [Cr(CO)4(NC5H4CHNPri)]. The difference between the photochemical behaviour of [Cr(CO)5(C4H4N2)] in solution at 243 K and in an argon matrix at 10 K

Irradiation of [Cr(CO)4(NC5H4CHNPri)] in solution within the lowest Cr-to-di-imine charge-transfer band causes photosubstitution of a cis carbonyl ligand. Free CO and fac-[Cr(CO)3(NC5H4CHNPri)] are formed when the complex is irradiated within this absorption band in matrices at 10 K. On the other hand, irradiation of a solution of [Cr(CO)5(C4H4N2)] within the Cr-to-pyridazine charge-transfer band mainly causes photosubstitution of pyridazine, whereas free CO and cis-[Cr(CO)4(C4H4N2)] are formed upon irradiation of this complex in a matrix. These results are discussed in relation to the lifetimes of the metal-to-ligand charge-transfer excited states and to the resonance-Raman spectra of these complexes.