Transition metal compounds, vibrational spectroscopy

Vibrational Spectroscopy of Hydride-Bridged Transition Metal Compounds... 

For example, see D. F. Shriver and C. B. Cooper, Vibrational spectra of metal-metal bonded transition metal compounds, in Advances in Infrared and Raman Spectroscopy (R. J. H. Clark and R. E. Hester, eds.), Vol. 6. John Wiley, New York, 1979. 

Pd(2-thpy)2 was investigated by applying the methods of time-resolved emission (Fig. 8) and phosphorescence-microwave double resonance (PMDR) spectroscopy (Fig. 10). By both methods, the vibrational satellite structures are resolved and reveal spin selectivity in these satellites. Thus, vibrational satellites can be assigned to the respective triplet substates. The complementary character of these two methods is demonstrated for the first time for transition metal compounds [58,61] (Compare Sects. 3.1.4 and 3.1.5.). 

The purpose of this chapter is to provide an overview of a rather wide array of experimental techniques that can tell us about the electronic structure of molecules. Some of these techniques, such as photoelectron (PE) spectroscopy, which is based on Einstein s photoelectric effect, are generally applied to gas-phase molecules. They can give high-resolution spectra, providing information about molecular vibrations and even, in a few cases, rotations. At the other end of the scale, UV/vis spectroscopy, particularly as applied to transition-metal complexes in solution, involves broad bands, and although it is an important and widely-used method, the information it gives is limited. Emission spectroscopy of transition-metal compounds has also become important. 

We have recently reviewed the use of vibrational spectroscopy in supercritical fluids [2] and the theme common to most of our projects is the use of spectroscopy for real-time optimisation of processes in supercritical solution. Such optimisation is considerably more important in supercritical fluids than in conventional solvents because the tunability of the fluids results in a greater number of parameters which can affect the outcome of a reaction. Thus, the chances of hitting the optimal conditions purely by trial and error are much less in supercritical solution than in conventional reactions. Below, we give three examples of our approach, synthesis of polymers, transition metal hydrogen compounds, and the use of flow-reactors. 

The carbon atoms are in a nearly sp hybridized state which relates these compounds strongly to the conjugated polymers and guarantees the same colorful vibrational spectroscopy as described in the above subsections. Also, the fullerenes can be doped to a metallic state which in some cases even goes superconducting with a rather high transition temperature (Rosseinsky et al., 1991). The state of the art in fullerene research up to 1993 is well described in (Dresselhaus et al., 1993). 

The molecular structures and bonding of [M(R)(CO)5] species (M = Mn, Tc, Re R = alkyl) have received considerable attention over the years, mainly because they contain a simple cr bond between a transition metal and an alkyl group. The structures of the methyl compounds [Mn(CH3)(CO)s] and [Re(CH3)(CO)5] have been investigated by a wide variety of techniques, including X-ray, electron, and incoherent inelastic neutron diffraction, as well as spectroscopic techniques, namely, vibrational, NMR, and mass spectroscopy. Several theoretical studies have also been reported. 

Not surprisingly, vibrational spectra have proven to be an invaluable tool for experimental chemists in the characterization of transition metal and actinide sandwich compounds (98). Most known actinocenes have been characterized early on by vibrational spectroscopy (99). The IR and Raman spectra of thorocene and the IR spectra of protactinocene and uranocene were reported in the 1970s (100,101). However, normal coordinate analysis of these vibrational spectra is difficult because of the large number of vibrational modes involved. So far only a tentative assignment of the vibrational spectra of thorocene and uranocene, based on a qualitative group theory analysis, has been advanced (102). 

A review on vibrational and n.m.r. spectroscopy of compounds containing phosphorus-fluorine bonds has been published. The chemistry of phos-phorus(in) fluorides and their co-ordination compounds has been briefly reviewed, and rearrangements of the five-co-ordinate PFg-transition-metal hydride species HM(PF3)4 (M = Co, Rh, or Ir) and [HM(PF3)4] (M = Fe, Ru, or Os), together with M(PFa)5 (M = Fe, Ru, or Os) have been studied. ... 

The lanthanide elements and their compounds have a number of properties that are particularly attractive for application of electron energy loss spectroscopy (EELS). The conduction band electrons in the metals are mixed s-d in character as in the lower part of the transition metal series, and the monitoring of both one-electron transitions and plasmons by EELS is a convenient indicator of changes in outer electronic structure in such metals, alloys and compounds. The partially filled 4f shells show strong atomic-like correlation even although the electrons are bound by only a few eV, and core excitations may reach localised atomic-like states. By appropriate variation of EELS reveals a richness in excited state structure that remains hidden from other spectroscopies such as X-ray absorption. Electrons may also cause vibrational excitation (Ibach and Mills 1982), but no such experiments have as yet been carried out on rare earth systems. 

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