RAMAN SPECTROSCOPY OF TRANSITION METAL COMPLEXES: MOLECULAR VIBRATIONAL FREQUENCIES, PHASE TRANSITIONS, ISOMERS, AND ELECTRONIC STRUCTURE

Raman spectroscopy is less commonly used than infrared absorption spectroscopy for the vibrational characterization of inorganic compounds, but its applications have significantly increased over the past decade due to high-performance instrumentation. This Comment describes the use of Raman spectroscopy for the characterization of inorganic compounds. We illustrate the application of Raman techniques with the spectra of a series of classic transition metal complexes recorded at variable temperature and pressure. Illustrative examples include [Ni(NH3)(6)]X-2 compounds (X=Cl- or [NO3](-)), thermochromic squareplanar or tetrahedral [CuCl4](2-) complexes, the cis and trans [Cu(glycinato)(2)] center dot H2O complexes, square-planar [Pt(dithiocarbamate)(2)] and [Pd(dithiocarbamate)(2)] complexes, as well as metal-oxo and trans-dioxo complexes of metals with the d(2) electron configuration, such as molybdenum(IV), rhenium(V), and osmium(VI). The variation of the symmetric metal-ligand stretching frequencies with temperature or pressure is presented. Resonance Raman spectroscopy provides a detailed characterization of the electronic structure for the [Ru(BQDI)(NH3)(2)Cl-2] complex with the observation of overtones and combination bands at the excitation wavelength of 488 nm. Time-dependent theoretical calculations for the [Ru(BQDI)(NH3)(2)Cl-2] complex are used to rationalize the resonance Raman intensities and to determine excited-state properties. Molecular lanthanide clusters are used to illustrate the applications of Raman spectroscopy to polymetallic complexes.