Ligands, transition-effecting action

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

In condensed phases, spectra are commonly measured in absorption. Three main types of transitions are observed in the absorption spectra of the actinide ions (1) Laparte-forbidden f to f transitions, (2) orbitally allowed 5/ to 6d transitions, and (3) metal to ligand charge transfer. Of these, study of internal f to f transitions has found wide use in the investigation of actinide chemistry. These band usually in the visible and ultraviolet regions, can be easily identified because of their sharpness, and are sensitive to the metal environment. As discussed earlier, the 5/ orbitals of the actinide elements are more exposed than the lanthanide 4/ orbitals, and therefore, crystal field effects are larger in the 5/ series. The f to f transitions for actinide elements may be up to 10 times more intense and twice as broad as those observed for the lanthanides, due to the action of crystal fields. In addition, extra lines resulting from vibronic states coupled to / / states have been observed. 

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