Transitions metal-centered

The spectral properties of this excited state species are of great interest since they represent the effects of the (d 2) (d 2 y2), d 2 metal centered transition upon... 

Comparison of the diffuse reflectance spectra of TH and 2.0% Rh(III)/TH clearly indicates novel absorptions at 400-500 and 500-700 nm (Fig. 6). The shoulder at about 500 nm compares well with the lowest metal-centered transition of [RhClg] observed in hydrochloric acid at 518 nm 47). At wavelengths shorter than about 550 nm a strong absorption increase suggests that it does not originate exclusively from the second metal-centered transition occiuring in [RhClel at 410 nm with about the... 

Finally, for the sake of completeness, it must be considered that metal ions of catalytic interest can exhibit absorption bands due to metal centered transitions of other types, as summarized in Table 2.3. 

Table 2.3 Types of metal-centered transitions other than d-d. ...

Werner s evidence for these structures required a theory to rationalize these metal-ligand bonds, and how more than four atoms could bond to a single metal center. Transition-metal... 

Endicott, J. F. Ramasami, T Tamilarasan, R. Lessard, R. B. Brubaker, G. R. Structuie and Reactivity of the Metal-Centered Transition Metal Excited States, Coord. Chem. Revs. 1987,77,1. Ferraudi, G. Thotochemical Properties of Copper Complexes, Coord. Chem. Revs. 1981,36,45. Ford, P. C. Vogler, A. Photochemical and Photophysical Properties of Tetranuclear and Hexanuclear Clusters of Metals with and Electronic Configurations, Acc. Chem. Res. 1993,26,220. 

PM3/TM is an extension of the PM3 method to include d orbitals for use with transition metals. Unlike the case with many other semiempirical methods, PM3/TM s parameterization is based solely on reproducing geometries from X-ray diffraction results. Results with PM3/TM can be either reasonable or not depending on the coordination of the metal center. Certain transition metals tend to prefer a specific hybridization for which it works well. 

One way that molecular mechanics methods have been adapted to transition metal applications is by including one orbital-based term in the force field to describe the metal center. These terms are typically based on semiempirical methods or even some variation of ligand field theory. 

A number of transition-metal complexes of RNSO ligands have been structurally characterized. Three bonding modes, r(A,5), o-(5)-trigonal and o (5 )-pyramidal, have been observed (Scheme 9.1). Side-on (N,S) coordination is favoured by electron-rich (et or j °) metal centers, while the ff(S)-trigonal mode is preferred for less electron-rich metal centres (or those with competitive strong r-acid co-ligands). As expected ti (N,S)... 

The author anticipates that the further development of transition metal catalysis in ionic liquids will, to a significant extent, be driven by the availability of new ionic liquids with different anion systems. In particular, cheap, halogen-free systems combining weak coordination to electrophilic metal centers and low viscosity with high stability to hydrolysis are highly desirable. 

Metal-centered emission is found in several rare-earth complexes, for example europium diketone chelates [29]. These compounds emit relatively narrow spectra associated with the d-f transitions of the metal ion. Relatively few comprehensive studies of this class have been carried out to date, but the available evidence [30] seems to indicate that the photoluminescent efficiencies are somewhat lower than the ligand-centered emitters. 

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