Transition metal complexes photochemical data

To date, most of the photochemical data available for transition metal complexes comes from condensed phase studies (1). Recently, the primary photochemistry of a few model transition metal carbonyl complexes has been investigated in gas phase (5.). Studies to date indicate that there are many differences between the reactivity of organometallic species in gas phase (5.6) as conq>ared with matrix (7-10) or solution (11-17) environments. In most cases studied, photoexcitation of isolated transition metal... 

The information available is discussed in light of the effects of excitation energy and the environment on the photofragmentation process of several transition metal cluster complexes. The photochemical information provides a data base directly relevant to electronic structure theories currently used to understand and predict properties of transition metal complexes (1,18,19). 

Many reviews of this area are available and no attempt will be made to be comprehensive here. Instead the focus will be on the relationship between photochemistry and photophysics with emphasis on the use of directly measured kinetic data (lifetimes and quantum yields) to draw mechanistic interpretations. The photochemical discussion will be limited to substitution, especially solvation, reactions. The effect of solvent motion will be explicitly treated. Reactions of a few hexacoordinated transition metal complexes will be used to illustrate the important ideas. 

The kinetic approach to the analysis of photoprocesses has been summarized. Kinetic data are seldom sufficient for evaluation of all the primary rate constants. Nonetheless, it has been possible by combining photophysical and photochemical results to determine some of the rate constants in a variety of transition metal complexes. 

The characterization of electronic excited states has attracted much attention in connection with photochemistry. For example, transition metal complexes are characterized by a variety of absorption spectra in the visible and ultraviolet (UV) regions. The absorption spectra essentially give us information about the electronic excited states corresponding to dipole-allowed transitions due to their high symmetries, while some of the data in crystalline fields indicate the existence of several excited states to which dipole transitions are forbidden in the absence of perturbation. Most photochemical reactions of metal complexes, which are occasionally important as homogeneous photocatalytic reactions, involve both allowed and forbidden excited states. Thus, the systematic understanding of the nature of these excited states is essential in designing photochemical reactions. 

We and others have used pulse radiolysis methods to clarify a number of complex photochemical mechanisms. In the course of these studies we have also been able to learn a great deal of new chemistry, including the electronic absorption spectra, thermodynamics, and reaction mechanisms of highly reactive transition-metal centers in both unusually high and low oxidation states. As these data pertain to aqueous media, they contribute in an important way to future work on solar photoconversion in water (the ideal medium from both economic and environmental points of view) and to catalysis in aqueous media in general. 

Solar energy conversion functions often depend on dynamic structures in solution or on a supporting matrix where a transiently appearing dynamic structure could evolve into a precursor for catalytic intermediates. Such dynamic structures are implicitly depicted by the Debye-Weller factor in the conventional XAS data analysis in Equation (12.1), without specific description of the structural origin. In many homogeneous photochemical reactions, metal complexes interact with solvent molecules to form transient dynamic solvated structures, such as dynamic bonding between the catalyst molecule and the solvent or substrate molecules. These dynamic structures may well be the precursor or transition states in catalytic reactions, but were unfortunately obscured in the conventional data analysis. 

The photochemical displacement of the arene is also observed for the complexes ( / -cp)(7 -arene)M (M = Fe, Ru). This reaction occurs from the photoactive a E ligand field excited state. A linear correlation exists between log (0/(l-0)) and o-p, the Hammett parameter for a series of complexes with chloro-and methyl-substituted arenes. The data indicate that a small amount of negative charge builds up at the arene in the transition state for the reaction that results in arene dissociation in these systems. The temperature dependence for the photodissociation of the arene ligand indicates that the metal-arene bond is almost broken in the a E excited state. When the analogous pentamethylcyclopentadienyl complexes (7 -cp )( / -arene)M (M = Fe, Ru) are photolyzed, the quantum yields for arene release are lower than those found for the unsubstituted compounds... 

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