Transition metal complexes photoreactions

Daniel C (2004) Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes Quantum Chemistry and Wave Packet Dynamics. 241 119-165... 

Three steps are Involved In predicting the photoreactions of a transition metal complex with this model. First, the molecular axes that are labillzed In the excited state are determined. 

In contrast to the typical behavior of organic compounds discussed above, many photoreactions of transition metal complexes have wavelength-dependent quantum yields (7). Generally, these wavelength effects have been interpreted in terms of more than one reactive excited state of the photolyzed species. The photoreactivity of V(CO) L (L = amine), for example, has been interpreted in this manner with the previously mentioned model of substitutional photoreactivity proposed by Wrighton et al. (42, 49,73). Assuming ligand dissociation to be the only primary photochemical process (Section III-B-1), photolysis of W(C0)5L could produce three primary products ... 

An excited state of the substrate S reacts with the transition metal complex, whereas S in the ground state would not react. Such a situation was labeled catalyzed photoreaction by Wubbels [1], Salomon [9], Mirbach [29], and Hennig [8]. 

One specific feature of transition metal complex spectra remains to be discussed a band in Mo(CO)a [28850 c-1, e 350] and W(CO)e [28300 cm-1, e 1000], which is not observed in Cr(CO)6 has been assigned to a spin-forbidden d-d transition l1 A ig- 3Tig] S8>. The intensity of this band increases with the atomic number of the central metal due to increased spin-orbit coupling 512h The increased probability of intersystem crossing in transition metal complexes should be kept in mind in a discussion of their photoreactivity. 

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes ... 

Abstract The most significant developments in quantum chemistry and wave packet dynamics providing the theoretical tools to study the electronic spectroscopy and photoreactivity of transition metal complexes are presented. The difficulties inherent to this class of molecules as well as the degree of maturity of the computational methods are discussed. Recent applications in transition metal coordination chemistry are selected to outline and to illustrate the necessity for a strong interplay between theory and experiments. 

The next section devoted to the quantum chemical methods and concepts gives a survey of the computational schemes and theoretical tools adapted to the investigation of electronic spectroscopy and photoreactivity in transition metal complexes. The solvent and other environmental effects are not discussed here and are not taken into account in the selected applications described in the later sections dedicated to the electronic spectroscopy and photoreactivity, respectively. 

The difficulty for the quantum chemist interested by the description and the understanding of electronic spectroscopy and photoreactivity in transition metal complexes will be to take into account in a coherent and consistent way these different effects which are not always easily distinguishable from each other. It is noteworthy that a full Cl approach which would include in the expansion the full set of determinants of the appropriate spin and space symmetry generated by distributing all electrons among all molecular orbitals would be impractical. 

One fundamental aspect in the understanding of the photochemical behaviour of transition metal complexes is the role of the triplet states on the photoreactivity. The calculation of Spin-Orbit Coupling (SOC) effects is mandatory and should be performed in connection with highly correlated methods. The zero-field splitting of triplet molecular states can be calculated by the means of perturbation theory until the spin-orbit effects are not of... 

In order to illustrate the complexity of excited states reactivity in transition metal complexes two selected examples are reported in the next section dedicated to the ab initio (CASSCF/MR-CI or MS-CASPT2) study of the photodissociation of M(R)(CO)3(H-DAB) (M=Mn, R=H M=Re, R=H, Ethyl) complexes. Despite the apparent complexity and richness of the electronic spectroscopy, invaluable information regarding the photodissociation dynamics can be obtained on the basis of wave packet propagations on selected 1-Dim or 2-Dim cuts in the PES, restricting the dimensionality to the bonds broken upon visible irradiation (Metal-CO or Metal-R). The importance of the intersystem crossing processes in the photoreactivity of this class of molecules will be illustrated by the theoretical study of the rhenium compound. 

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