Mini-Symposium, Paper M3

Metal-to-ligand charge-transfer photochemistry: Quantum chemistry and dynamics of the model system HMn(CO)₃(H-DAB)

C. Daniel
Laboratoire de Chimie Quantique
UMR 7551 CNRS / Université Louis Pasteur
F-67000  Strasbourg

Until recently, the MLCT (Metal-to-Ligand-Charge-Transfer) states were mostly assumed to be unreactive and their long lifetime made them suitable for spectroscopic analysis. However, according to a number of experiments reported for a family of a-diimine mono- and di-nuclear transition metal carbonyls [1], these molecules may either behave like transition metal complexes without low-lying MLCT states, undergoing ligand dissociation or radical formation upon irradiation or manifest the photophysics of MLCT complexes. The duality between these two different behaviors may be used to promote different applications like energy and electron transfer processes or formation of reactive intermediates used in substitution reactions and catalytic processes.

The two-dimensional CASSCF/MR-CCI potential energy surfaces calculated for the homolysis of the metal-hydrogen bond and for the CO_ax loss in the system HMn(CO)₃(a-diimine) point to the complexity and the richness of the photochemistry of the transition metal complexes with low-lying MLCT states. The interaction between the quasi-bound MLCT states and the low-lying SBLCT (Sigma-Bond-to-Ligand-Charge-Transfer) (s_Mn-H ® p*_H-DAB) state which is dissociative for the metal-hydrogen bond breaking is the key of the photochemical behavior of this class of coordination complexes. The excited states dynamics are analyzed on the basis of wavepacket propagtions on nine coupled (non-adiabatically or by spin-orbit) potentials corresponding to the electronic ground state and to the excited states lying in the visible and UV domain of energy [2]. The main features of the absorption spectrum are also reported.

[1]   D. J. Stufkens, Comments Inorg. Chem. 13 (1992) 359;  D. J. Stufkens, Coord. Chem. Rev. 104 (1990) 39;  H. V. van Dijk, B. D. Rossenaar, T. van der Graaf, R. van Eldik, C. H. Langford, D. J. Stufkens, A. Vlcek,  Inorg. Chem. 33 (1994) 2865;  B. D. Rossenaar, C. J. Kleverlaan, M. C. E. van de Ven, D. J. Stufkens, A. Vlcek, Chem. Eur. J. 2 (1996) 228.

[2] D. Guillaumont, K. Finger, M. R. Hachey, C. Daniel Coord.Chem. Rev. 171 (1998) 439;  D. Guillaumont, C. Daniel. Coord. Chem. Rev., in the press;   D. Guillaumont, M. P. Wilms, C. Daniel, D. J. Stufkens. Inorg. Chem. 37 (1998) 5816.