Photochemistry and photophysics of coordination compounds of the main group metals

The photoproperties of main group metal complexes with the electron configurations s2 (e.g. Tl+, Sb3+, Te4+) and so (e.g. T13+, Pb4+) were studied on the basis of a general concept which relates characteristic excited states to typical photophysical and photochemical processes. The photochemistry is dominated by metalcentered sp (s2) and ligand to metal charge transfer (so) excited states which are capable of inducing interand intramolecular redox reactions. INTRODUCTION The photophysics and photochemistry of transition metal complexes has become an important branch of inorganic chemistry as well as photochemistry.l The excited state properties of these compounds are now fairly well understood. On the contrary, very little is known about the photophysics and photochemistry of coordination compounds of the main group metak2 Although this lack of knowledge seems to be quite surprising there are some obvious reasons which hamper the study of main group metal complexes. Frequently, these compounds are kinetically labile and can exist with variable coordination numbers and structures in the solid state and in solution. In addition, the structures of many complexes deviate from highly symmetrical geometries. These properties complicate the spectroscopic identification and characterization of main group metal complexes. On the other side, there are also features which facilitate the investigation of these compounds. Since the valence shell includes only s and p orbitals the d orbitals have not to be considered for low-energy electronic transitions. Compared to transition metals the variation of stable oxidation states of main group metals is much smaller. This confines the number of possible products which may be generated in photoredox reactions. A few years ago we started a systematic investigation of the photophysics and photochemistry of the main group metal complexes.2 In this short account it is not attempted to treat this subject comprehensively. The current state is illustrated by a few selected examples. STRUCTURE AND BONDING Before we enter the discussion of the photoproperties it is appropriate to give a short description of the structure and bonding of main group metal complexes. These compounds can be classified according to the electron configuration of the free main group metal ions. The so and s2 configurations are most important and correspond to the highest oxidation states and those which are smaller by two units, respectively. The other configurations such as sl, s2p2 and s2p4 are less important and will not be discussed here. Generally, so ions form highly symmetrical complexes and their bonding pattern is illustrated by a qualitative MO scheme of an octahedral complex (Figure l).3 The n interaction is omitted for clarity. The six ligands are kept to the metal by only four bonds. 131 1 1312 A. VOGLER AND H. NIKOL