COMMON FEATURES OF VARIOUS MECHANISMS OF ELECTRON-TRANSFER ACROSS A 4,4'-BIPYRIDINE BRIDGE - A THEORETICAL EVALUATION OF RESONANCE STRUCTURES OF THE TRANSITION-STATE

The transition state of electron transfer occurring in mixed-valence s ystems, which involve the 4,4'-bipyridine as bridge, is examined in th e framework of three mechanisms: a 'double exchange' (or 'two state') and two 'super exchange' mechanisms involving positive or negative pol aron components. The common features of these mechanisms as well as ef fects which are due to the topology of the bridge are investigated. Th e electron transfer is described within 'Mesomeric or Resonance' theor y, and the corresponding weights of various resonance structures are c alculated by means of the poly-electron population analysis. Even thou gh the one-electron characteristics (as charge and spin densities) are totally different for the three considered mechanisms, common feature s are revealed from the multi-particle structures. It is shown that in the field of two equally resonating charge centers, the electron pair s (ionic + covalent) of 4,4'-bipyridine show a trend towards quinoid l ocalization. This effect occurs regardless of the mechanism with which the thermal electron transfer is realized; it is more pronounced for the two 'super exchange' mechanisms than in the 'double exchange' one. The main observed effects are essentially controlled by the topology.