Valence instability in a dimer of two-orbital system: possible charge separation due to `negative Uʹ effect

A novel mechanism of charge separation in a dimer of two-orbital molecular system is proposed. The effective electron–electron repulsive energy in the dimer, Ueff, has been estimated by means of valence-bond (VB) model calculations for the ground state of the dimer. It has been found that the HOMO–LUMO interplay leads to negative Ueff in the strong dimerization condition, indicating spontaneous separation of charges on dimers (2 dimer− → dimer0 + dimer2−). Possible relevance to experimental features in some [Pd(dmit)2] salts (dmit=1,3-dithiol-2-thione-4,5-dithiolate) is discussed.