Donor–acceptor molecular compounds (essentially localized interactions)

This chapter discusses components that are linked mainly by localized interactions. There is a variety of donor and acceptor types ranging from n-Donors and σ*-Acceptors, through n-Donors and p-Acceptors, n-Donors and p*-Acceptors, π-Donors and s,p-Acceptors to p-Donors and σ*-Acceptors. The first of these categories includes some self-complexes. A distinction has been made between ‘pure acceptors’ and ‘self-interacting acceptors’. The ‘pure acceptors’ are involved only in donor-acceptor interaction with the donor component of the molecular compound. In the ‘self-interacting acceptors’, the principal acceptor atom (e.g., Ag⁺ in AgClO₄ or Sb in SbCl₃), in addition to its donor-acceptor interaction, is also involved in interactions with other atoms of the acceptor moiety (e.g., the oxygens of AgClO₄ or chlorines of SbCl₃). These additional interactions are essential to the stability of the crystalline molecular compounds. Despite widespread use of the term ‘charge transfer’ to describe these compounds, physical measurements (e.g., NQR, Mössbauer effect) suggest that there is very little actual transfer of charge in the ground state, as Mulliken often emphasized.