Electron Donor–Acceptor (EDA) Complexes80
In EDA complexes,81 there is always a donor and an acceptor molecule. The donor
may donate an unshared pair (an n donor) or a pair of electrons in a p orbital of a
double bond or aromatic system (a p donor). One test for the presence of an EDA
complex is the electronic spectrum. These complexes generally exhibit a spectrum
(called a charge-transfer spectrum) that is not the same as the sum of the spectra of
the two individual molecules.82 Because the first excited state of the complex is
relatively close in energy to the ground state, there is usually a peak in the visible
or near-uv region and EDA complexes are often colored. Many EDA complexes are
unstable and exist only in solutions in equilibrium with their components, but others
are stable solids. In most EDA complexes the donor and acceptor molecules are
present in an integral ratio, most often 1:1, but complexes with nonintegral ratios
are also known. There are several types of acceptor molecules; we will discuss complexes
formed by two of them.