From an analysis of empirical and theoretical information on ionic and covalent interactions, a relationship is established between the coupling matrix elements (which measure the nonadiabatic splitting at avoided crossings and strongly influence transition probabilities) and basic quantities of separated partners (ionization potentials, electron affinities, polarizabilities). Although the systems considered here are limited to alkali atoms interacting with simple atoms and molecules, the proposed correlation rule paves the way for extensions to deal with electron transfer reaction dynamics for more general donor-acceptor couples.