Models of interaction 

Interaction  among  a Lewis acid and a base may have an electrostatic contribution like donor atoms are frequently electronegative and possess some partial  negative  change,  where acceptor  atoms  may be positively  charged.  Also there is an orbital interaction, which can be denoted by the simple molecular orbital (MO) diagram of diagram 1. On the left side and right are represented correspondingly the lowest unoccupied  MO (LUMO) of the acceptor A and the highest occupied MO (HOMO) of the donor D. The levels within the center depict the formation  of a more stable bonding MO and a destabilized antibonding  MO in the complex. The electron pair from the donor takes the bonding MO, that is partially shared among the two species.

Interaction among the orbitals in diagram. 1 will be strongest when the energy variation between the acceptor LUMO and the donor HOMO is the minimum. In this model the best donors filled orbitals at high energies, the best acceptors will have empty orbitals at low energies. By difference, the strongest electrostatic interactions will occur between the smallest one and most highly charged (positive) acceptor and (negative) donor atoms.