Crystal field theory.

In the ionic CFT, it is assumed that the
ions are simple point charges . When
applied to alkali metal ions containing a
symmetric sphere of charge, calculations
of bond energies are generally quite
successful. The approach taken uses
classical potential energy equations that
take into account the attractive and
repulsive interactions between charged
particles (that is, Coulomb's Law
interactions).
The bond energy between the charges is
proportional to q1 * q2 /r
where q1 and q2 are the charges of the
interacting ions and r is the distance
separating them. This leads to the correct
prediction that large cations of low
charge, such as K + and Na + , should form
few coordination compounds.
For transition metal cations that contain
varying numbers of d electrons in orbitals
that are NOT spherically symmetric,
however, the situation is quite different.
The shape and occupation of these d-
orbitals then becomes important in an
accurate description of the bond energy
and properties of the transition metal
compound.