Zinc chemistry is similar to the chemistry of the late first-row transition metals nickel and copper, though it has a filled d-shell, so its compounds are diamagnetic and mostly colorless. The ionic radii of zinc and magnesium happen to be nearly identical. Because of this some of their salts have the same crystal structure and in circumstances where ionic radius is a determining factor zinc and magnesium chemistries have much in common. Otherwise there is little similarity. Zinc tends to form bonds with a greater degree of covalency and it forms much more stable complexes with N- and S- donors. Complexes of zinc are mostly 4- or 6- coordinate although 5-coordinate complexes are known.
What is the determining factor where zinc and magnesium are very different chemically?