**5. Conclusions**

The theory of hard and soft acids and bases (HSAB) has proven to be a useful tool in predicting the outcome of bionorganic substitution reactions. According to this principle, electrophile such as complex compounds of transition metal ions reacts preferentially with donor atoms of biologically relevant nucleophiles of similar hardness or softness. Strong bonds are forming between hard acids and hard bases, soft acids and soft bases or borderline acids with borderline. Thus, platinum (II) belong to soft acid, and prefer soft bases. Kinetically are preferred reactions with sulfur donor biomolecules but more thermodynamically stable are Pt-N products. On other hand zinc(II) is borderline hard/soft ions and readily complexes with ligands containing a range of donor atoms, e.g., hard O-, intermediate N- and soft S-donors according to coordination numbers.
