Different Tribes - Donor Group Variation

What should already be obvious is that there can be different types of donors, since we have by now introduced examples of molecules where N, O, S, P and even C atoms bind to the 

Variation with chelate ring size of the stability of complexes for various metal(II) ions for binding of the 0,0-chelates oxalate (ox, 5-membered ring), malonate (mat, 6) and succinate (sue, 7). 

Most donor atoms in ligands are members of the main group (p block) of the Periodic Table. Of these, common simple ligands you are likely to meet will be the following  

Those ligands with N, O, P and S donors, as well as the halogen anions, are particularly common. These donor atoms cover the large majority of ligands you are ever likely to meet, including in natural biomolecules. There are others, of course even carbon, as II3C, for example, is an effective donor, and there is an area of coordination chemistry (organometallic chemistry) devoted to compounds that include M—C bonds, addressed later in Section 2.5. 

Very often, ligands contain more than one potential donor group. Where these are not identical, we have a mixed-donor ligand. These are extremely common, indeed the dominant class classic examples are the amino acids [H2N—CH(R)—COOH], which present both N (amine) and O (carboxylate) donors. Where there are choices of donor groups available to a metal ion, it is hardly surprising that some preference may exist - we shall return to a discussion of this aspect in Chapter 3.5. 

---