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
Figure 2.11 Variation with chelate ring size of the stability of complexes for various metal (II) ions for binding of the O.O-chelates oxalate (ox. 5-membered ring) malonate (mal.6) and succinate (suc.7).
metal (Figure 2.10. for example). In the same way that different tribes are all members of the same human species molecules with different donors are all members of the same basic species– in this case ligands. You expect different tribes to have distinctive differentiating features and the same applies to ligands with different donors. Because the donor atom is directly attached to the central metal the metal feels the influence of the donor atom much more than any other atoms and groups linked to that donor atom. This is reflected in the chemical and physical properties of a complex. 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 H3C, 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 .
