Most of the elements in the periodic table have a coordination number of 4. The ability to expand the coordination number allows a strucutre to expand as it polymerizes. The third factor is their ability to form double bonds with with the unshared oxygen atoms. Oxygen isn't readily oxidized to higher oxidation numbers. They keep the POM structure stable.
POMs are able to lose electrons via oxidation and remain stable.
Toxicity of POMs One of the earliest POMs to be somewhat successful in fighting the AIDS virus was HPA-23. Researchers had success with using HPA-23 in vitro against certain viruses. As a result, they tested it with the AIDs virus in vitro and it was successful. They decided to administer it to humans intraveneously. However, the POM was found to be toxic to the liver, spleen, kidney, and brain.