Re: What is the most complex ion in the world?

Dear Thomas,
first let me correct a certain linguistic misunderstanding that has come 
about in the course of time: The word "complex" (noun and adjective), 
especially in its chemical application, originally had little to do 
with "complicated". It simply means assembled from more or less 
independent units. In the case of hexacyanoferrate, the Fe++ ion was 
considered able to exist independently, and so was cyanide ion 
(the "ligand"). If the two were brought together, a four-fold negatively 
charged octahedral unit Fe(CN)6 was observed to form and to move about as 
such, and remain so in crystalline salts, and since this cannot be 
constructed using electron pair bond "sticks", it was called a complex 
ion. The type of bonding in complexes has since developed into a science 
of its own embracing myriads of compounds.

Most certainly you will not be able to find a linear "ranking" of 
complexity of complex ions. The complexity can grow in so many directions:
the ligands can be "multidentate" or "chelating" or "bridging", you can 
have polynuclear complexes, including those containing metals neighboring 
(bonding) to other metals etc. A nice class are the old silicowolframates 
and -molybdates. A pretty complex complex, enjoying renewed popularity, is 
Vitamin B12:
 http://www.chem.vt.edu/chem-dept/helm/2514/project/b12/b12.html

Best regards
Werner Sieber