ABSTRACT
Why do we not extend our inventory beyond S = 9/2? A practical answer is that halfinteger spin systems with S > 9/2 appear to be rare in biology. In fact, at the time of writing I know of no example, but note that in the field of molecular magnets the number of identified S > 9/2 systems is rapidly increasing. We can also argue why we do not expect to find very high spins in biology. For mononuclear systems (one metal ion) the maximal spin is S = 5/2 for d5 systems (FeIII, MnII). For pairs of d-ions the maximal half-integer spin S = 9/2 is found for the mixed-valence configuration (MIIIMII). Such a pair should be in a fairly symmetrical coordination surrounding in order for double exchange to be dominant and thus to afford such a high spin. Apparently,
the intrinsic low symmetry of biomolecules combined with the limited size of metal clusters in biomolecules ensures that S ≤ 9/2.
