Nature and mechanism of high spin–low spin crossovers and the existence of intermediate spin states

Temperature- and pressure-dependent spectroscopic and magnetic, and X-ray crystallographic studies show that, in some FeIII and FeII crossover systems, rapid thermal equilibria occur between high spin, low spin and sometimes intermediate spin states, with solvent molecules sometimes inducing specific spin states; it is of relevance to biological systems that the single ‘spin-mixed’ state model is inapplicable in every well studied case.