Trapping and manipulating excited spin states of transition metal compounds.

This tutorial review describes how complexes of iron(II) (and, rarely, other metal ions) can be switched between their high- and low-spin states by different physical stimuli. At low temperatures, it is possible to trap a compound in a metastable excited spin-state which, in favourable cases, may be stable to thermal relaxation below temperatures as high as 130 K. The selective switching and trapping of individual spin centres in polynuclear compounds, and the interplay between spin centres as they relax back to their ground states, are also discussed. Similar phenomena, in which spin transitions are coupled to charge transfer phenomena, can also occur in inorganic and metal-organic cyanometallate compounds and in cobalt-semiquinonate complexes.