As indicated above this peroxo ion. This [O 2 ] 3-ion is stabilised by antiferromagnetic interaction with the Mn ions of the complex. Combined computational, crystallographic and spectroscopic data show that an equilibrium exists between an O5 oxo and O6 hydroxo form, S=3 spin state and a deprotonated O6 form containing a two-centre one electron bond in [O5O6] 3-which we identify as the form detected by XFEL crystallography. This form gives rise to an S=6 spin state which gives rise to a low intensity EPR spectrum compared with the accompanying S=3 state, making its detection via EPR difficult and overshadowed by the S=3 form. Simulations assuming a 70% contribution of the S=6 form give rise to a superior fit to the experimental EPR spectrum compared with an S=3 only form. The study reveals the key electronic, magnetic and structural design features of Nature's catalyst which allows water oxidation to O 2 to be uniquely performed under ambient conditions.