Our key finding is that for the Ms=6 oxo-oxo form an electronic state corresponding to Mn4(↑↑↑↑)Mn3(↑↑↑)Mn2(↑↑↑)Mn1(↑↑↑)[O5O6](↓) is found as a shallow local minimum at an O5O6 distance of 2.0 Å. The IBOs, Figure , show that electron movement has occurred from O5 to Mn4 leading to a high spin Mn4 (III) and formation of a nascent two centre one electron O5-O6 bond. This species was identified previously by us as a shoulder on the Ms=3 state, see Figure . While a shoulder on the Ms=3 PES, it corresponds to a broad minimum energy structure on the Ms=6 surface due to the favourable antiferromagnetic coupling with all four Mn ions. We note that this species has been referred by us and others previously as an O5 oxo O6 oxyl form but is best and more appropriately described as [O5O6] 3-since negative spin density is present on both O5 and O6, clearly demonstrated by the spin density plot for this form in Figure .