The systematic computational study of the mechanism for water oxidation in four different complexes confirms the existence of an alternative mechanism to those previously reported: the single electron transfer - water nucleophilic attack (SET-WNA). The new mechanism relies on two SET steps, and features the existence of an intermediate with a (HO•••OH)-moiety attached to the metal center. It is operative in at least three representative copper based complexes, and is the only option that explains the experimentally observed efficiency in two of them. The proposal of this new reaction pathway redefines the mechanistic scenario and importantly, generates a completely new avenue for designing more efficient water oxidation catalysts based on first row transition metals.