Oxidation state is an important index to evaluate the charge distribution within molecules. The most common definition of oxidation state was established by IUPAC, which let the atom with higher electronegativity takes all the bonding electrons and calculated the difference between the number of electrons and protons around each atom to assign the oxidation states. However, the definition doesn't thoroughly consider the distribution of the bonding electrons and further restricts the applicability of oxidation states.
To precisely assign the oxidation state for each component in the molecule, especially for organometallic complexes, several different research groups, including Pedro Salvador and Martin Head-Gordon, have developed different methods to determine the oxidation states. In 2009, Martin Head-Gordon group established a new method called localized orbitals bonding analysis (LOBA) to assign the electrons associated with each localized orbitals. However, this method failed to provide reasonable oxidation states since the orbitals cannot be localized for some complicated systems.
To overcome this problem to get the correct assignment of oxidation states, in 2022, Martin Head-Gordon and Pedro Salvador decide to localize the electrons based on different fragments rather than atoms. Thus, they developed the method known as oxidation state localized orbitals (OSLOs), which can accurately assign electrons to different fragments to obtain the oxidation states of each fragment.