Brown ring molecule defies the very definition of oxidation state

For the first time, the iron complex formed in the classic ‘ring test’ for nitrate ions has been isolated.¹ The team behind the work hoped to settle the question over the metal’s oxidation state that has baffled chemists for half a century. But the brown ring compound turned out to break the rules and, according to the researchers, challenge the concept of ‘oxidation state’ as defined by the International Union of Pure and Applied Chemistry (Iupac).