[Fe(H2 O)5 (NO)]2+ , the "Brown-Ring" Chromophore.

Although the "brown-ring" ion, [Fe(H2 O)5 (NO)]2+ (1), has been a research target for more than a century, this poorly stable species had never been isolated. We now report on the synthesis of crystals of a salt of 1 which allowed us to tackle the unique bonding situation on an experimental basis. As a result of the bonding analysis, two stretched, spin-polarised π-interactions provide the Fe-NO binding-and challenge the concept of "oxidation state".