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âBrownâRingââRelated Coordination Polymers of the Quartetâ{FeNO}7 Chromophore
A conspicuous detail of the soâcalled brownâring test (the analytical test on nitrate) is the reddish color of the bottom layer of concentrated sulfuric acid, which develops upon the bleeding of the brown layer into the acid. Crystals of the same color form from a solution of ferrous sulfate in concentrated sulfuric acid on saturation with gaseous nitric oxide. The structure of this H3O[{Fe(NO)(ÎŒ4âSO4)(ÎŒ2âSO4)0.5}n/n] (1a) is made up from infinite chessboardâtype layers with sulfur on the field junctions and Fe(NO) moieties below the black and above the white fields. An FeâNâO angle of about 160° causes disorder in the tetragonal space group I4/mmm. A similar crystal pathology was found in the related [{Fe(MeOH)(NO)(ÎŒ4âSO4)}n/n] (1b) in the same crystal class. A oneâdimensional coordination polymer is formed in crystals of a third compound that comprises the Fe(NO)O5 coordination pattern, namely the brown oxalato species [{Fe(H2O)(NO)(ÎŒ2âox)}n/n·H2O] (2). A still larger NO tilt of about 156° is not obscured by disorder in the triclinic crystals of 2