Chromium reactions with thiocyanate

Cobalt, nickel, chromium and copper salts reduce the sensitivity of the iron test because of the color of their ions, or of their reaction products with thiocyanate. However 1.25 y iron can be detected in the presence of 320 times the amount of cobalt or chromium, and 0.025 y iron in the presence of 640 times the amount of nickel or copper. ... 

The basic process can be exemplified by the reaction of the cobalt(III) complex [Co(NH3)5(NCS)]2+ (the oxidant) with the chromium(II) complex [Cr(OH2)6]2+ (the re-ductant), where thiocyanate acts as the bridging ligand. The overall reaction in aqueous acidic solution is shown in Equation (5.58). 

Two studies have been made of the chromium(vi) oxidation of thiocyanate, - in which the reported dependence of the ligand concentration is substantially different. In keeping with the earlier observation that a complex between Cr and SCN is sufficiently stable with respect to redox at low [H+] to allow temperature-jump studies to be made (and hence to yield a value for the formation constant), Muirhead and Haight found an immediate increase in absorbance on mixing the reactants in the stopped-flow apparatus. The spectrum of the intermediate was derived using the continuous-flow method. Assuming the oxidation reaction to proceed via this complex, the rate law may be written in the form... 

Many 7i-cyclopentadienyl complexes may be prepared from the reaction of alkali metal cyclopentadienides with ammonia-soluble transition metal salts such as nitrates and thiocyanates in liquid ammonia (2-30). The amine complexes, [M(NH3) ] (C5H5)2, lose ammonia when heated in vacuo, and the uncharged dicyclopentadienyl complexes of iron, cobalt, nickel, chromium, and manganese are obtained. 

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