Unexpected phenomena in the mercury(II) - Chlorite ion system: Formation and kinetic role of the HgClO2+ complex

On the basis of time-resolved spectrophotometric measurements, the formatio n of a novel chlorito complex, HgClO2+, was confirmed under acidic conditio n in the mercury-(II)-chlorite ion system. The complex formation occurs wit hin the dead time of the stopped-flow instrument applied (tau(d) similar to 1 ms). The stability constant of HgClO2+ was determined from the initial p art of the kinetic traces recorded at the characteristic 325 nm absorbance maximum of the complex: K = 41.7 +/- 1.9 M-1, epsilon(325) = 727 +/- 24 M-1 cm(-1) (I = 1.0 M (NaClO4), 25.0 degrees C). An intense formation of chlor ine dioxide was observed which is due to mercury(II)-catalyzed decompositio n of chlorite ion. At longer reaction times, when most of the chlorite ion is consumed, the concentration of chlorine dioxide abruptly drops to its fi nal value. This unique feature of the reaction is attributed to an autocata lytic redox cycle which is initiated by an electron transfer step between t he coordinated chlorite ion and chlorine dioxide. The proposed mechanism po stulates the formation of a Cl(II) intermediate which rapidly oxidizes both ClO2- and ClO2.