Dehydration kinetic studies of HCO3− catalyzed by three half-sandwich nickel(II) complexes in the presence of inhibitors NO2−, N3− and NCS−

This communication reports the dehydration kinetic measurements of HCO3− catalyzed by three inhibitor-containing half-sandwich nickel(II) complexes [TpMe]NiX ([TpMe]=hydrotris(3,5-dimethylpyrazolyl)borate; X−=η2-NO2− (1), N3− (2), NCS− (3)) by employing the stopped-flow techniques at pH <7.9. The five- or six-coordinated aqua complexes must be the reactive catalytic species in the catalyzed bicarbonate dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3−. The dehydration rate constants k of the rate-determining step decrease in the order: 1 (186 M−1 s−1) > 2 (169 M−1 s−1) > 3 (120 M−1 s−1), which is correlated with an increase of the activation energy barrier heights, this being the origin of the variation in the inhibition abilities among the different inorganic anions.