Two inhibitor-containing half-sandwich cobalt(II) complexes [Tp(Me)]CoX ([Tp(Me)] = hydrotris(3,5-dimethylpyrazolyl)borate; X-: N-3(-) (1), NCS- (2)) have been synthesized and characterized. The structure of 2 was determined by X-ray crystallographic analysis. The dehydration kinetic measurements of HCO3- catalyzed by the cobalt(II) complexes are performed by the stopped-flow techniques at pH <7.9. The five-coordinated aqua complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3-. The acid dissociation constants (expressed as pK(a) values) for 1 and 2 are 7.9 and 7.8, respectively, which are obtained by pH titration. The dehydration rate constant k of the rate-determining step of 2 (95.9 M-1 s(-1)) is lower than that of 1 (129 M-1 s(-1)), which can be correlated with a higher activation energy barrier, this being the origin of the variation in the inhibition ability between N-3(-) and NCS-. The inhibition ability of NCS- is strong than that of N-3(-), which is also confirmed by the decrease in effective atomic charges of the Co(II) ions as revealed by the theoretical calculations. (C) 2003 Elsevier B.V. All rights reserved.