Two inhibitor-containing half-sandwich cobalt(II) complexes [TpMe]CoX ([TpMe] = hydrotris(3,5-dimethylpyrazolyl)borate; X−: N3− (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 pKa 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 N3− and NCS−. The inhibition ability of NCS− is strong than that of N3−, which is also confirmed by the decrease in effective atomic charges of the Co(II) ions as revealed by the theoretical calculations.