The electrochemical behaviors of three half-sandwich iron(II) complexes, [CpFe(CO)(2)X] (Cp = cyclopentadienyl, 1: X = Cl; 2: X = Br; 3: X = I) were investigated. For the three complexes, two reduction processes at ca. -1.3 V and -2.0 V were observed. They are assigned to the reductions of Fe(II) -> Fe(I) couple and the reduction of the dimer, [CpFe (mu-CO)(CO)](2) (Fp(2)), which formed via the dimerization of the neutral intermediate "CpFe(CO)(2)(") (Fp) generated from the first reduction, respectively. Our results ruled out the possibility of two-electron reduction for the first process. Using stop-flow infrared spectroscopic technique (SFIS) allowed us to trace the rapid chemical reactions and the first-order reaction rate constants for the formations of both Fp and Fp(2) were derived as 0.27 and 0.43 s(-1), respectively. Both complexes 1 and Fp(2) could be reduced by LiHBEt3. In the reduction, ligand exchange reaction occurred between the chloride and hydride, which was evidenced by the observation the absorption bands of [CpFe(CO)(2)H] (FpH). Their reduction reactions abided first-order model for the complex and the reducing agent. By assuming that equal moles of the complex and the reducing agent were used in the reduction, a rate equation was established. Again, the SFIS technique was employed to follow the reductions and linear plots were observed to give the rate constants as 14 and 18 mol(-1) L s(-1), respectively. The catalysis on proton reduction of the complexes was also examined. Sufficient experimental evidences in our work and the analysis of literature data suggested strongly that the catalytic species was not the neutral species Fp. Instead the anion, FpX(-), was responsible for the catalysis. The poor performance in catalysis of these complexes resulted from the competing reaction coupled to the first reduction, loss of the halide upon reduction. The resultant neutral species, Fp dimerized rapidly into Fp(2). (C) 2018 Elsevier Ltd. All rights reserved.