Mechanism of oxidation of tris-(1,10-phenanthroline)iron (II) by 6-molybdocobaltate(III) ion

Oxidation of [Fe(phen)(3)](2+) by 6-molybdocobaltate(III), [(COMo6O21)-Mo-III](3-) is studied in acid perchlorate solution at constant ionic strength(mu = 1 mol dm(-3), NaClO4). The kinetics, studied under pseudo-first order conditions ([(CoMo6O21)-Mo-III](3-) much greater than [Fe(phen)(3)](2+)), has indicated that the reaction is defined by the rate law -d[Fe(phen)(3)](2+)/dt = (k + k(1)[(CoMo6O21)-Mo-III](3-))[(CoMo6O21)-Mo-III](3-) [Fe(phen)(3)](2+) [H+] here k and k, are the composite rate constants. A similar rate law is also obtained by considering the dimeric species [((MCoMo)-Mo-III)(21)](4-) as an additional reactive entity in addition to the monomeric [(MCoMo)-Mo-III](2-) ion which is an ion pair formed by the alkali metal ion M+ and [(CoMo)-Mo-III](3-) ion. The existence of the dimeric species [(CoO6-MoO6O15)(2)](6). 20H(2)O in solid state is indicated by X-ray crystallographic data. The presence of the dimeric species [H4Co2Mo10O38](6-) in solution is also proposed from the study of the oxidation of Co-II by HSO5- in the presence of molybdate. The ratio k(1)/k approximate to 10(3). The high value of k(1) is attributed to the relatively greater polarisability of the transition state whereas the relative stability explains the low value of k. The rapid scan spectra of [(CoMo6O21)-Mo-III](3-) and [Fe(phen)3]21 in the presence of perchloric acid suggest the relatively much weaker probability of the protonation of [Fe(phen)(3)](2+) than that of [(CoMo6O21)-Mo-III](3-). The presumption of a complex between [(CoMo6O21)-Mo-III](3-) and [Fe(phen)(3)](2+) is obligated to avoid the termolecular collision between the reacting metal complex ions and the proton and to explain the second order dependence in [(CoMo6O21)-Mo-III](3-) ion.