An alternative mechanism for the formation of the cobalt(III) molybdate cation, Co(NH₃)₅MoO₄ ⁺

An alternative mechanism is proposed for the formation of Co(NH₃)₅MoO₄ ⁺ from Co(NH₃)₅H₂O³⁺ and MoO₄ ^2- in slightly basic solution (pH 7.1-8.0). The observed second-order rate dependence upon [MoO₄ ^2-] may be explained by HMoO₄ ^- acting as a proton-donating catalyst, in contrast to the previously postulated mechanism involving the formation of dimeric molybdenum(VI) species. An increase in reaction rate was also caused by the addition of other protonated anions (HCO₃ ^-, H₂PO₄ ^-) to the reaction solution, thus supporting the contention of a general proton-assisted mechanism. No evidence of dimeric molybdenum(VI) was found.