Substitution reactions on arsenic(V). The formation and hydrolysis of pentaamminearsenatocobalt(III) in aqueous solution

Abstract

The formation of Co(NH₃)₅HAsO₄⁺ and Co(NH₃)₅H₂AsO₄²⁺ conforms to the rate law:             rate = [Co(NH₃)₅H₂O³⁺](k_-1[HAsO₄^2-]+k_-2[H₂AsO₄^-]) where k_-1 = (11±1)x10^-2 l. mol^-1 s^-1 and k_-2 = (120±15)x10^-4 l. mol^-1 s^-1 at 295 K and unit ionic strength. The hydrolysis of the arsenato complex conforms to the rate law:        rate = [total arsenato complex](K_a2k₁+k₂[H⁺]+k₃[H⁺]²)(K_a2+[H⁺])^-1 where k₁ = (290±15)x10^-7s^-1, k₂ = (408±20)x 10^-6 s^-1, k₃ = (670±34)x10^-3 l. mol^-1 s^-1, and pK_a2 = 3.30±0.05 at 295 K and unit ionic strength. The formation and hydrolysis reactions proceed through bond formation and cleavage between oxygen and arsenic. The mechanisms of the reactions characterized by k₁ and k_-1 are considered to be associative substitutions on arsenic(v), but the mechanisms of the other reactions are less certain.