Contrast in the effect of penta-amminecobalt(III) on the chemical properties of nicotineamide and isonicotineamide ligands

The specific rates of reaction of hydroxyl radicals with L, HL⁺, and [CO(NH₃)₅L]³⁺ [L = pyridine (py), nicotineamide (na), and isonicotineamide (ina)] have been measured. The order k(OH + L) > k{OH + [Co(NH₃)₅L]³⁺} > k(OH + HL+) occurs with the exception k{OH + [Co(NH₃)₅(na)]³⁺} > k(OH + na). The spectra of the intermediates L·OH and [Co^III(NH₃)₅(L·OH)] formed in these reactions have been measured. The spectra of L·OH are similarto those of [Co^III(NH₃)₅(L·OH)] for L = py and ina. However, λ_max = 385 nm for the intermediate formed in the reaction OH + [Co(NH₃)₅(na)]³⁺ whereas λ_max = 315 nm for na·OH. Furthermore the radical [(NCS)]₂^- reacts with [Co(NH₃)₅(na)]³⁺ with k = 3.3 × 10⁸ dm³ mol^-1 s^-1 although it does not react with na and with py, ina, and their complexes. These results are discussed.