Contrast in the effect of penta-amminecobalt(III) on the chemical properties of nicotineamide and isonicotineamide ligands
Abstract
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 [CoIII(NH3)5(L·OH)] formed in these reactions have been measured. The spectra of L·OH are similarto those of [CoIII(NH3)5(L·OH)] for L = py and ina. However, λmax= 385 nm for the intermediate formed in the reaction OH +[Co(NH3)5(na)]3+ whereas λmax= 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.