Reactions of Cr2+ with the pentaamminecobalt(III) complexes of pyridine, nicotinamide, and isonicotinamide were studied. All the complexes are reduced at rates described by a rate law of the form k(CoIII)• (Cr2+). The pyridinepentaammine complex produces only Cr(H2O)63+ from Cr2+, and k for the reaction at 25° is 4.0 ± 0.1 X 10-3 sec-1 (∆Hǂ = 9 ± 2 kcal/mole, ∆Sǂ = – 39 ± 7 eu). The nicotinamidepentaammine complex yields two products by parallel reaction paths: Cr(H2O)63+ with k = 1.4 ± 0.1 X 10-2 M-1 sec-1 at 25° (∆Hǂ = 9 ± 2 kcal/mole, ∆Sǂ = – 36 ± 7 eu) and an amide-bound CrIII-nicotinamide complex with k = 3.3 ± 0.2 X 10-2 M-1 sec-1 at 25° (∆Hǂ = 10 ± 1 kcal/mole, ∆Sǂ = -31 ± 3 eu). The isonicotinamidepentaam-mine complex reacts to produce an amide-bound CrIII-isonicotinamide complex with k = 17.4 ± 0.5 M-1 sec-1 at 25° (∆Hǂ = 3.9 ± 0.3 kcal/mole and ∆Sǂ = -40 ± 1 eu). The latter CrIII complex reacts further under the influence of Cr2+ to produce a pyridinyl nitrogen-bound Cr-isonicotinamide complex according to a (H+) dependent equilibrium. Aquations of the CrIII products were also studied. The structures of CrIII products were assigned on the basis of their infrared spectra. These structures demonstrate that the CrIII products are formed via activated complexes in which the oxidizing and reducing centers are coordinated to remote positions of the bridging ligand. Arguments based on rate comparisons indicate that complexes of isonicotinamide react with Cr2+ by a radical intermediate mechanism. © 1968, American Chemical Society. All rights reserved.
