Spontaneous and base-catalyzed nitrito to nitro linkage isomerization of trans-[Co(NH3)4(NH2CH3)ONO]2+ complex

The nitrito ion trans-[Co(NH3)(4)(NH2CH3)ONO](2+) rearranges to its nitro form trans-[Co(NH3)(4) (NH2CH3)NO2](2+) in aqueous base according to the rate law k(obs) = k(s) + k(bc)[OH-]. At mu = 1.0 M, 25 degrees C k(s) = 1.01 x 10(-4) s(-1) Delta H-s* = 98 kJ mol(-1); Delta S-s* = 7 JK(-1); k(bc) = 3.3 x 10(-2) M(-1)s(-1); Delta H-bc* = 121 kJ mol(-1); Delta S-bc* = 132 JK(-1) mol(-1). The base-catalyzed isomerization proceeds enterely via an intramolecular conjugate-base mechanism. The reaction is fully retentive and trans-[Co(NH3)(4)(NH2CH3)NO2](2+) is the only species that could be detected by H-1 nmr after the reaction was completed. The results are discussed in the light of the reaction mechanisms proposed so far and in relation to the normal substitution chemistry of pentaammine-cobalt(III) complexes. (C) 1998 Elsevier Science Ltd. All rights reserved.