SPONTANEOUS AND BASE-CATALYZED NITRITO TO NITRO LINKAGE ISOMERIZATIONOF TRANS-[CO(NH3)(4)(NH2CH3)ONO](2+) COMPLEX

The nitrito ion trans-[Co(NH3)(4)(NH2CH3)ONO](2+) rearranges to its ni tro 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 t he only species that could be detected by H-1 nmr after the reaction w as completed. The results are discussed in the light of the reaction m echanisms proposed so far and in relation to the normal substitution c hemistry of pentaammine-cobalt(III) complexes. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.