 Solution

Low-spin Co(III) is substitution-inert. In acid, coordinated ammine deprotonation is suppressed, so the strongly donating amido conjugate base that assists ligand loss is not generated appreciably. Direct aquation/linkage change consequently remains very slow.

The persistence of the starting S-bound complex therefore supports a base-activated kinetic pathway. It is consistent with the rapid rearrangement in base and does not mean that the S-bound form must be the thermodynamically preferred linkage isomer under every condition.