By introduction of a deuterium label selectively as shown and use of both H-1 and high-resolution C-13 NMR spectroscopy, the facile rearrangement from the amide-N to nitrile-N bonding mode has been shown to proceed with the label apparently migrating from a position adjacent to the amide to next to the nitrile. This reaction is considerably faster than conventional linkage isomerization, where capture of the remote nitrile group would have led to no label migration with respect to the functional groups. Actually the label does not move with respect to the metal ion-the Co-N bond remains intact, while the amide functional group dehydrates to a nitrile and the terminal nitrile concomitantly hydrates. A plausible mechanism involving neighboring group participation is shown, similar to that first suggested by Balahura and Purcell for the analogous o-phenylene rearrangement.