The natural route is conjugate-base substitution. Hydroxide reversibly removes an ammine proton:The amido donor promotes loss of X, producing a short-lived five-coordinate intermediate. That leaving-group-sensitive loss is the slower stage. Subsequent trapping/proton transfers are fast and no longer depend on which halide left.
Water capture followed by proton transfer gives the hydroxo complex. Competing SCN capture can occur at either end of this ambidentate ligand, producing Co-SCN and Co-NCS. Any initially captured S-bound material can undergo the base-assisted linkage rearrangement considered next. Water is overwhelmingly abundant, so the stable N-bound product can remain a minor overall product. The slow X-dependent and fast X-independent stages fit loss of X followed by chemistry of a common intermediate; a single rate-limiting direct attack by thiocyanate would not explain that separation naturally.
Begin with the same ammine deprotonation and amido-assisted weakening/cleavage of the Co-S bond, but now SCN is already beside the metal. A solvent-caged pair of five-coordinate cobalt amido species and the departing thiocyanate can reorient and recombine through nitrogen before the ligand diffuses into bulk solution. Reprotonation restores the five ammines and gives A.
Thus the essential sequence isWater capture competes to give Co-OH, but rapid internal return of the already associated ligand accounts for the high yield of A. The slower entry into the reactive conjugate-base/dissociation sequence is followed by faster reorientation, capture and proton transfer. This is linkage isomerization, not oxidation or reduction of cobalt.
The near absence of incoming C label means the coordinated thiocyanate usually retains its molecular identity during linkage change. A mechanism that releases it into a fully mixed solution pool and then captures an arbitrary external thiocyanate would predict much more exchange.
Rapid geminate return within a solvent cage explains the result: the original ligand turns and binds through N before an external labeled ligand can intercept the cobalt centre. A continuously bound intramolecular rearrangement is also compatible with low exchange. Therefore the label result supports an intramolecular/cage-local process but does not alone prove that the Co-S bond stays unbroken throughout.
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.
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