Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / 37 / b / iii

The near absence of incoming ${}^{14}$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.