= Solution
These are <inner-sphere electron transfer> pathways distinguished by where Cr(II) attacks the coordinated thiocyanate. Cr(II) is substitution-labile and can replace an aqua ligand to form a bridged Co(III)/Cr(II) precursor.
* \b[Remote attack at nitrogen:] form $\mathrm{Co^{III}{-}S{-}C{-}N{-}Cr^{II}}$. <Electron transfer> through the bridge gives Co(II)/Cr(III). Cleavage on the now labile Co(II) side leaves the N-bound product $\mathrm{[Cr(NCS)(OH_2)_5]^{2+}}$. This is the faster $2.0\times10^5\,\mathrm{M^{-1}s^{-1}}$ channel.
* \b[Adjacent attack at sulfur:] Cr binds the same sulfur that is attached to cobalt, giving a shared-S bridge, $\mathrm{Co^{III}{-}S(CN){-}Cr^{II}}$. <Electron transfer> and Co-S cleavage leave $\mathrm{[Cr(SCN)(OH_2)_5]^{2+}}$, the $0.8\times10^5$ channel.
In both cases the product Co(II) rapidly loses its ammines to give the hexaaqua <ion>, whereas substitution-inert Cr(III) retains the bridge-end information. The two chromium linkage isomers are therefore a record of two precursor geometries, rather than evidence that one outer-sphere electron-transfer event somehow selects two arbitrary new ligands afterwards.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-q37-bridged-transfer.png]
{title=Remote and adjacent bridges retain different thiocyanate linkage information}
Back to article page
