= Solution
Cleavage of the <disulfide> supplies one <methylthiolate> per cobalt. A monomer $\mathrm{CpCo(SMe)}$ would have only $7+6+2=15$ <electrons> by ionic counting. Dimerization with two bridging <thiolates> supplies a second <sulfur> interaction at each cobalt, and a direct Co–Co bond completes each count. Thus a suitable structure is \b[$[\mathrm{CpCo}(\mu\!\text{-}\mathrm{SMe})]_2$ with a Co–Co <single bond>], containing an $\mathrm{Co_2S_2}$ diamond:
$$
\mathrm{Co}:\quad d^7+6\ (\mathrm{Cp})+2+2\ (\text{two S donors})+1\ (\mathrm{Co-Co})=18.
$$
Each cobalt is formally $+2$: Cp and <methylthiolate> each carry one negative charge. The stoichiometry is
$$
2\mathrm{CpCo(CO)_2}+\mathrm{MeSSMe}\longrightarrow[\mathrm{CpCo}(\mu\!\text{-}\mathrm{SMe})]_2+4\mathrm{CO}.
$$
The <molecular formula> $\mathrm{C_{12}H_{16}Co_2S_2}$ reduces to the required empirical formula. The <sulfur>–<sulfur> bond has been cleaved, both <sulfur> atoms now bridge the <metals>, and the loss of two CO molecules per cobalt is balanced. Formally cobalt(I) is oxidized to cobalt(II), while the <disulfide> is reduced to <thiolate>.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2021/paper-2-thiolate-dimer.png]
{title=Thiolate bridges and a direct cobalt–cobalt bond satisfy the electron count}
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