= Solution
Place four terminal <trifluorophosphine> donors on each <cobalt> and join the <cobalt> centers by a single <metal-metal bond>, as in the first panel. The P atoms bind to <cobalt>; the fluorines remain attached to <phosphorus>. Each neutral donor supplies two <electrons>. Neutral <electron counting> gives
$$
\boxed{\mathrm{VE(Co)}=9+4(2)+1=18.}
$$
The ionic description is Co(0), $d^9$, with eight <electrons> from the four donors and one from the Co–Co bond. \b[A Co–Co single bond supplies the missing <electron> at each center.] No bridging PF$_3$ is required by this count.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-2-clusters.png]
{title=Proposed metal-connectivity and ligand arrangements for the three electron counts}
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