 Solution

Write $L=[{\mathrm{C}}_2{\mathrm{B}}_9{\mathrm{H}}_{11}{]}^{2-}$ for the open-face dicarbollide ligand. The structure consistent with the stated attachment is $[L\mathrm{Mo}(\mathrm{CO}{)}_3\to \mathrm{Mo}(\mathrm{CO}{)}_5{]}^{2-}$, with a direct Mo–Mo connection, three carbonyls on the cage-capping Mo and five on the outside Mo. The first Mo has an ${\eta }^5$ interaction with the five-vertex face and completes a 12-vertex metallacarborane cage; the second is exo to that cage. The two tetramethylammonium ions are counterions.

The unsaturated $\mathrm{Mo}(\mathrm{CO}{)}_5$ fragment is a 16-electron Lewis acceptor; $[L\mathrm{Mo}(\mathrm{CO}{)}_3{]}^{2-}$ is an 18-electron metal-base fragment in ionic counting, $d^6+6e(L)+6e(3CO)$. Donation from a filled Mo-based orbital supplies the external fragment with a pair. This donor–acceptor representation avoids the mistaken claim that both initially saturated 18-electron fragments simply add a new conventional bond without changing anything.

An alternative formal oxidation-state allocation for a single covalent Mo–Mo bond is cage Mo(I) and external Mo(−I): the local counts are $d^5+6+6+1=18$ and $d^7+10+1=18$. These are electron-bookkeeping alternatives to the metal-base/metal-acid picture, not a measurement of integral site charges. With terminal carbonyls, the external $\mathrm{Mo}(\mathrm{CO}{)}_5$ group can be approximately octahedral when the Mo–Mo direction occupies its sixth coordination direction. The cage-containing unit must accommodate an exo metal interaction in addition to its facial cage/CO environment; its geometry need not be a perfectly undistorted isolated half-sandwich.