= Solution
\b[A is the Ru$_3$ triangle capped by $\mu_3$-AlEt, with three edge-bridging hydrides and one Cp\* ligand at each Ru:] $\mathrm{[\{Cp^*Ru\}_3(\mu_2-H)_3(\mu_3-AlEt)]}$. The 45-proton singlet accounts for three equivalent Cp\* ligands, the three-proton upfield singlet for the equivalent bridging hydrides, and the two-proton quartet plus three-proton triplet for one ethyl group.
The revised source has \b[two] face-bridging hydrides in the precursor. Substituting the AlEt cap then balances as
$$
\boxed{\mathrm{(Cp^*Ru)_3H_5+AlEt_3\longrightarrow(Cp^*Ru)_3H_3(AlEt)+2C_2H_6}.}
$$
Thus the gas is \b[ethane]. Multicentre Ru–H–Ru and Ru$_3$–Al bonding stabilizes the capped cluster; describing the AlEt unit as a face-bound low-valent aluminium fragment is more informative than drawing three unrelated localized donor bonds. The canonical precursor formula lacks the second face hydride and would not balance to this product plus two ethanes. The revised formula explicitly repairs that discrepancy.<Revised question 33 of the 2022 Cambridge Part III Chemistry Paper 2>[physical page 81, part c]{ref}
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