The revised question page explicitly gives the cluster as . Its charge supplies two electrons, soa jellium shell closure, through 1G. The reported structure has a Ga atom inside an eleven-atom Ga shell, with ten further ligand-bearing Ga units outside that core.[ref]
A Metalloid Cluster Compound with a Variation of the Ga22 Framework by Andreas Schnepf and others. 2002, central Ga@Ga11 core and ten external GaR units
The canonical exam copy omits the 2− charge. Taken literally as a neutral formula it gives 56 electrons, which is not a listed shell closure; assigning 58 without identifying this difference would be unjustified. The revised public source resolves the discrepancy rather than requiring an invented charge.[ref]
Revised question 33 of the 2022 Cambridge Part III Chemistry Paper 2. physical page 81, part b(iv)
A is the Ru triangle capped by -AlEt, with three edge-bridging hydrides and one Cp* ligand at each Ru: . 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 two face-bridging hydrides in the precursor. Substituting the AlEt cap then balances asThus the gas is ethane. Multicentre Ru–H–Ru and Ru–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.[ref]
Revised question 33 of the 2022 Cambridge Part III Chemistry Paper 2. physical page 81, part c
The requested equality is electrons if each unsaturated unit is counted as a four-electron donor to the chosen extended cluster framework. It needs that additional convention; a count confined to the localized skeleton gives instead.
For , an fragment contributes two skeletal electrons, a singly substituted cap contributes three, and the bridging hydrogen contributes one:For the second formula, let be the contribution assigned to each unsaturated unit. The comparison isThus requires . A bookkeeping partition that includes the two alkene pairs in the extended cluster electron pool givesIn contrast, assigning the double bond entirely to the exoskeletal substituent leaves each corresponding cage carbon with two electrons for the cage:The difference is exactly the two pairs, four electrons, rather than an arithmetic discrepancy. These are different electron partitions: do not compare from one partition with from the other as though both were the same skeletal count, and do not infer a structural classification merely by inserting the extended count into Wade's rules for twelve localized skeletal vertices.
Both the original and revised question pages print the same neutral molecular formula; the revised copy does not supply a charge, a drawing, or an explicit instruction about these electrons.[ref]
Revised question 33 of the 2022 Cambridge Part III Chemistry Paper 2. Part (d)(iii), physical page 81 of the later public bundle; the formula is unchanged from the original question
The primary study identifies the open cluster with the two genuine groups and reports their hydroalumination to give the capped aluminium cube. Its accessible abstract does not establish the orbital partition needed to select the four-electron convention.[ref]New Carbaalanes Containing Clusters of Aluminum and Carbon Atoms by Werner Uhl and others. Abstract: open cluster 2 and its conversion to compound 4; Organometallics 19 (2000), 4536–4543
Conditional conclusion: the advertised -electron equality follows under the explicit extended-framework convention above. Without evidence that those pairs belong in the cluster electron pool, retain the -electron localized count and this qualification. The source checks do not justify asserting a typo or inventing a charge to force agreement.