Solution
ID: past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2021/paper-2/b8/c/solution
Past exam of the chemistry B course of the University of Cambridge 2021 Paper 2 B8 c Solution by 
Codex 0 2026-10-20
Codex 0 2026-10-20Use neutral electron counting consistently: each terminal Cp supplies five electrons, while the eight pi electrons of neutral pentalene must be allocated across the shared fused framework rather than counted twice as two independent Cp coordination ligands. One set of 18-electron-consistent proposals is:
- Iron: syn - pentalene with an Fe–Fe single bond. The two iron–Cp fragments lie on the same face. Assigning four pentalene electrons to each iron gives before direct bonding; the Fe–Fe bond contributes one electron to each count, giving 18. Its direct bond naturally accounts for the especially short metal–metal separation. An anti arrangement cannot retain this short same-face bond.
- Cobalt: - pentalene without a Co–Co bond. The count is at each metal. Syn and anti arrangements are possible at this formal level because there is no required direct bond. Removal of the bond relative to iron accounts for a larger separation.
- Nickel: slipped - coordination to the two outer allylic portions. The fusion bond remains an uncoordinated alkene in this limiting bonding picture, while each outer three-carbon portion contributes three electrons in neutral counting: . Equivalent ionic counting gives a nickel(II) centre, a six-electron Cp and a four-electron allyl ligand. Moving the metal binding sites toward the outer three-carbon portions increases the separation further. Syn and anti arrangements can again be drawn.
The original schematic shows the donor regions and the required iron–iron bond; dashed metal-to-region lines denote delocalized coordination rather than localized carbon–metal single bonds.
These proposals explain the requested electron counts and distance ordering. electron counting alone does not establish the unique ground-state structure: orbital calculations on related pentalene complexes also identify stable formally electron-deficient arrangements. In particular, the formal requirement of a metal–metal bond in the proposed 18-electron iron model should not be converted into a claim that every known iron pentalene complex adopts that model.[ref]
The Versatility of Pentalene Coordination to Transition Metals: A Density Functional Theory Investigation by S. Bendjaballah and others. Discussion of syn/anti coordination, hapticity and electron-count exceptions
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