Interestingly, although C=C bond formation represents the key thermodynamic driving force for the overall homologation reaction (with the final dimerization process being energetically downhill by ca. 70 kcal mol -1 ), experimental evidence for the lability of the central C=C bond in [C4O4] systems can be obtained from the reactions of isolated samples of [2-K(C7H8)]2 with alkali metal hydroborate reagents. In the case of the reaction with Li[ s Bu3BH] in benzene/THF, simple cation metathesis is observed, generating [Li(THF)]2[{(NON)Al}2(C4O4)], [2-Li(THF)]2, the structure of which closely resembles both [2-K(C7H8)]2 and [2-Li(OEt2)]2 (see Supporting Information). However, thermolysis with K[Et3BH] at ca. 65-70 °C in the same solvent system leads to 70 % conversion over a period of 14 d to a new species, the apparently analogous potassium salt [2-K(THF)]2. The structure of this species, however, can be shown by X-ray crystallography to feature the alternative syn disposition of substituents about the central C=C double bond (Scheme 2 and Figure ). The C=C/C-C and C=O/C-O bond lengths within the carbon chain are not significantly different from the anti form, but the [2-K(THF)]2 units are linked in the solid state via bridging K + cations into tetraaluminium 'dimer of dimers' (see Supporting Information). The lower solubility of this aggregate presumably drives the anti-to-syn isomerization process.