The chemoselective cleavage of a six-membered aromatic ring in biphenylene is reported using an aluminium(I) complex. This type of selectivity is unprecedented. In every example of transition metal mediated C-C sigma-bond activation reported to date, the reaction occurs at the central four-membered ring of biphenylene. Insight into the origin of chemoselecitivty was obtained through a detailed mechanistic analysis (isolation of an intermediate, DFT studies, activation strain analysis). We conclude that the divergent reactivity can be attributed to differences in both the symmetry and radial extension of the frontier molecular orbitals of the aluminium(I) fragment compared to common transition metal fragments.