0.1 kcal mol -1 , (ii) benzene has a calculated homolytic bond dissociation energy of 147.0 kcal mol -1 . The increased thermodynamically stability of C-C bonds in aromatic rings is unsurprising given to their character and electron delocalisation across the ring. C-C bonds are also sterically protected. They are often buried within the molecular framework and the orbitals involved in bonding are kinetically inaccessible. As such chemoselectivity becomes a key issue, with surrounding C-H bonds often the first sites to react with reagents and catalysts that would otherwise be capable of breaking C-C bonds. C-C bond activation has been achieved on the surface of heterogenous catalysts, within the active sites of enzymes, and under homogenous conditions using metal complexes. The systems which are best understood are arguably those that contained well-defined transition metal sites (Co-Ir, Ni-Pt), where partially occupied valence d-orbitals facilitate C-C bond breaking. Often model substrates that contain weakened C-C bonds and/or extensive ring strain are studied. For example, hydrocarbons with smaller ring sizes are routinely investigated as the C-C bond strength decreases across the series cyclohexane > cyclopentane > cyclobutane > cyclopropane. Reactivity tends to follow established mechanisms (Scheme 1).