Figure 1.

Key intermediates in the proposed mechanism for low-temperature gas generation in marine shales. M denotes a low-valent transitions metal capable of forming two metal-carbon bonds. The central complex is a metallocyclobutane interconverting with olefin-carbene-metal complexes, an ethylidene carbene complex to the left and a methylene carbene complex to the right. This is the Chauvin mechanism proposed for olefin metathesis [19].