The vast amount of research on the B-12 coenzyme catalysed 1,2-bond shift reactions now provides strong evidence for the conclusion that spin-spin coupled biradical intermediates (R-....(.)[Co]) are responsible for the facile rearrangements involved via pi -complexed (charge-transfer complexed) half-reaction states. The biradical intermediates are also likely to be involved in stereospecific [2+2] activation of the substrates.