Nature Chemistry 1, 302–309 (2009); published online: 7 June 2009; corrected after print 24 June 2009

In the version of this article originally published, corrections were needed to some units and values of thermodynamic parameters: (1) page 303, penultimate sentence of column 1 should read: (ΔS) = −11.2 ± 9.4 cal K−1; (2) page 305, top of column 2 should read: (ΔG298 = 20.3 ± 0.1 kcal mol−1, ΔH = 23.4 ± 3.4 kcal mol−1, and ΔS = 10.2 ± 11.4 cal K−1). The authors wish to confirm that the corrections have no implications for their conclusions or the mechanistic proposal they have put forward. These changes have now been corrected on the HTML and PDF versions of this article.