Nature Chemistry 3, 19–27 (2011); published online 15 December 2010; corrected after print 18 April 2012.

In the version of this Article originally published, in Fig. 1d the energy levels 4P and 6P were interchanged, and should have appeared as shown below. Also, the start of the final sentence of the section 'Interplay of exchange and orbital promotion energies' should have read: “Finally, S = 5/2 becomes virtually degenerate with the ground state...”. These errors have been corrected in the online versions of the Article.