The hexaaminecobalt(III) complex [Co(tmptacn)](3+) ( tmptacn) 1,4,7-tris(2'- pyridylmethyl)-1,4,7-triazacyclononane) undergoes a novel base-catalyzed N- to C-bonded rearrangement in which a tacn nitrogen is displaced by the alpha-carbon which deprotonates and binds to the metal ion as a carbanion. The X-ray structure establishes the configuration for the regio- and stereoselectively (100%) formed product. The reaction involves both ring expansions and ring contraction. The carbanion is part of a strained four-membered ring. The kinetics are reported for the N-to C-rearrangement, shown to be retentive for the optically resolved (+)-[ Co( tmptacn)](3+) reactant, and also the kinetics for a competitive and somewhat faster base-catalyzed racemization reaction of this complex. The reaction is completely but very slowly reversed in acid, also with retention, and in D2O/D + there is 1: 1 D-incorporation into the two sets of inequivalent tacn carbons. Extensive 1D and 2D NMR studies establish mechanistic details, and alternative mechanisms are proposed for the forward and reverse reactions. In neutral solution, there is a competitive oxidation reaction for the reverse C- to N-bonded process, involving the regio- and stereoselective (100%) incorporation of an R-OH substituent into the tacn ring.