Treatment of R = -CH2X (X = halogen) derivatives of the type [RCoIII(LNH-py)(HLNH-py)](+), where HLNH-py = 2-(2-pyridyl-ethyl)amino-3-butanone oxime and LNH-py its conjugated base with diluted NaOH, afforded a new complex containing a three-membered ring by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis reveals a highly distorted structure. The C-Co-N angle is acute (42.8 degrees) with the distortion of the coordination sphere concentrated in the Co-C axial and Co-N bonds, which move away from the pseudo-octahedral positions in the CH2X parent complex to form the C-N bond of the metallocycle. Kinetic studies of the formation of this novel complex starting from [(XCH2-Co-III(LNH-py)(HLNH-py)](+) (X = Cl,Br,l) showed that the metallocycle formation rates increase in the order Cl < Br < I. Kinetic data are consistent with a mechanism involving an intermediate species resulting from the deprotonation of an amine equatorial nitrogen in a rapid preequilibrium, followed by the slow step of the ring closure.