UB3LYP calculations with the TZVP, DGDZVP and 6-31+G(2d,p) basis sets have been used to explore the potential energy surfaces of CoHn (n = 1-3). A hydrogen rearrangement process with a low activation energy was found in the H-Co center dot center dot center dot H-H complex. This process requires elongation of the H-H bond and a concomitant shortening of the distance between the hydride and the vicinal hydrogen of the H-2 ligand. In addition, we present a classification of cobalt-hydrogen interactions in the CoHn complexes based on topological parameters of the charge density. (C) 2010 Elsevier B.V. All rights reserved.