Density functional calculations are used to determine structures and stabilities of Pd-6 and hydride complexes Pd-6(H)(n) (n = 2, 4). Calculations show that a triplet state of Pd-6 with an elongated octahedral structure is the ground state and the Pd6 cluster can activate the second H-2 molecule. Barriers of the rate-determining steps for the first H-2 and the second H-2 activation process are ca. 11 kcal/mol and ca. 7-9 kcal/mol, respectively. In the dihydrogen complexes, the H-2-ligand behaves as donor in the molecular bonding between H-2 and Pd-6. Chemisorbed hydrogen atoms prefer to bridge the Pd-Pd edges in the most stable hydride complexes Pd-6(H)(n) (n = 2,4). (C) 2003 Elsevier Science B.V. All rights reserved.