Halide clusters, [(M6Cl12)Cl-2(H2O)(4)].4H(2)O (M: Nb, Ta) and (H3O)(2)[(M6Cl8)Cl-6].6H(2)O (M: Mo, W) with an octahedral metal framework, develop catalytic activity for the isomerization and slight hydrogenation of olefinic double bonds when the halide clusters are treated in a stream of hydrogen at elevated temperatures. In the case of Nb clusters, treatment above 200degreesC produces the catalytic activity. Raman spectra of such heat-treated clusters show that the cage breathing motion of Nb-6 remains intact, whereas the breathing vibrations of the edge bridging Nb-Cl of [Nb6Cl12] disappear. In the case of W clusters, the catalytic activity appears at 300degreesC, at which temperature the structure Of [(W6Cl8)Cl-4(H2O)(2)] begins to decay to form W6O12. Maximum activity emerges at 450degreesC, at which temperature the structure of W6Cl2 begins to decay. Elemental analyses of both clusters show the loss of Cl atoms. Thus, activation of these clusters occurs by partial loss of the halogen ligands, leaving the cluster framework intact. (C) 2002 Elsevier Science B.V. All rights reserved.