When a tantalum chloride cluster [(Ta6Cl12)Cl-2(H2O)(4)]center dot 4H(2)O (1) was treated in a nitrogen stream at temperatures above 300 degrees C, a catalytic activity developed for the isomerization of pentynes and hexynes to the corresponding conjugated dienes with smaller amounts of allenes. Niobium and tungsten clusters with the same metal framework also catalyzed the reactions. In a hydrogen stream, niobium, molybdenum, and tungsten clusters, and a trinuclear thenium cluster catalyzed the partial hydrogenation of alkynes to the corresponding alkenes. The isomerization activity was ascribed to the Bronsted acidity of hydroxo species of the activated cluster complexes, and the hydrogenation activity was attributed to the nature of the metal atoms incorporated in the cluster. (c) 2006 Elsevier B.V. All rights reserved.