Transition-metal-promoted ring-opening reactions of vinylcyclopropenes. 1,2,3,5-.eta.-Penta-2,4-dienediyl- and 1,5-.eta.-penta-2,4-dienediyl(1-metallacyclohexa-2,4-diene) complexes of rhodium(III) and iridium(III) and their conversion to (.eta.5-cyclopentadienyl)hydridometal compounds