The reactions of bulky terminal alkynes, RC=CH [R = Bu-t, 1a; Me3Si, 1b; (cod)Rh(eta (5)-Me2C5H2), 1c] with [RhCl(cod)](2) in the presence of Et3N have been found to provide novel hydropentalenylrhodium complexes, (cod)Rh(eta (5)-R4C8H3) (2, 4 and 5). The structures have been determined by a single-crystal X-ray diffraction analysis. A mechanism involving the migration of an alpha -tert-butyl group or an alpha -hydrogen atom of the metal-vinyl intermediates is proposed.