Multisite-bound vinylidenes on heterometallic clusters. Reformulation of (.eta.5-C5H5Ni)Ru3(CO)9(.mu.4-.eta.2-C:CHR) (R = tert-Bu) as hydrido-bridged (.mu.-H)(.eta.5-C5H5Ni)Ru3(CO)9(.mu.4-.eta.2-C:CHR). Molecular structure of the isopropyl derivative (R = iso-Pr) and an improved synthetic method