Reactions of the title heterodinuclear fulvalene complex with alkynes give rise to distinct products, including complexes contg. two- and four-electron-donating terminal and bridging alkyne ligands. Bound terminal alkynes rearrange to the corresponding alkenylidenes, allowing the first observation of the equil. between I (contg. a side-on bound alkenylidene) and II (featuring a semibridging alkenylidene). The asymmetry of the mixed metal systems permits the detection and kinetic anal. of various fluxional processes. The structures of I and II were detd. by x-ray crystallog.