Reaction of vinylacetylene with [Mo(.eta.5-C5H5)(CO)2]2. Reactivity of [{Mo(.eta.5-C5H5)(CO)2}(.mu.-HC.tplbond.CCH:CH2)]. Crystal structures of [{Mo(.eta.5-C5H5)(CO)2}2(.mu.-.eta.2,.eta.3-HC.tplbond.CCHCH3)][BF4] and of [[{Mo(.eta.5-C5H5)(CO)2}2]2-.mu.-(.mu.-HC.tplbond.CCH2CH2)2]