Metal alkoxides: models for metal oxides. 2. Addition of ethyne, propyne, and 2-butyne to Mo2(OR)6 (M.tplbond.M) compounds (R = tert-Bu, iso-Pr, and neopentyl). Characterization of .mu.-alkyne and .mu.-C4H4 adducts and an evaluation of their role in alkyne oligomerization reactions