Metal-metal multiple bonds. 20. Structural, kinetic, and theoretical studies of the reactivity of dimolybdenum .mu.-alkylidenes toward small molecules. Structures of M2(.mu.-CAr2), M2(.mu.-CAr2'), and M2(.mu.-.eta.1,.eta.2-iso-PrNC:CAr2') [M = Cp(or Cp')Mo(CO)2; Ar = p-tolyl, Ar2' = 2,2'-biphenylenyl, and Cp' = C5H4Me]