Reaction of [Mo,(CO)~(~-HC,H) (?-C,H,),] with Me0,CC-CC0,Me affords several products, one of which, of molecular formula [Mo,((HC,H) (Me02CC2C0,Me),)(q-C5H5)2] ( l ) , is here the subject of an X-ray diffraction study. Crystals of (1) are triclinic, space group P7 (no. 2) with Z = 2 in a unit cell of dimensions a = 11.519(7), b = 18.602(9), c = 8.709(3) A, ct = 86.65(4), p = 100.99(4), y = 97.54(5)', and they encapsulate two molecules of solvent (CH,CI,) per molecule of complex. The structure has been refined to R 0.054 for 3 450 reflections. The acetylenic moieties have joined to form a c 8 chain ; within this chain the terminal atoms and the two central atoms are attached to one of the metal atoms, while the first three and the last three form x-ally1 attachments to the other metal atom. The (C,H,)Mo=Mo(C,H,) unit and the C8 chain together form a framework of mirror symmetry, but the molecule as a whole does not possess C, symmetry because of the methoxycarbonyl ligands which are bonded to C(11), C(12), C(13), C(14), C(17), and C(18). When a similar reaction is carried out with [MO,(CO)~(~-MeO,CC,CO,Me)(q-C5H5),] as the starting material, one of the products is again a complex containing a c8 chain with methoxycarbonyl ligands on all eight carbon atoms, [Mo,{p-( Me0,CC,C02Me),}(q-C5H5)2] (2). An X-ray diffraction study of (2) reveals a remarkable difference in the bonding mode. Crystals of (2) are monoclinic, space group P2,lc (no. 14) with Z = 4 in a unit cell of dimensions a = 14.737(2), b = 11.224(2), c = 23.882(5) A, p = 11 5.92(1)', and the structure has been refined to R 0.088 for 4 897 reflections. The molecule (2) has idealised C, symmetry (not crystallographically required), and the C, chain forms a ' flyover ' from one metal atom to the other. The two groups of four carbon atoms are each planar (and co-planar with the o-bonded metal atom) and form a diene-type interaction with the other metal atom. A further marked difference from ( 1) is that the central bond of the C8 chain, which in (1) is orthogonal to the Mo=Mo link, is in ( 2 ) parallel to that direction. The metalmetal distances, 2.618(1) A in (1) and 2.635(1) A in ( 2 ) , are indicative of Mo-Mo double bonding.