Cp2Mo2(CO)4, which contains a Mo-Mo triple bond, reacts with acetylenes RC=CR' in hydrocarbon solvents to form 1:1 adducts Cp2Mo2(CO)4(RCCR') in which the acetylenic moiety RCCR', acting as a four-electron donor ligand, bridges two molybdenum atoms that are also joined by a Mo-Mo single bond. The reactivity order HC=CH > MeC=CMe ~PhC=CH > CF3C=CH > CF3C=CCF3 ~PhC=CPh has been found for 1:1 adduct formation. Three of the Cp2Mo2(CO)4(RCCR) compounds, where R = H, Et, and Ph, have been structurally characterized by X-ray studies. There is a common Cp2Mo2(CO)4C2 moiety having (1) a quasi-tetrahedral M02C2 core with Mo-Mo distances of 2.96-2.98 Á and(2) an asymmetric 2 2(€0)4 unit, in which one molybdenum atom has two terminal carbonyl ligands while the other has one terminal and one semibridging carbonyl ligand. The occurrence of the semibridging carbonyl group is attributed to internal crowding. A number of these acetylene adducts have been characterized by infrared, NMR ( , 13C, and 13C[!H]), and mass spectrometry. The dynamical solution behavior of Cp2Mo2(CO)4(RCCR') compounds can be interpreted on the following assumptions. (1) A low-energy process interconverts Mo(l) and Mo(2), Cp(l) and Cp(2), and the carbonyl ligands a s d, b ^c, This process allows the Cp2Mo2(CO)4 moiety to adopt a time-averaged structure having C2 symmetry. (2) A higher energy process effects rotation about the Mo-Mo bond.(3) The central M02C2 moiety is rigid on the NMR time scale up to 90 °C. ( 4) Carbonyl ligand scrambling between molybdenum atoms is not observed on the NMR time scale at temperatures up to 90 °C. ( 5) For certain acetylene adducts, e.g., R = R' = Et, an isomer different from that found in the crystalline state is also present In equilibrium in solution. In Cp2Mo2(CO)4(HCCH) the acetylene is not labile toward ligand displacement reactions (with CO and MeC=CMe) or hydrogenation. 1.989 (3) C(l,3)-C(l,4) 1.395 (8) -C(3) 38.72 (8) -C(3) 2.902 (4) C(l,4)-C(l,5) 1.415 (7) -C(5) 45.34 (9) -C(5) 2.190(3) C(l,5)-C(l,l) 1.408 (7) -C(6) 47.50 (9) -C(6) 2.172 (3) C(1,1) 111.1 (1) -C(l,l) 2.308 (4) C(2,l)-C(2,2) 1.386 (9) C( 1,2) 93.0(1) -C( 1,2) 2.364 (4) C(2,2)-C(2,3) 1.41 (1) C( 1,3) 108.8 (2) -C( 1,3) 2.363 (4) C(2,3)-C(2,4) 1.30 (1) C( 1,4) 143.4 (2) -C( 1,4) 2.322 (4) C(2,4)-C(2,5) 1.36(1) C( 1,5) 146.6 (1) -C( 1,5) 2.310 (4) C(2,5)-C(2,l) 1.39(1)Mo(l )-Mo(2)-C(3) 68.5 (1) C( 5)-H(5)

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35) Calculations were performed using the Molecular Structure Corp. computer and the Enraf-Nonlus Structure Determination Package with programs written chiefly by Frenz and Okaya. (36) At the suggestion of a referee, solution was attempted In the space group P2,/n. The fractional coordinates of Table I do correspond to a nearly centrosymmetric molecule if the central aliene carbon, C( 16), and two of the carbonyl oxygen atoms, 0(2) and 0(4), are disregarded. A disordered model of the structure is possible in space group P2,/n with a crystallographic Inversion center relating the two halves of the molecule. After full-matrix least-squares refinement, final discrepancy values of R, = 0.046 and fi2 = 0.089 were obtained using the disordered centrosymmetric model. The results of this refinement led us to prefer the original description of the molecule which gave discrepancy values of R, = 0.030 and R2 = 0.046. In either case, however, the chemically significant structural parameters will be as reported here on the basis of the Pn space group. (37) D. T.

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