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The title compound, (η 5 -C 5 H 5 ) 2 Mo 2 (CO) 4 (μ-EtCCEt) in its crystalline form, has a molecular structure that lacks symmetry. Two (ηs-C 5 H 5 )Mo groups are connected by an MoMo bond 2.977(1) A in length with the usual sort of crosswise acetylene bridge. There are two terminal CO groups on Mo(1) and one on Mo(2). The fourth CO group is in a semi-bridging posture with Mo(2)—C = 1.936(6) and Mo(1)—C = 2.826(6). The 13 CO NMR spectrum at −126°C has six lines, indicative of the presence of two isomers, in each of which two CO's are either statically or dynamically equivalent. Complex changes occur in the spectrum as the temperature is increased so that at −40°C there is only a single line. A detailed interpretation of these spectra is not yet available.

Tris(trifluoromethyl)thallium, Tl(CF3)3, as well as other less defined trifluoromethyl-Group III compounds have been prepared for the first time by condensing metal atoms with trifluoromethyl radicals on a cryogenic surface. The unusually reactive nature of several of the products necessitated that a very unique and elaborate metal-atom reactor be designed and constructed which would allow the isolation of the products at sub-ambient conditions.

The compound (η 5 -Me 5 C 5 )Co(μ-CO) 2 Co(η 5 -Me 5 C 5 ) has been prepared and characterized. The crystal structure has been determined and the CoCo distance found to be 2.327(2) A. This structure completes the information on MM distances in the series of compounds CpM(EO)(E′O)MCp, where Cp = C 5 H 5 or Me 5 C 5 , M = Fe or Co and E = C or N, several members of which were previously studied by others. The EAN rule would predict a double bond here as in the isoelectronic compound CpFe(NO) 2 FeCp and the MM distances are virtually identical in the two molecules. Disordered packing of the molecules limits the accuracy with which the molecular dimensions, other than CoCo, may be determined. However, the structure is in generally close agreement with those of the other molecules in the set previously mentioned. The crystals are monoclinic with a 7.433(2) A, b 14.704(8) A, c 9.671(2) A, β 101.84(2)° and V 1034.6(7) A 3 ; Z = 2. The structure was refined to R 1 = 0.062 and R 2 = 0.074 using a disordered model in space group P 2 1 / m .

The title compound, (η5-C5H5)2Mo2(CO)4(μ-HC CH) has a molecular structure practically identical to that of its previously described analog containing μ-EtC CEt. Its13C NMR at −144°C has a broad doublet in the terminal CO region; this sharpens at −118°C then again broadens (-100°C) and finally coalesces below −58°C to a single resonance. The appearance of a semi-bridging CO (SBCO) ligand in the title compound and its EtC CEt analog, but not in a related compound with μ-H2C C CH2 is attributed to internal crowding and it is suggested that these compounds may provide the most unambiguous examples of such an effect.

The title compound was first obtained photolytically from (η 5 -Me 5 C 5 )Co(CO) 2 (by a mechanism not understood) and later by refluxing C 5 Me 5 H with Co 3 (CO) 9 CCH 3 . It was identified and characterized structurally by X-ray crystallography. The crystals belong to space group P 2 1 /n with a 9.529(2), b 18.325(4), c 14.801(3) A, β 95.69(2)° with Z = 4, making one entire molecule the asymmetric unit. Using 2641 reflections with F 0 2 > 3σ (F 0 2 the structure was solved and refined to R 1 = 0.051 and R 2 = 0.067. It consists of a triangular pyramid with two (η 5 -C 5 Me 5 )Co groups and one Co(CO) 3 group in the base and CCH 3 at the apex. The basal edge between the (η 5 -C 5 Me 5 )Co groups has a bridging CO group and a Co—Co distance of 2.405(1) A, while the other two unbridged Co—Co distances are 2.484(1) and 2.501(1) A. The distances from the apical C atom to the bridged Co atoms are 1.857(6) and 1.866(6) but 1.932(6) A to the unique Co atom. The C—C distance in the ethylidyne group is 1.497(9) A. The structure is closely similar to that of the analogous (η 5 -C 5 H 5 ) 2 Co 3 (CO) 4 CCH 3 compound.