The sequential linking of alkynes at dichromium and dimolybdenum centres; X-ray crystal structure of [Cr 2 (CO)(µ-C 4 Ph 4 )(η-C 5 H 5 ) 2 ]

In heptane at reflux, the alkynes RCCR (R = Ph, H, or CO2Me) react with [Cr2(CO)4(η-C5H5)2] to give complexes [Cr2(CO)(µ-C4R4)(η-C5H5)2]. An X-ray diffraction study on the product from PhCCPh shows that the crystals of [Cr2(CO)(µ-C4Ph4)(η-C5H5)2] when grown from dichloromethane–hexane incorporate ¼CH2Cl2 per molecule of complex and are orthorhombic, with Z= 8 in a unit cell of dimensions a= 19.569(4), b= 19.731(5), c= 16.637(2)Å, and space group Pbcn(no. 60). The structure has been solved by heavy-atom methods from 3 166 data for which l 3.0σ(l), collected on a four-circle diffractometer, and refined to R 0.066. The axis of the molecule comprises a (η-C5H5)CrCr(η-C5H5) moiety which is non-linear, with the cyclopentadienyl rings in an unsymmetrical trans relationship to one another, and with a carbonyl ligand semi-bridging in a plane which is effectively a mirror plane for the whole molecule. Two PhCCPh molecules have joined to form a four-carbon chain, of which the two terminal atoms form a quasi-tetrahedral group with the two chromium atoms [CrCr 2.337(2), Cr–C 2.025(7) mean, C ⋯ C(non-bonded)ca. 2.7 Å], and the two central atoms are π-bonded to that Cr atom which does not carry the carbonyl ligand. The two metal atoms lie on opposite sides of the plane through the four-carbon portion of the CrC4Ph4 ring. The alkynes RCCH (R = Ph or Me) react with [Cr2(CO)4(η-C5H5)2] to yield as major products the complexes [Cr2(CO)(η-C4H2R2)(η-C5H5)2] in which there is a head-to-tail arrangement of the Ph or Me groups in the C4Cr rings. However, with phenylacetylene the isomer containing the ring system [graphic omitted]Ph was also detected. The compounds [Mo2(CO)n(η-C5H5)2](n= 4 or 6) react with PhCCPh in octane at reflux to give [Mo2(CO)(µ-C4Ph4)(η-C5H5)2], and similarly [Mo2(CO)4(η-HC2H)(η-C5H5)2] affords [Mo2(HC2H)(PhC2Ph)2(η-C5H5)2], a ‘fly-over’ complex containing a six-carbon chain bridging a MoMo bond. Reaction of [Mo2(CO)4(η-HC2H)(η-C5H5)2] with RCCR (R = CO2Me) yields the complexes [Mo2(CO)2(µ-C6H2R4)(η-C5H5)2](two isomers) and [Mo2(η-C8H2R6)(η-C5H5)2](two isomers) in which C6 and C8 chains bridge Mo–Mo and MoMo bonds respectively. The structures of these species were deduced from 1H and 13C n.m.r. spectra. Reaction of [Mo2(CO)4(µ-RC2R)(η-C5H5)2] with RCCR (R = CO2Me) gives [Mo2(CO)2(µ-C6R6)(η-C5H5)2] and [Mo2(µ-C8R8)(η-C5H5)2](two isomers), characterised inter alia by their 1H and 13C n.m.r. spectra. Treatment of [Mo2(HC2H)(PhC2Ph)2(η-C5H5)2] with MeO2CCCCo2Me gives two isomers of compositon [Mo2(HC2H)(PhC2Ph)2(MeO2CC2CO2Me)(η-C5H5)2], thereby establishing that the complexes with four linearly linked alkynes are formed from the species with three such linked groups. Possible mechanisms for carbon-chain growth on the dichromium and dimolybdenum centres are discussed and are related to the changes in multiplicities of the metal–metal bonds.