Synthesis of tetranuclear cobalt—molybdenum butterfly clusters with alkyne ligands: crystal structure of [Co 2 Mo 2 (µ 4 -C 2 Me 2 )(µ-CO) 4 (CO) 4 (η-C 5 H 5 ) 2 ]

The dimolybdenum alkyne complexes [Mo2(µ-R1CCR2)(CO)4(η-C5H5)2](R1= R2= H, Me or CO2Me; R1= H, R2= Me, Ph or CO2Me) reacted with dicobalt octacarbonyl in refluxing toluene to give the 60-electron tetranuclear clusters [Co2Mo2(µ4-R1C2R2)(µ-CO)4(CO)4(η-C5H5)2] in moderate to good yields. The crystal structure of the complex derived from but-2-yne (R1= R2= Me) has been determined: monoclinic, space group Cc, a= 34.74(2), b= 8.7970(10), c= 15.910(5)Å, β= 102.93(4)°, Z= 8. The cluster contains a butterfly-type core in which the two molybdenum atoms form the wingtips and the two cobalt atoms the hinge, with the alkyne ligand bridging the four metals to form an octahedral structure. All of the cobalt–molybdenum edges are asymmetrically bridged by carbonyl ligands which are bonded more strongly to molybdenum (average Mo–µ-Co 1.980 Å;) than to cobalt (average Co–µ-CO 2.267 Å).