ACETYLENE-VINYLIDENE TRANSFORMATION AND C-EQUIVALENT-TO-C BOND RUPTURE IN THE COORDINATION SPHERE OF MIXED-METAL CLUSTERS

The regularities of metal-assisted acetylene-vinylidene rearrangement are investigated. It is shown that in the coordination sphere of mixed -metal clusters electron-compensating processes occur after partial el imination of ligands (CO or Cp). In the first step PhC=CH disrupts the C-H bond with Re2(CO)8(mu-C-CPh)(mu-H) formation. This is followed by the addition of new electron deficient fragments (CpNi or Fe2(CO)6) t o give the clusters containing a mu3, eta2-coordinated phenylvinyliden e. Further decarbonylation results in the mu4, eta2-coordination of ph enylvinylidene and its dimer in the clusters with Re2Fe2- or Re2Ni2- c ores, respectively, where the multicentered pi-interaction takes place . Upon further decarbonylation the ortho-metallation of phenylvinylide ne is observed with the formation of metalindene-containing clusters a nd the formal electronic saturation of each metal atom. It is supposed that electron-compensating processes are the cause of the C=C triple bond cleavage in coordinated diphenylacetylene upon the addition of el ectron-deficient metal-containing fragments. The phenylcarbyne bridged clusters are isolated with Ni2Mo2- or Mo4Co3C- cores (in the latter c ase cleavage of the C-0 bond also takes place). These reactions are co mpared with carbyne-coupling processes, on the one side, and with clea vage of the Fe2S2(CO)6 molecule to give coordinated S=Fe(CO)3 fragment s, which are isolobal to RC=CR, on the other. The step by step formati on of the nonanuclear mixed-metal cluster, Mo2Fe7S8(CO)22, through iso lated clusters with MoFe2S2- and MoFe4S4-cores, is observed and discus sed in view of nitrogenase MoFe-cofactor modelling. It is shown that t he final cluster appears to be able to catalyse the transformation of N2 to NH3.