Scandium, Yttrium, and Lanthanide Compounds

X-ray diffraction studies on yttrium and ytterbium compounds (C5H5)2MR2A1R2 have established their structures as of type 58 with the characteristically acute M-C-Al bond angle at the hypercarbon atom that shows involvement in a 3c-2e bond to the metal atoms while bonding normally, by three 2c-2e bonds, to the methyl hydrogen atoms. 

The scandium, yttrium, or lanthanide atoms in these compounds evidently form such bridges to make better use of their valence shell electrons and orbitals. (C5H5)2MR monomers would contain only 14 valence shell electrons (five from each cyclopentadienyl ligand, one from the alkyl group in addition to the three metal electrons), well short of the 16 or preferably 18 valence shell electrons normally present in stable organotransition metal compounds dimerization to (C5H5)2M(p-R)2M(C5H5)2 raises the valence shell electron count to 16 and uses a metal orbital that would be vacant in the monomer. 

The relative strength of the methyl bridges in the mixed metal compound (C5H5)2Y(p,-Me)2AlMe2 (58) was demonstrated by variable temperature ll NMR studies of the exchange of methyl groups between bridging and terminal positions. These afforded a value of about 16 kcal mol for the activation 

In recent years, there has been an upsurge of interest in bimetallic mixed M-Al complexes (where M is an early transition metal) with structures related to 58, because of their activity in Ziegler-Natta or Kaminsky-type alkene polymerizations.  

Two further complexes containing alkylaluminate residues linked through alkyl bridges to samarium or yttrium, respectively, are complexes 62 and 63  

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