Dimeric structures tetravalent complexes

Tetravalent tungsten complexes contain the structural units WO +, or dimeric configuration having W= W double bonds, as well as trinuclear clusters with three W atoms bound together in a triangular configuration. 

In connection with the mechanistic interpretations of stereocontrol, either (LXII) or (LXIII) is consistent with the effect of a bulky alkoxide group. This is because alkyl-alkoxide exchange reactions could place an alkoxide group on aluminum where growth occurs. Structure (LXIII) seems reasonable in view of the known tendency of aluminum alkoxides to dimerize (McElvain and Davie, 1951 Hoffmann, 1960) however, a monomeric structure (LXII) cannot presently be excluded. The metal alkoxides, like alkyllithiums, probably involve equilibria among monomeric and associated species. Therefore, one or more active species may be involved. Whatever the species, it is not too difficult to imagine a complex composed of a tetravalent aluminum, in which monomer-chain or monomer-chain-alkoxide interactions favor one path for monomer addition. 

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