The most recent results obtained on organometallic derivatives of Ti-Zr-Hf and V-Nb-Ta in low oxidation states, especially with CO and arene ligands, are reviewed. The primary reaction products in the MX(n)/Al/AlX3/arene system, have been shown to contain the eta(6)-arene metal complexes in the oxidation state (I) (vanadium) or (II) (niobium, zirconium, hafnium). Stoicheiometric and catalytic reactions of the eta(6)-arene complexes of titanium and zirconium with disubstituted acetylenes have been studied: the zirconium(II)-tetrachloroaluminato complex Zr(eta(6)-benzene)(AlCl4)2 was shown to catalyze the trimerization of butyne-2 to hexamethyl-dewar-benzene initially, followed by its isomerization to hexamethylbenzene.
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