AROMATIC-HYDROCARBONS, CARBOCYCLIC LIGANDS SPANNING SEVERAL OXIDATION-STATES IN BOTH MAIN-GROUP AND TRANSITION-ELEMENTS - RECENT ADVANCES WITH EARLY TRANSITION-D ELEMENTS

In this paper some synthetic procedures to obtain (eta(6)-arene)metal derivatives are reviewed. The metal-atom-arene-vapour co-condensation technique is the most appropriate to generate complexes of polycyclic aromatic hydrocarbons or heterocycles. As far as the aluminium halide-mediated synthesis is concerned, two classes of reaction are observed. When AlX(3) is used with a metal halide in the presence of an aromatic hydrocarbon in the absence of any reducing agent, AlX(3) can function as a dehalogenating agent, to give ionic compounds of general formula [M(eta(6)-arene)n](AlX(4))(m), or it can add across the M-X bond with formation of M(mu-X)(n)AlX(4-n), systems. In both cases the metal displays its typical oxidation state. However, the use of AlX(3) in combination with aluminium (the Fischer-Hafner reducing system) affords ionic or covalent low-oxidation-state metal(eta(6)-arene) complexes. Attention is focused on our most recent results concerning the synthesis, properties and reactivity of eta(6)-arene derivatives of Group 4 and 5 elements, showing, inter alia, the first example of a tetraarylborate anion behaving as a 12-electron donor to one metal atom and low-valent eta(6)-arene compounds as useful reagents in the inorganic and coordination chemistry of the corresponding metal in nonaqueous systems.