New findings in the arene chemistry of the 3d transition metals

This article surveys some new organometallic chemistry with the bicyclic aromatic hydrocarbon naphthalene. Aspects to be covered include the synthesis and properties of mononuclear complexes having q2, q 4 or q6-bonded naphthalene ligands. Examples of complexes of the latter type are CpM(q6-C,,H,) (M = Cr, V), which because of the lability of the arene ligands (naphthalene ligand effect) are valuable sources of CpM fragments. Also reported are the synthesis and structures of a series of homoand heterodinuclear complexes in which the bicyclic arene functions as a bridging ligand. Finally, the two-step preparation of CpFe(qS-Cl,H8-H) and CpFe(q '-C1,H8-D) starting from ferrocene is presented. These compounds, which can be regarded as a naphthalene molecule hydroor deuterometallated by CpFe-H or CpFe-D fragments, undergo the retro-hydro(deutero)metallation very easily when reacted with suitable substrates, liberating naphthalene. Thus, unlike the parent compound CpFe(q '-C,H,-H), these two benzocyclohexadienyl iron(I1) complexes are ready sources of CpFe fragments with metal-H or metal-D bonds. INTRODUCTION In the past couple of years our research work on the arene chemistry of the 3d transition metals has concentrated mainly on systcms involving mononuclear arenes such as benzene and alkylbenzenes"'c). The results can be summariied under the following aspects: 1. The first concerns the modes of coordination of monocyclic arenes in homoand heterodinuclear complexes. We have found that arene bridges having all six ring carbon atoms metal-coordinated are capable of bonding two metal centres in at least four different ways. Two examples with benzene as the bridging l iand are shown in Fig. 1. Fig. 1. Benzene as a bridging ligand in the triple-decker sandwich complex 1 and in the dicobalt complex 2. 2. The second aspect relates to the addition of the metal-hydrogen bond to unsaturated hydrocarbons (hydrometallation), which was first established unambiguously for aromatic hydrocarbons when we demonstrated this type of reaction by the hydrocobaltation of benzene with diphosphinemonohydridocobalt fragments.