Homoleptic complexes in which alkyl or aryl groups are η1-bound to a transition metal form one of the cornerstones of organometallic chemistry. For example, most transition metals form both neutral and anionic complexes with σ-bonding aryl ligands. This chapter discusses the synthesis of the dimesityl iron (II) dimer (FeMes2)2 and its pyridine complex. It describes a series of homoleptic aryl complexes of the first row transition elements manganese, iron, and cobalt. The chapter features the transition metal elements iron and cobalt coordinated to naphthalene or anthracene in an η4 fashion. This coordination mode is indicative of weakened metal ligand bonding. Complexes with naphthalene or anthracene bonded in this way are of interest primarily because of their utility as synthons. It has been recently shown that sterically demanding m-terphenyl ligands can stabilize two-coordinate transition metal centers, in homoleptic complexes.

The tetrylenes Ge(ArMe6)2 and Sn(ArMe6)2 (ArMe6 = C6H3-2,6-(C6H2-2,4,6-(CH3)3)2) reacted with dimethylzinc to afford the insertion products (ArMe6)2Ge(Me)ZnMe (1) and (ArMe6)2Sn(Me)ZnMe (3), which feature Ge–Zn and Sn–Zn bonds as well as two-coordinate zinc atoms. Crystals of 1 were found to be unsuitable for X-ray crystallography, so the ethyl-substituted (ArMe6)2Ge(Et)ZnEt (2) was synthesized in a parallel way to provide crystals suitable for X-ray studies. These showed the structure to be similar to that of 3. The reaction of Pb(ArMe6)2 with dimethylzinc yielded ArMe6ZnMe with a linearly coordinated zinc atom via ligand exchange but no characterizable, new lead product was obtained. The reaction of Sn(ArMe6)2 with dimethylzinc is reversible in hydrocarbon solution at room temperature and displayed a dissociation constant Kdiss and a ΔGdiss of 0.0028(5) and 14(4) kJ mol–1 at 298 K, respectively. Compounds 1–4 were characterized by NMR and IR spectroscopy as well as by X-ray crystallography for 1, 3, and 4.

Reactions of the divalent germylene Ge(Ar-Me6)(2) (Ar-Me6 = C6H3-2,6-{C6H2-2,4,6-(CH3)(3)}(2)) with water or methanol gave the Ge(IV) insertion product (Ar-Me6)(2)Ge(H)-OH (1) or (Ar-Me6)(2)Ge(H)OMe (2), respectively. In contrast, its stannylene congener Sn(Ar-Me6)(2) reacted with water or methanol to produce the Sn(II) species {(ArSn)-Sn-Me6(mu-OH)}(2) (3) or {(ArSn)-Sn-Me6(mu-OMe)}(2) (4), respectively, with elimination of (ArH)-H-Me6. Compounds 1-4 were characterized by IR and NMR spectroscopy as well as by X-ray crystallography. Density functional theory calculations yielded mechanistic insight into the formation of (Ar-Me6)(2)Ge(H)OH and {(ArSn)-Sn-Me6(mu-OH)}(2). The insertion of an m-terphenyl-stabilized germylene into the O-H bond was found to be catalytic, aided by a second molecule of water. The lowest energy pathway for the elimination of arene from the corresponding stannylene involved sigma-bond metathesis rather than separate oxidative addition and reductive elimination steps. The reactivity of Sn(Ar-Me6)(2) with water or methanol contrasts with that of Sn{(CH(SiMe3)(2)}(2), which affords the Sn(IV) insertion products {(Me3Si)(2)CH}(2)Sn(H)OH and {(Me3SO2CH}(2)Sn(H)OMe. The differences were tentatively ascribed to the Lewis basicity of the employed solvent (Et2O vs THF) and the use of molar vs millimolar concentrations of the substrate.