Chemistry of o-xylidene-metal complexes. Part 4. Stereospecific synthesis of the early transition metal meso-metallacycles: [M{CH(SiMe3)C6H4CHSiMe 3-o}(η-C5H5)2] (M = Ti, Zr, Hf, or Nb), their reversible one-electron reduction (M = Ti, Zr, Hf, or Nb) and oxidation (M = Nb); and the X-ray crystal structure of the zirconium complex

The reaction of the organodilithium reagent [{o-C₆H₄(CHSiMe₃)₂}{Li(tmen)} ₂] (tmen = Me₂NCH₂CH₂NMe₂) and the appropriate metallocene(IV) chloride [M(η-C₅H₅)₂Cl₂] in OEt₂ at ca. 35 °C yields the corresponding crystalline, thermally stable, stereospecifically pure meso-metallaindane meso-[[graphic omitted]HSiMe₃-o}(η-C₅H₅)₂] [M = Ti (2), Zr (3), Hf (4), or Nb (5)]; by-products in two of these reactions are [{Ti(η-C₅H₅)₂Cl}₂] or the binuclear zirconium compound (6) formulated as meso-[[graphic omitted]HSiMe₃-o}(μ-σ: η-C₅H₄)₂Zr(η-C₅H ₅)₂]. Compounds (2) - (5) are sublimable at ca. 140 °C (10^-3 Torr), and on pyrolysis afford o-C₆H₄(CH₂SiMe₃)₂; they are reasonably air-stable and inert to CO under ambient conditions. Compound (6) is also accessible from (3) in OEt₂ and successively Li(tmen)Buⁿ in n-C₆H₁₄ and [Zr(η-C₅H₅)₂Cl₂]. Assignment of the meso (rather than rac) diastereoisomeric configuration for complexes (2) - (4) rests on their n.m.r. spectra and for (3) on X-ray data. E.s.r. data on complex (5), or the Na(C₁₀H₈)-tetrahydrofuran reduction products of each of (2) - (4), are consistent with each being a d¹ complex. The electrochemical reduction [-E†^red = 1.46 (Ti), 2.02 (Zr), or 2.26 V (Hf)] of each of the complexes (2) - (4) is pseudo-reversible, but the anions [[graphic omitted]HSiMe₃-o}(η-C₅H₅)₂] ^- tend to lose C₅H₅^-. The niobium complex (5) undergoes reversible one-electron oxidation (E†^ox = -0.47 V) or reduction (-E†^red = 1.63 V). The X-ray structure of the title metallaindane (3) shows pseudo-tetrahedral co-ordination of Zr with a centroid-Zr-centroid' angle of 125.1° and bite angle C^α-Zr-C^α′ of 80.2(2)°, with <Zr-C^α, C^α′> 2.305(4) and <Zr ⋯ C^β,C^β′> 2.71(1) Å; hence the o-C₆H₄{CH(SiMe₃)}₂-metal bonding is best described as intermediate between that appropriate for metalla(IV)cyclic and an η⁴-5,6-dimethylenecyclohexa-1,3-diene-metal(II) structure; consistent also is the fold angle, Φ, of 66.7° (Φ being the dihedral angle between the ZrC^αC^α′ plane and the C₈ extension of the aromatic plane).