Chemistry of o-xylidene-metal complexes. Part 3. Tungste o-xylidene complexes derived from tetrachloro(oxo)tungsteh(VI); X-ray crystal structures of [W(CH₂C₆H₄CH₂-o) ₃]·0.5C₆H₆ and [{W(CH₂C₆H₄CH₂-o)₂O} ₂Mg(C₄H₈O)₄]

Reaction of WCl₄O with the di-Grignard reagent o-C₆H₄(CH₂MgCl)₂ or the chloride-free 'o-xylidene' complex Mg(CH₂C₆H₄CH₂-o)(thf) in tetrahydrofuran (thf) yields either the thermally stable tris(chelate), [[graphic omitted]H₂-o)₃] (7), or the W^V complex [{[graphic omitted]H₂-o)₂[graphic omitted]g(thf)₄] (8) (a non-electrolyte in thf), in a manner critically dependent on reaction conditions. In both reactions a reduction step precedes alkylation, as determined by e.s.r., with probable intermediates being [WCl₄O]^-, [WCl₅O]^2-, and [WCl₃O(thf)₃]. The complex (7) undergoes reversible one-electron reduction as shown by cyclic voltammetry, E†^red = -1.68 V, and the W^V radical anion, g_av. = 2.005, is also accessible from (7) and sodium dihydronaphthylide. X-Ray analysis shows that the o-xylenediyl ligand (1) behaves as a chelating ligand in both (7) and (8), but in (8) the two ligands (1) are bonded differently. One is typically metallacyclic in nature, W-C^α 2.16(2) Å, W-C^α-C(aromatic) 109(1)° with a fold angle (Φ) of 42.4°, whereas the other has a larger Φ (66.1°) with <W-C^α> 2.16(4) Å, and W-C^α-C(aromatic) 98(2) and 93(1)°, possibly arising from an aromatic π interaction with the tungsten atom. For the tris(chelate) (7) all ligands are equivalent, the tungsten symmetry is quasi-3/m, with a more pronounced W-π-arene interaction, possibly of a stabilising influence [W-C 2.2₁, W-C(aromatic) 2.4₉ Å; W-C-C(aromatic) 82.₇°, Φ = 83.3°]. In complex (8), the magnesium atom is octahedrally co-ordinated, with the four thf ligands disposed equatorially; the WO bond is short, 1.71(1) Å.