Tetra(7-azaindolato)ditungsten: effect of ligand shape and rigidity on the length of a tungsten-tungsten quadruple bond

Ditungsten(II) tetrabenzoate reacts with the lithium salt of 7-azaindole to form tetrakis( 7-azaindolato) ditungsten in 75% yield. The compound crystallizes as the tetrahydrofuran solvate, W-2(C7H5N2)(4).2THF and its structure has been determined by X-ray crystallography. It forms crystals in space group P2(1)/c, with the following unit cell dimensions: a=9.185(2), b=13.263(3),c=14.168(8) Angstrom,beta=104.46(3)degrees, V=1671(4) Angstrom(3), Z=2. The molecules reside on crystallographic centers of inversion and the ligands are disordered, necessitating refinement by a method employing rigid-body treatment of oppositely oriented half-molecules of the ligand at each position. The W-W distance, 2.227 2) A, is the longest one so far observed for a ligand of this type and it is argued that this is due to the geometry and rigidity of the bidentate bridging 7-azaindolate ligand. The greater W-W bond length, in turn, causes the delta-->delta* electronic transition to fall at a relatively low frequency, 16 000 cm(-1), where it is very clearly resolved from strong absorption at higher energies.