A tungsten complex containing a highly delocalized metal-ligand system.

Nucleophilic attack of (triphenylphosphonio)cyclopentadienide on the dichlorodiazomethane-tungsten complex trans-[BrW(dppe)(2)(N(2)CCl(2))]PF(6) [dppe is 1,2-bis(diphenylphosphino)ethane] results in C-C bond formation and affords the title compound, trans-[W(C(24)H(18)ClN(2)P)Br(C(26)H(24)P(2))(2)]PF(6) x 0.6CH(2)Cl(2). This complex, bis[1,2-bis(diphenylphosphino)ethane]bromido{chloro[3-(triphenylphosphonio)cyclopentadienylidene]diazomethanediido}tungsten hexafluorophosphate dichloromethane 0.6-solvate, contains the previously unknown ligand chloro[3-(triphenylphosphonio)cyclopentadienylidene]diazomethane. Evidence from bond lengths and torsion angles indicates significant through-ligand delocalization of electron density from tungsten to the nominally cationic phosphorus(V) centre. This structural analysis clearly demonstrates that the tungsten-dinitrogen unit is a powerful pi-electron donor with the ability to transfer electron density from the metal to a distant acceptor centre through an extended conjugated ligand system. As a consequence, complexes of this type could have potential applications as nonlinear optical materials and molecular semiconductors.