In organometallic chemistry, bent metallocenes are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel, but are tilted at an angle. A common example of a bent metallocene is Cp2TiCl2. Several reagents and much research is based on bent metallocenes. Synthesis Like regular metallocenes, bent metallocenes are synthesized by a variety of methods but most typically by reaction of sodium cyclopentadienide with the metal halide. This method applies to the synthesis of the bent metallocene dihalides of titanium, zirconium, hafnium, and vanadium: 2 NaC5H5 + TiCl4 → (C5H5)2TiCl2 + 2 NaCl In the earliest work in this area, Grignard reagents were used to deprotonate the cyclopentadiene. Niobocene dichloride, featuring Nb(IV), is prepared via a multistep reaction that begins with a Nb(V) precursor: NbCl5 + 6 NaC5H5 → 5 NaCl + (C5H5)4Nb + organic products (C5H5)4Nb + 2 HCl + 0.5 O2) → [{C5H5)2NbCl}2O]Cl2 + 2 C5H6 2 HCl + [{(C5H5)2NbCl}2O]Cl2 + SnCl2 → 2 (C5H5)2NbCl2 + SnCl4 + H2O Bent metallocene dichlorides of molybdenum and tungsten are also prepared via indirect routes that involve redox at the metal centres. Structure and bonding Bent metallocenes have idealized C2v symmetry. The non-Cp ligands are arrayed in the wedge area. For bent metallocenes with the formula Cp2ML2, the L-M-L angle depends on the electron count. In the d2-complex molybdocene dichloride (Cp2MoCl2) the Cl-Mo-Cl angle is 82°. In the d1 complex niobocene dichloride, this angle is more open at 85.6°. In the d0-complex zirconocene dichloride the angle is even more open at 92.1°. This trend reveals that the frontier orbital, which is dz2, is oriented in the MCl2 plane but does not bisect the MCl2 angle. Reactivity Salt metathesis reactions Since bent metallocenes typically have other ligands, often halides, these additional sites are centers of reactivity. For example, reduction of zirconocene dichloride gives the corresponding hydrido chloride called Schwartz's reagent: (C5H5)2ZrCl2 + 1/4 LiAlH4 → (C5H5)2ZrHCl + 1/4 "LiAlCl4" This hydride reagent is useful for organic synthesis. Related titanium-based complexes Petasis reagent and Tebbe's reagent also feature bent metallocenes. Titanocene pentasulfide is used in research on polysulfur rings. Alkyne and benzyne derivatives of titanocene are reagents in organic synthesis. Reactions of Cp rings Although the Cp ligands are generally safely considered spectator ligands, they are not completely inert. For example, attempts to prepare titanocene by reduction of titanocene dichloride affords complexes of fulvalene ligands. Bent metallocenes