organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

, bent metallocenes are a subset of

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...

s. 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

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

s 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 Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pin ...

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 reagent Grignard reagents or Grignard compounds are chemical compounds with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromi ...

s were used to deprotonate the

cyclopentadiene Cyclopentadiene is an organic compound with the chemical formula, formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−. This colorless liquid has a strong and unpleasant odor. At room temperature, ...

. 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) → sub>2Ol₂ + 2 C₅H₆ :2 HCl + sub>2Ol₂ + 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. TebbeRgt.png,

Tebbe's reagent Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylidenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. ...

Cp4Zr2H2Cl2.png,

Schwartz's reagent Schwartz's reagent is the common name for the organozirconium compound with the formula (C5H5)2ZrHCl, sometimes called zirconocene hydrochloride or zirconocene chloride hydride, and is named after Jeffrey Schwartz, a chemistry professor at Princet ...

. Ansa-Metallocene.svg, Ansa-metallocene, X is the linker group, often (CH₂)_n or R₂Si (R = alkyl) Cp2Ti(CO)2.png,

Titanocene dicarbonyl Titanocene dichloride is the organotitanium compound with the formula ( ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slo ...

Structure and bonding

Bent metallocenes have idealized C_2v 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 Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air. Preparation and structure Zircono ...

the angle is even more open at 92.1°. This trend reveals that the frontier orbital, which is d_z2, is oriented in the MCl₂ plane but does not bisect the MCl₂ angle.

Reactivity

Salt metathesis reactions

Bent metallocenes typically have other ligands, often halides, which are centers of reactivity. For example, reduction of

gives the corresponding hydrido chloride called

: :(C₅H₅)₂ZrCl₂ + 1/4 LiAlH₄ → (C₅H₅)₂ZrHCl + 1/4 "LiAlCl₄" Related titanium-based complexes

Petasis reagent The Petasis reagent, named after Nicos A. Petasis, is an organotitanium compound with the formula Cp2Ti(CH3)2. It is an orange-colored solid. Preparation and use The Petasis reagent is prepared by the salt metathesis reaction of methylmagnesiu ...

and

also feature bent metallocenes. 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. GreenTitanoceneWeakM-M

Bent metallocenes derived from

pentamethylcyclopentadiene 1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic compound, cyclic diene with the formula , often written , where Me is Methyl group, . It is a colorless liquid. 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand ''1,2,3,4,5-penta ...

can undergo reactions involving the methyl groups. For example, decamethyltungstocene dihydride undergoes dehydrogenation to give the tuck-in complex. Tuck-inMLHGtung

The original example proceeded via sequential loss of two equivalents of H₂ from decamethyltungstocene dihydride, Cp*₂WH₂. The first dehydrogenation step affords a simple tuck-in complex: :(C₅Me₅)₂WH₂ → (C₅Me₅)(C₅Me₃(CH₂)₂)W + 2 H₂

Redox

When the non-Cp ligands are halides, these complexes undergo reduction to give carbonyl, alkene, and alkyne complexes that are useful reagents. A well-known example is

titanocene dicarbonyl Titanocene dichloride is the organotitanium compound with the formula ( ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slo ...

: :Cp₂TiCl₂ + Mg + 2 CO → Cp₂Ti(CO)₂ + MgCl₂ Reduction of vanadocene dichloride gives

vanadocene Vanadocene, bis(η5-cyclopentadienyl) vanadium, is the organometallic compound with the formula V(C5H5)2, commonly abbreviated Cp2V. It is a violet crystalline, paramagnetic solid. Vanadocene has relatively limited practical use, but it has been ex ...

Olefin polymerization catalysis

Although bent metallocenes are of no commercial value as olefin polymerization catalysts, studies on these compounds were highly influential on the industrial processes. Already in 1957 there were reports on the polymerization of

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

using a catalyst prepared from Cp₂TiCl₂ and trimethyl aluminium. Reactions involving the related Cp₂Zr₂Cl₂/Al(CH₃)₃ system revealed the beneficial effects of trace amounts of water for ethylene polymerization. It is now known that the partially hydrolyzed organoaluminium reagent methylaluminoxane ("MAO") gives rise to families of highly active catalysts. Work in this are led to

constrained geometry complex In organometallic chemistry, a "constrained geometry complex" (CGC) is a kind of catalyst used for the production of polyolefins such as polyethylene and polypropylene. The catalyst was one of the first major departures from metallocene-based cata ...

es, which are not bent metallocenes, but exhibit related structural features.