Organometallic compound

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Organometallic compounds are those compounds having bonds between an one or more metal atoms and one or more carbon atoms of an organyl group. They are classified by prefixing the metal with organo-, e.g. organopalladium compounds. In addition to the traditional metals and semimetals, elements such as boron, silicon, arsenic and selenium are considered to form organometallic compounds. Examples include organomagnesium compounds such as iodo(methyl)magnesium MeMgI, diethylmagnesium (Et₂Mg); organolithium compounds such as butyllithium (BuLi), organozinc compounds such as chloro(ethoxycarbonylmethyl)zinc (ClZnCH₂C(=O)OEt); organocopper compounds such as lithium dimethylcuprate (Li⁺[CuMe₂]^–); and organoborane compounds such as triethylborane (Et₃B).

The status of compounds in which the canonical anion has a delocalized structure in which the negative charge is shared with an atom more electronegative than carbon, as in enolates, may vary with the nature of the anionic moiety, the metal ion, and possibly the medium; in the absence of direct structural evidence for a carbon–metal bond, such compounds are not considered to be organometallic.

Depending mostly on the nature of metallic ion and somewhat on the nature of the organic compound, the character of the bond may either be ionic or covalent. Organic compounds bonded to sodium or potassium are primarily ionic. Those bonded to lead, tin, mercury, etc. are considered to have covalent bonds, and those bonded to magnesium or lithium have bonds with intermediate properties.

Organometallic compounds with bonds that have characters in between ionic and covalent are very important in industry, as they are both relatively stable in solutions and relatively ionic to undergo reactions. Two important classes are organolithium and organomagnesium compounds. In certain organometallic compounds such as ferrocene or dibenzenechromium, the pi-orbitals of the organic moiety ligate the metal.

[edit] Organolithium compounds and Grignard reagents

These two classes of compounds generally involve having a high electron density on the carbon. This makes the carbon highly nucleophilic. These compounds are widely used in organic chemistry for carbon-carbon bond forming reactions. With less electropositive elements such as many transition metals, a wide variety of reactivites are observed.

[edit] Organolithium compounds

Organolithium compounds have the general formula R:Li. They can be prepared by the reduction of an organic halide with lithium metal.

[edit] Organomagnesium compounds

Main article: Grignard reagents

Organomagnesium compounds are also called Grignard reagents in tribute to the French scientist Victor Grignard, who discovered them. Grignard reagents have the general formula R:MgX (X being the halogen; as Mg has +2 charge, R has -1 and X has -1) and can be prepared by the free radical reaction between an organic halide and magnesium metal:

[edit] Behavior towards acidic media

Since both organolithium compounds and Grignard reagents are very basic in nature, they react with any slightly acidic compounds, including water. Care must be taken during their preparation or in the reactions which they are used. Dry, distilled ether or tetrahydrofuran (THF) are often used as a solvent because they are able to solvate the Mg²⁺ cation through the oxygen atom.

When they react with compounds that contain an acidic hydrogen (that is usually a hydrogen bonded to an electronegative atom), they abstract the hydrogen to form the conjugate alkane from the organometallic compounds. When studying such reactions the partial electronegativities must be taken into consideration as they are essential in the reaction mechanism. The organic part of the organometallic compound has partially negative charge, while the metal part has partially positive charge).

A general reaction:

R:MgX + H:Cl → R:H + Cl^- + Mg⁺² + X^-

This basic nature of Grignard reagents and organolithium compounds cause some problems in their preparation and usage. Trying to prepare a Grignard reagent from an organic halide that contains a acidic hydrogen is futile and results in other compounds.

[edit] Formation of alcohols using organometallic compounds

Grignard reagents can also act as nucleophiles due to the negative partial charge at the organic part. In such, they can be used to synthesize alcohols in few ways.

One such way is to react an Grignard reagent with an epoxide. Due to the oxygen atom, the organic chains attached usually carry a partially positive charge. Grignard reagents being nucleophiles attack epoxides from these sites. Being basic they attack the less substituted carbon atom of the expoxides as the reaction mechanism is SN2 (substitution, nucleophilic, bimolecular). The result of this reaction (whether tertiary, secondary or primary alcohol) depends on the type of expoxide used.

The other method of preparing an alcohol is to react the Grignard reagent with carbonyl compounds. Because carbonyl compounds contain an electronegative oxygen and a double bond, the carbonyl carbon is partially positive and is prone to nucleophilic attacks. Grignard reagents react with;

1. formaldehydes to give primary alcohols
2. all other aldehydes to give secondary alcohols
3. ketones to give tertiary alcohols
4. esters to give ketones and then tertiary alcohols

Organolithium compounds can also be used to synthesize alcohols from carbonyl compounds.

The electronegativity of Grignard reagents and organolithium compounds also cause problems in their preparation. A Grignard reagent from any alkyl halide containing, carbonyl, epoxy, nitro or cyano groups can not be prepared.simple:Organometallic compound