Organometallic chemistry

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Organometallic chemistry


<p>UNIT 2</p> <p>ORGANOMETALLIC COMPOUNDSOrganic compounds like alkanes, alkenes, alkynes and aryl compounds combine with certain ketals like Mg, Al, B, Zn, Pb and Na etc., to form a class of compounds called organometallic compounds. These compounds contain at least one C M bond, where C = carbon and M = metal. e.g., CH3MgI Methyl magnesium iodide CH3Li - Methyl lithium (CH3CH2)4Pb Tetraethyl lead</p> <p>ClassificationOrganometallic compounds are classified according to the nature of bonds present in them. They are classified as, 1. Ionic organometallic compounds 2. Covalent (or) bonded organometallic compounds 3. Complex (or) non-classically bonded organometallic compounds</p> <p>1. Ionic organometallic compoundsIn these compounds the carbon atom of the alkyl group carries negative charge. This negatively changed carbon is linked to a metal cation by a strong electrostatic force of attraction called ionic bond. They exhibit the properties of ionic compounds. e.g., CH3Na, C6H5K, RCCNa, (CH3)2Mg, (CH3)2Zn.</p> <p>2. Covalent (or) bonded organometallic compoundsThe alkyl or aryl carbon of these compounds are linked to the metal with a covalent bond. They exhibit the properties of covalent compounds. e.g., (C2H5)4Pb, B(CH3)3, C6H5OLi.</p> <p>3. Complex (or) non-classically bonded organometallic compoundsThese compounds are formed by the reaction of alkenes, alkynes and aryl groups with the metals. The metal atoms in these compounds are lined to the alkyl or aryl carbon by a -bond (or) coordinate bond. e.g., K[Pt(C2H4Cl3], Li2[Mg(C6H5)4</p> <p>Nature of carbon-metal bondThree kinds of carbon-metal bonds are encountered in organometallic compounds.</p> <p>1. Ionic bond: Ionic bond is found in the compounds formed by alkali and alkaline earth metals,sometimes by lanthanide elements.</p> <p>2. Covalent bond: These are formed by metalloids. Most of times covalent bond is heteropolar andsometimes the reactivity of these compounds is based on polarity.</p> <p>3. -bond: This may be localized or delocalized. General Methods of Preparation of Organometallic compounds 1. By Oxidative additionIn oxidative addition, a metal complex with vacant coordination sites and a relatively low oxidation state is oxidized by the insertion of the metal into a covalent compound like H-H.</p> <p>2. By reductive eliminationIt involves the elimination of a molecule from a transition metal complex. In the process of this elimination, the metal center is reduced by two electrons.</p> <p>The groups being eliminated must be in a cis position.</p> <p>3. By TransmetallationTransmetallation is a general reaction type describing the exchange of ligands between two metal centers.</p> <p>4. CarbometallationIt involves the nucleophilic addition to alkenes and alkynes.</p> <p>Properties of Organometallic compoundsThese undergo reactions with both inorganic and organic compounds.</p> <p>Reactions with inorganic reagents1. Many ionic and -bonded organometallic compounds are oxidized spontaneously in air. 2. Hydrogenation occurs at room temperature in absence of catalyst.</p> <p>3. Many organometallic compounds are hydrolysed by water.</p> <p>4. Organometallic compounds react with halogen acid in benzene to give hydrocarbons and metal salts.</p> <p>5. With halogens:</p> <p>Reactions with organic reagents1. Addition with unsaturated groups:</p> <p>2. Alkyls of metals react with aryl halides.</p> <p>3. Organometallic compounds behave as Lewis acids and form adducts with donors like amines.</p> <p>Organolithium compounds Preparation1. Lithium metal with alkyl halide:</p> <p>2. By transmetallation (metal-metal exchange)</p> <p>3. By metallation (metal-hydrogen exchange)</p> <p>Properties1. Nucleophilic attack on multiple bonds</p> <p>2. With aldehydes and ketones Alkyl lithium compounds react with aldehydes and ketones to give corresponding alcohols.</p> <p>3. With alkyl iodides Alkyl lithium compounds react with alkyl iodides and give the compounds having C C bonds.</p> <p>4. With halogens Alkyl lithium compounds react with halogens and give the parent alkyl or aryl halide.</p> <p>5. With cyanogen chloride alkyl lithium compounds give alkyl cyanides.</p> <p>6. Alkyl lithium compounds react with chloramine to form a primary amine.</p> <p>Structure</p> <p> Lithium alkyls are polymeric and exist as tetrameric units such as (LiCH 3)4. In methyl lithium there is a tetrahedral set of four lithium atoms with a methyl group placed symmetrically above each lithium face. The structure clearly shows a methyl group bridging three lithium atoms. The Li C bond distance is 231 pm and the Li Li bond distance is 268 pm.</p> <p>UsesOrganolithium compounds are as reactive as Grignard reagents. Hence, many useful organic compounds can be prepared using organolithium compounds.</p> <p>Organoboron compounds Preparation1. Alkyl borons can be easily prepared by hydroboration.</p> <p>2. The alkyl and aryl borons can be prepared from boron halides by lithium or Grignard reagents.</p> <p>Properties1. Hydrolysis Hydrolysis of alkylboron compounds forms many organic compounds. The nature of products depends on the reagent used for hydrolysis.</p> <p>2. With carbon monoxide Alkylborons react with carbon monoxide and form alcohol and ketones.</p> <p>3. Alkylboranes join among themselves to produce alkanes in presence of alkaline silver nitrate.</p> <p>4. With halogenacids Alkylboranes react with halogen acids and give the corresponding alkanes.</p> <p>5. With iodine</p> <p>Olefin (or) Alkene ComplexesAlkene complexes are extremely important because they are the first step of olefin functionalization.</p> <p>Virtually all transition metals form olefin complexes or react with olefins.</p> <p>Zeises salt Zeises salt is a coordination compound. The molecular formula of Zeises salt is K[PtCl3(C2H4)].H2O K+ ion and water molecule is present outside the coordination sphere. Both, Cl- and ethylene are coordinated with platinum ion, hence inside the coordination sphere.</p> <p>SynthesisIt is a complex between platinum metal and ethylene, obtained when ethylene is passed through aqueous solution of potassium tetrachloroplatinate.</p> <p>Structure</p> <p> In the structure of Zeises salt, the ethylene occupies the fourth coordination site of the square planar complex with the C-C axis perpendicular to the platinum-ligand plane. In ths compound, the dsp2 hybridized orbital of platinum overlaps with -bonding molecular orbital of ethylene. Simultaneously, the filled d orbital of platinum overlaps with * orbital of ethylene.</p> <p>Cyclopentadienyl Complexes These are also called as metallocenes and are the compounds of cyclopentadienyl groups with the transition elements. They are generally represented as (.C5H5)2M. The first compound discovered was di--cyclopentadienyl iron (or) ferrocene, (.C5H5)2Fe.</p> <p>Preparation of Ferrocene1. By the reaction of alkali metal cyclopentadienides with iron.</p> <p>2. Reaction of anhydrous metals (II) halide with cyclopentadiene in an amine.</p> <p>3. Reaction of cyclopentadiene with metal or metal oxides.</p> <p>4. From Grignard reagent by reacting with the metal halides.</p> <p>Physical properties1. It is an orange yellow solid, neutral, soluble in organic solvents and is sublimable at 100 oC. 2. It is stable in air and towards hydrolysis. 3. It is easily oxidized in the acid solution to give the ferrocinium ion.</p> <p>Chemical propertiesAs cyclopentadienyl rings in the molecule of ferrocene is aromatic in nature and hence can undergo aromatic substitution. 1. Friedel crafts acylation Ferrocene readily undergoes acylation with acetyl chloride as follows.</p> <p>2. Friedel crafts alkylation Alkylation of ferrocene can be carried out as shown below.</p> <p>3. Nitration Direct nitration of ferrocene leads to its decomposition, so nitration is carried out indirectly as described below.</p> <p>4. Halogenation Direct halogenation of ferrocene leads to its decomposition, so halogenation is carried out indirectly as described below.</p> <p>5. Carboxylation</p> <p>6. Vilsmeir reaction Ferrocene undergoes Vilsmeir reaction to yield ferrocene carboxyaldehyde which is a useful starting material for the preparation of other ferrocene derivatives.</p> <p>7. Mannich condensation</p> <p>Structure</p> <p> Ferrocene has a structure in which the iron atom is sandwiched between two C5H5 organic rings. It has been observed by X-ray diffraction that in gas phase the structure of ferrocene is eclipsed while at low temperature the structure of ferrocene is staggered. The staggered configuration in the solid phase exists because of the crystal packing forces so that C-C and H-H repulsions between the two rings are minimum.</p> <p>Uses1. Ferrocene and its derivatives are used as antiknocking agents in petrol. 2. Some ferrocinium salts exhibit anticancer activity. 3. Ferrocene, being readily decomposed to iron nanoparticles, can be used as a catalyst for the production of carbon nanotubes. 4. Chiral ferrocenyl phosphines are employed as ligands for transition metal catalyzed reactions.</p> <p>Uses of Organometallic compounds1. The soluble organometallic compounds of transition metals act as homogeneous catalysts. Some examples are: (a) Selective hydrogenation of certain double bonds using Wilkinsons catalyst, (Ph3P)3RhCl. (b) (Et3P)2NiCl2 acts as catalyst for the isomerization of alkenes. 2. Organometallic compounds can also act as heterogeneous catalysts. For example, Zeigler-Natta catalyst (a solution of TiCl4, containing triethylaluminium) for the polymerization of ethylene and the other alkenes. 3. Organometallic compounds of magnesium (R Mg X), cadmium (R2Cd) and lithium (R Li) are extensively used in organic synthesis. 4. Tetraethyl lead is used as an antiknock compound. 5. A number of organometallics also find application in agriculture. For example, ethyl mercury chloride, C2H5HgCl is used as a fungicide for the protection of young plants and seeds against fungal infection. 6. Aryl arsenic compounds are used as chemotherapeutic agents. 7. Silicon rubbers because of their high thermal stability, resistance to oxidation and chemical attack are used in modern surgery for the purpose of production of artificial body parts.</p>


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