31-01-2013, 02:52 PM
SYNTHESIS,STRUCTURE& BONDING ASPECTS OF ORGANOMETALLIC COMPLEXES OF FOUR & SIX ELECTRON CYCLE
[attachment=49091]
INTRODUCTION
Organometallic compounds are also known as organo-inorganics, metallo-organics and metalorganics. Organometallic compounds are distinguished by the prefix "organo". .Organometallic compounds are those compounds which contain one or more metal carbon bonds. The common example of organometallic compounds are Pb(C2H5)2 (tetraethyl lead), (C5H5)2 (ferrocene)
It may be noted that metal cyanide(M-CN) and metal carbides contain the metal carbon bond, but they are not considered as organometallic compounds because their properties are not similar to organometellic compounds. In contrast the complexes formed by the ligand CO2 known as metal carbonyl are considered to be organometallic because their properties resemble with these compounds.
CONCEPT
Electron counting is the key in understanding organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of organometallic compounds. Organometallic compounds which have 18 electrons (filled s, p, and penultimate d orbitals) are relatively stable. This suggests the compound is isolable, but it can result in the compound being inert.
To understand chemical bonding and reactivity in organometallic compounds the isolobal principle should be used. NMR and infrared spectroscopy are common techniques used to determine structure and bonding in this field. Scientists are allowed to probe fluxional behaviors of compounds with variable-temperature NMR.
SYNTHESIS OF ORGANOMETALLICS COMPOUNDS
First reported organometallic compounds were prepared by the reductive substitution of alkyl halides, as shown in the following three equations. All these metals have strong or moderate negative reduction The potentials, with lithium and magnesium being the most reactive. Halide reactivity increases in the order: Cl < Br < I. Alkylsodium and potassium compounds are not made in this way because Wurtz coupling of the alkyl moiety ( giving R_R ) tends to predominate. This can also be a problem when allyl or benzyl halides are converted to Grignard or lithium reagents.
REACTIONS OF SIMPLE ORGANOMETALLICS COMPOUNDS
Reactions of organometallic compounds reflect the nucleophilic (and basic) character of the carbon atom bonded to the metal. Consequently, the most common reactions are electrophilic substitutions and additions to unsaturated electrophiles. The electropositive nature of the metal atom or group is an important factor influencing the reactivity of these reagents. Alkyllithium (and sodium) compounds are the most reactive of the commonly used compounds in this class, having metal-carbon bonds that are roughly 30% ionic. The carbon-magnesium bond of Grignard reagents is about 20% ionic, and they have proven to be somewhat less reactive.
Dialkylzinc reagents have significantly reduced reactivity, and fail to react with carbon dioxide, esters and many aldehydes and ketones. Alkylmercury and lead compounds are the least reactive commonly studied organometallics. The ionic character of the carbon-mercury bond is estimated to be less than 10%. Such compounds react with mineral acids, but not with water or alcohols.
Organic compounds incorporating carbon-metal bonds have been studied for nearly 200 years, and their unique properties have been widely used to effect synthetic transformations. Depending on the reduction potential of the metal, the reactivity of organometallic compounds varies markedly, the most reactive requiring low to moderate temperatures and inert conditions (atmosphere and solvents) for preparation and use. In general, the reactivity parallels the ionic character of the carbon-metal bond, which may be estimated from the proton and carbon chemical shifts of methyl derivatives.
BONDING OF ORGANOMETALLICS COMPOUNDS
Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry, because organometallic compounds are chemical compounds containing bonds between carbon and a metal or metalloid element. Organometallic bonds are different from other bonds in that they are not either truly covalent or truly ionic, but each type of metal has individual bond character. Cuprate (copper) compounds, for example, behave quite differently than Grignard reagents (magnesium), and so beginning organic chemists should concentrate on how to use the most basic compounds mechanistically, while leaving the explanation of exactly what occurs at the molecular level until later and more in-depth studies in the subject.
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 organ lithium and Grignard reagents. In certain organometallic
compounds such as ferrocene or dibenzenechromium, the pi orbitals of Organometallic the organic moiety ligate the metal.
STRUCTURE & PROPERTIES
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 Grignard reagents.
[attachment=49091]
INTRODUCTION
Organometallic compounds are also known as organo-inorganics, metallo-organics and metalorganics. Organometallic compounds are distinguished by the prefix "organo". .Organometallic compounds are those compounds which contain one or more metal carbon bonds. The common example of organometallic compounds are Pb(C2H5)2 (tetraethyl lead), (C5H5)2 (ferrocene)
It may be noted that metal cyanide(M-CN) and metal carbides contain the metal carbon bond, but they are not considered as organometallic compounds because their properties are not similar to organometellic compounds. In contrast the complexes formed by the ligand CO2 known as metal carbonyl are considered to be organometallic because their properties resemble with these compounds.
CONCEPT
Electron counting is the key in understanding organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of organometallic compounds. Organometallic compounds which have 18 electrons (filled s, p, and penultimate d orbitals) are relatively stable. This suggests the compound is isolable, but it can result in the compound being inert.
To understand chemical bonding and reactivity in organometallic compounds the isolobal principle should be used. NMR and infrared spectroscopy are common techniques used to determine structure and bonding in this field. Scientists are allowed to probe fluxional behaviors of compounds with variable-temperature NMR.
SYNTHESIS OF ORGANOMETALLICS COMPOUNDS
First reported organometallic compounds were prepared by the reductive substitution of alkyl halides, as shown in the following three equations. All these metals have strong or moderate negative reduction The potentials, with lithium and magnesium being the most reactive. Halide reactivity increases in the order: Cl < Br < I. Alkylsodium and potassium compounds are not made in this way because Wurtz coupling of the alkyl moiety ( giving R_R ) tends to predominate. This can also be a problem when allyl or benzyl halides are converted to Grignard or lithium reagents.
REACTIONS OF SIMPLE ORGANOMETALLICS COMPOUNDS
Reactions of organometallic compounds reflect the nucleophilic (and basic) character of the carbon atom bonded to the metal. Consequently, the most common reactions are electrophilic substitutions and additions to unsaturated electrophiles. The electropositive nature of the metal atom or group is an important factor influencing the reactivity of these reagents. Alkyllithium (and sodium) compounds are the most reactive of the commonly used compounds in this class, having metal-carbon bonds that are roughly 30% ionic. The carbon-magnesium bond of Grignard reagents is about 20% ionic, and they have proven to be somewhat less reactive.
Dialkylzinc reagents have significantly reduced reactivity, and fail to react with carbon dioxide, esters and many aldehydes and ketones. Alkylmercury and lead compounds are the least reactive commonly studied organometallics. The ionic character of the carbon-mercury bond is estimated to be less than 10%. Such compounds react with mineral acids, but not with water or alcohols.
Organic compounds incorporating carbon-metal bonds have been studied for nearly 200 years, and their unique properties have been widely used to effect synthetic transformations. Depending on the reduction potential of the metal, the reactivity of organometallic compounds varies markedly, the most reactive requiring low to moderate temperatures and inert conditions (atmosphere and solvents) for preparation and use. In general, the reactivity parallels the ionic character of the carbon-metal bond, which may be estimated from the proton and carbon chemical shifts of methyl derivatives.
BONDING OF ORGANOMETALLICS COMPOUNDS
Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry, because organometallic compounds are chemical compounds containing bonds between carbon and a metal or metalloid element. Organometallic bonds are different from other bonds in that they are not either truly covalent or truly ionic, but each type of metal has individual bond character. Cuprate (copper) compounds, for example, behave quite differently than Grignard reagents (magnesium), and so beginning organic chemists should concentrate on how to use the most basic compounds mechanistically, while leaving the explanation of exactly what occurs at the molecular level until later and more in-depth studies in the subject.
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 organ lithium and Grignard reagents. In certain organometallic
compounds such as ferrocene or dibenzenechromium, the pi orbitals of Organometallic the organic moiety ligate the metal.
STRUCTURE & PROPERTIES
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 Grignard reagents.