Organo metallic

ORGANO METALLICS

 

Organometallic compounds are those compounds which contain one or more metal-carbon bonds. All the compounds containing carbon and a metal atom are not organometallic. We use this term for compounds that contain at least one M-C bond.

For example

An alkoxide such as (C3H7O4)Ti is not considered to be an organometallic compound because the organic group is bonded to Ti atom by oxygen and there is no Ti-C bond. The compound C6H5Ti(OC3H7)3 is an organometallic compound because it contains a Ti-C bond (C6H5-Ti) in the compound.

EC. Frankland was the first chemist to synthesise an organometallic Compound, dimethylzinc, (CH3)2Zn in 1848. He also prepared other compounds containing metal-carbon bonds such as :

Zn(C2H)5, Hg(CH3)2, Sn(CH3)4 and B(CH3)3

CLASSIFICATION OF ORGANOMETALLIC COMPOUNDS

Organometallic compounds can also be broadly classified into two types on the basis of the nature of metal-carbon bonds:


(1) σ-bonded organometallics

(2) π-bonded organometallics

 

(1) σ-bonded organometallics

The compounds containing metal-carbon covalent sigma bond are called σ-bonded organometallic compounds. The common examples are:


(i) Grignard reagents:
 R-Mg-X where X= halogen, R= alkyl or aryl group.

CH3MgBr (Methyl magnesium bromide), C2H5MgI (Ethyl magnesium iodide)


(ii) Diethyl zinc,
 (C2H5)2Zn, dimethyl magnesium, dimethyl beryllium, etc.

R= CH3 or C2H5

M= Be, Mg or Zn 


(iii) Trimethyl aluminium (CH3)3Al:
 It exists as a dimer [Al2(CH3)6] with two methyl groups acting as bridges between two aluminium atoms. In this structure, two methyl groups act as bridges between two Al-atoms. The alkyl bridge is formed by multicentre (2 Al-atoms and one C-atom) bonding.


Tri-aryl aluminium i.e., tri-phenylaluminium (C6H5)3Al also exists as dimer. 

(iv) Tetra ethyl tin, Sn(C2H5)4 and tetramethyl tin, Sn(CH3)tetramethylsilane ,  trimethylarsine, etc. 

(v) Lithium alkyl compounds are covalent compounds and generally have tetrameric structures. For example methyllithium, CH3Li exists in the solid state as tetrameric units (LiCH3)4]

(vi) Alkyl beryllium and alkyl magnesium [Mg(CH3)2 and Be(CH3)] exhibit varying degrees of polymerisation.

(2) π-bonded organometallics

The compounds containing metal-carbon double bond are called π-bonded organometallics. These are usually formed by transition elements. The common examples are :


(i) Zeise’s salt
 K[PtCl3(C2H4)]

In this structure the three Cl-atoms and the middle point of ethylene double bond form a square plane. The platinum atom is present in the centre of the square and C = C double bond of the ethylene molecule is perpendicular to plane containing Pt and Cl atoms.

It is written as K[PtCl32-C2H4)]- The number of carbon atoms bound to the metal is indicated by the Greek letter ‘η’ (eta) followed by a number. In this case, Pt is bound to two carbon atoms and therefore, it is written as η2.

(ii)Ferrocene or bis (cyclopentadienyl) iron

The structure of ferrocene is regarded as a sandwich structure in which the iron atom is sandwiched between two C2H5 organic rings. The planes of the rings are parallel so that all the carbon atoms are at the same distance from the iron atom.

Ruthenocene, Ru(C5H6) and osmoceneOs(C5H5)2 have structures similar to ferrocene.


(iii) Dibenzene chromium
, Cr(η6-C6H6)2 

Ligands of organometallic compounds

Organometallic compounds contain at least one metal-carbon bond, and are typically coordinated by one or more ligands. Ligands are molecules or ions that can donate electrons to the metal center to form a coordinate covalent bond. The choice of ligand(s) can significantly influence the properties and reactivity of the organometallic compound. Some common ligands for organometallic compounds include:

1.     Neutral ligands: These are molecules that have no charge, such as carbon monoxide (CO), ethylene (C2H4), and dienes. They can donate electron density to the metal center through their pi bonds.

2.     Anionic ligands: These are negatively charged ions, such as halides (Cl-, Br-, I-), alkoxides (RO-), and amides. They can donate electron density to the metal center through their lone pairs of electrons.

3.     Cationic ligands: These are positively charged ions, such as protonated amines (NH3+), phosphines, and N-heterocyclic carbenes (NHCs). They can donate electron density to the metal center through their lone pairs of electrons.

4.     Chelating ligands: These are ligands that can coordinate to the metal center through multiple donor atoms, such as bidentate ligands like ethylenediamine (en) or acetylacetonate (acac), or tridentate ligands like terpyridine.

18 electron rule

The 18 electron rule is a rule of thumb used in organometallic chemistry to predict the stability and reactivity of transition metal complexes. The rule states that complexes with 18 valence electrons are particularly stable.

Here are some key points and examples to explain the 18 electron rule:

1.     The 18 electron rule applies to transition metal complexes, which typically have a central metal atom surrounded by ligands.

2.     Valence electrons include the electrons in the metal atom's outermost d and s orbitals, as well as the electrons in the ligands that are directly bonded to the metal.

3.     The 18 electron rule is based on the fact that noble gas atoms have 18 electrons in their outermost shell, which is considered to be a particularly stable configuration.

4.     According to the 18 electron rule, complexes with 18 valence electrons are especially stable because they have a full outer shell of electrons, like a noble gas.

5.     Examples of complexes that follow the 18 electron rule include [Fe(CO)5], [Ni(CO)4], and [PtCl6]2-, all of which have 18 valence electrons.

6.     Complexes that deviate from the 18 electron rule can be less stable and more reactive. For example, [Fe(CO)4] has only 16 valence electrons, making it more reactive than [Fe(CO)5]. In contrast, [Fe(CO)6] has 20 valence electrons, making it less stable than [Fe(CO)5].

Hapticity

Hapticity refers to the coordination mode of a ligand to a metal center, describing how many atoms in the ligand directly bind to the metal and in what manner. Here are some points to explain hapticity with examples:

  • Hapticity is a term used in organometallic chemistry to describe how many atoms of a ligand are directly bound to a metal center.
  • The hapticity of a ligand can affect the reactivity and properties of an organometallic compound.
  • Hapticity is typically denoted by the Greek letter eta (η) followed by a superscript number that indicates the number of atoms directly bonded to the metal center.
  • For example, in the complex CpFe(CO)25-C5H5), the cyclopentadienyl (Cp) ligand binds to the iron center through five carbon atoms in a pentahapto, or η5, mode.
  • Another example is the complex Fe(CO)34-C4H6), in which the butadiene ligand binds to the iron center through four carbon atoms in a tetrahapto, or η4, mode.
  • Hapticity can also be used to describe ligands that are bridging between two or more metal centers, such as in the complex Fe2(μ-CO)2(CO)62-C2H4), in which the ethylene ligand bridges between two iron centers in a dihapto, or η2, mode.
  • Hapticity can also affect the stability of organometallic compounds, with higher hapticity generally leading to greater stability due to increased delocalization of electron density across the ligand.
  • In summary, hapticity is a useful way to describe the coordination mode of ligands in organometallic compounds, and it can have important implications for the reactivity and stability of these compounds.

Organolithium compounds

Organolithium compounds have a linear or planar structure with a tetrahedral arrangement around the carbon atom bonded to lithium. The carbon-lithium bond is highly polarized due to the large difference in electronegativity between the two atoms, with the carbon atom carrying a partial negative charge and the lithium atom carrying a partial positive charge. The polarity of the bond gives rise to a high reactivity of the organolithium compound, making it a strong nucleophile and a strong base. The lithium atom is also coordinated by one or more additional ligands, such as ethers or amines, which help to solvate the compound and stabilize the lithium ion in solution. The coordination of these ligands can also affect the reactivity and selectivity of the organolithium compound. In the solid state, organolithium compounds often form complexes or aggregates due to the strong interactions between the lithium ions and the ligands.

Organolithium compounds are highly reactive chemical species that contain a covalent bond between carbon and lithium atoms. Here are some of their important properties:

1.     Highly reactive: Organolithium compounds are highly reactive due to the polar nature of the C-Li bond. They react with a variety of electrophiles such as carbonyl compounds, alcohols, epoxides, and alkyl halides.

2.     Strong bases: Organolithium compounds are strong bases and readily react with acids to form lithium salts. They can also deprotonate a variety of acidic compounds such as alcohols, phenols, and carboxylic acids.

3.     Solubility: Organolithium compounds are highly soluble in nonpolar solvents such as diethyl ether and tetrahydrofuran (THF), but less soluble in polar solvents such as water and alcohols.

4.     Complex formation: Organolithium compounds can form complexes with Lewis bases such as ethers, amines, and phosphines. These complexes are important in controlling the reactivity and selectivity of organolithium reactions.

5.     Strong reducing agents: Organolithium compounds are strong reducing agents and can reduce a variety of functional groups such as carbonyl compounds, nitro compounds, and halogens.

6.     Stereoselectivity: Organolithium reactions can exhibit high stereoselectivity, meaning they can selectively produce one stereoisomer over another. This is due to the highly reactive and directional nature of the C-Li bond.

 

 Ferrocene

Ferrocene is an organometallic compound composed of two cyclopentadienyl rings bound to a central iron atom.

Ferrocene has a unique structure that consists of a central iron atom sandwiched between two cyclopentadienyl rings. Each cyclopentadienyl ring has five carbon atoms arranged in a planar pentagon, and each carbon atom donates one electron to form a ring of six delocalized electrons. These electrons interact with the iron atom in the center, which has eight valence electrons available for bonding.

The two cyclopentadienyl rings in ferrocene are parallel, with the iron atom positioned exactly halfway between them. This arrangement allows for the formation of strong covalent bonds between the iron atom and each of the carbon atoms in the rings.

Some of its key properties and structural features include:

1.     Structure: Ferrocene is a sandwich complex, meaning that the iron atom is sandwiched between two parallel cyclopentadienyl rings.

2.     Bonding: The bonding in ferrocene is often described as a combination of σ and π interactions. The iron atom forms σ bonds with the cyclopentadienyl rings, while the overlapping of the π orbitals of the cyclopentadienyl rings with the d orbitals of the iron atom leads to π bonding.

3.     Stability: Ferrocene is a remarkably stable compound. The presence of two cyclopentadienyl rings on either side of the iron atom contributes to its stability.

4.     Magnetic properties: Ferrocene is a diamagnetic compound, meaning that it is not attracted to a magnetic field.

5.     Redox properties: Ferrocene is a redox-active compound, meaning that it can readily undergo oxidation and reduction reactions. This is due to the presence of the iron atom, which can easily donate or accept electrons.

6.     Melting point: Ferrocene has a relatively high melting point of 172-174°C, which is attributed to the strong intermolecular interactions between the molecules.

7.     Solubility: Ferrocene is soluble in nonpolar solvents such as benzene, toluene, and hexane, but insoluble in polar solvents such as water.

METAL CARBONYLS

The homoleptic complexes in which carbon monoxide (CO) acts as the ligand are called metal carbonyls.
For example: Ni(CO)4

Structure and bonding in metal carbonyl

The structure of some important metal carbonyls are:

Bonding in metal carbonyls

•   The metal-carbon bond in metal carbonyls possesses both and p character. CO as a ligand binds itself to metal atoms through the carbon atom to form the metal-carbon (M-C) bond. It is a weak donor.

•   The M–C σ bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal.

•   The M–C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the vacant antibonding π* orbital of carbon monoxide. This characteristic property of back bonding which stabilises the metalligand interaction is termed as synergic effect.

Properties of metal carbonyls

•   They are generally solids at room temperature and pressure except Ni(CO)4 and Fe (CO)5.

•   Mononuclear carbonyls are volatile and toxic.

•   Mononuclear carbonyls are either colourless or light coloured

Iron carbonyl

Iron carbonyl refers to the coordination compound of iron with carbon monoxide (CO). The formula for iron carbonyl is Fe(CO)n, where n can vary from 1 to 6. The most common iron carbonyl compound is iron pentacarbonyl (Fe(CO)5).

The iron-carbon bond in iron carbonyl is a unique example of a metal-carbon bond, and it is a type of covalent bond known as a metal carbonyl bond. In iron carbonyl, the Fe atom is surrounded by 5 CO molecules arranged in a trigonal bipyramidal geometry. The CO ligands are attached to the Fe atom through their C atoms, forming a sigma bond between the Fe and the C atom of CO. Additionally, the CO molecule has a lone pair of electrons on its oxygen atom, which can form a dative bond (coordinate covalent bond) with the Fe atom, creating a stronger bond.

Iron carbonyl has several interesting properties, including:

1.     It is a volatile liquid at room temperature and atmospheric pressure.

2.     It has a high boiling point of 103°C, which is much higher than other simple metal carbonyls.

3.     It is highly toxic and can be lethal if ingested or inhaled.

4.     It is a useful precursor for the synthesis of other iron-containing compounds, including iron nanoparticles.

5.     It is a powerful reducing agent, and it can reduce metal ions to their corresponding metal atoms.

6.     It is used as a catalyst in certain organic reactions, such as the hydroformylation of alkenes to produce aldehydes.