In this Lecture class notes we want to show that organometallic chemistry is very much important topic for csir net chemical sciences, in the previous year examination it gives one of the Highest no of questions. So must prepare with it well. Hope this Study Material will be helpful for you. Thanks for sharing pdf available in below post
The term “Orgaometallic” generally denotes compound in which organic groups are directly linked to the metal through at least one carbon atom. The bonding is ionic or covalent or delocalized between organic groups and a metal atom.
Simple organometallic compounds are one which a metal-carbon bond which is typically similar with
respect to the derivative of associated constituent.
Further divided in to
a) Symmetrical: Example: [ Hg (C2H5)2] Diethyl mercury
b) Unsymmetrical : Example: CH3-Hg-C2H5 Ethyl methyl mercury
A mixed organometallic compounds are those in which a metal atom bonded with more than one identity of organic or inorganic constituent. Example: C2H5-Mg-Br Ethylmagnesiumbromide
Classification of organometallic compounds:
On this basis of nature of metal- carbon bond organometallic compounds are classified in to Ionic bonded organometallic compounds and Covalent bonded organometallic compounds as follows,
1) Ionic bonded organometallic compounds:
The organometallic compounds of alkali, alkaline earth metals, Lanthanides and Actinides are
predominantly form ionic compounds. These are generally colourless compounds extremely reactive,
non-volatile solids and insoluble in organic solvents. Examples: Ph3C –Na+ , Cp2Ca, Cs+Me– , Na+Cp–
2) Covalent bonded organometallic compounds:
A) σ- bonded organometallic compounds:
These are the compounds in which carbon atom of the organic ligand is bonded to the metal by a 2 electron, 2 centered ( 2e-2c ) covalent bond. Generally formed by most of the elements with values of electronegativity are higher than 1.
Examples: Ni(CO)4, Fe(CO)5
B) π- bonded organometallic compounds:
These are alkene, alkyne or some other carbon group has a system of electron in π-orbitals overlap of these π- orbitals with vacant orbitals of the metal atom gives rise to arrangement in which the metal atom is bonded to several carbon atom instead of to one. Examples:[(η5-C5H5)2Fe)], K[PtCl3(η2-C2H2)] (Zeise’s salt)
3) Multicenter bonded organometallic compounds:
The compounds in which a loosely bonded electron deficient species exist with the coordination of metal like Li,Be,Al…etc Examples: MeLi or (CH3)4Li4, Al2Me6
Classification of ligands based on hapticity:
In hapto-classification organic ligands are classified according to “number of carbon atoms within a bonding distance of metal atom or the no. of carbon atoms through which an organic ligand is attached to central metal atom” is known as “hapticity” of the ligand. This is represented by the symbol “ η ” appearing beforethe symbol of the ligand. Where, η- hapticity of the ligand.
Monohepto ligands (η1): Organic ligands attached to the metal through one carbon. Ex: -CH3, -C2H5
Dihepto ligands (η2) : Example: CH2=CH2, K[PtCl3(η2-C2H2)] (Zeise’s salt)
Trihepto ligands (η3): Example: Allylic radical CH2=CH-CH2-
Tetrahepto ligands (η4): Example: Butadiene CH2=CH-CH= CH2
Pentahepto ligands (η5): Example: Metallocene M (η5-C5H5)2
Hexahepto ligands (η6): Example: Dibenzene ChromiumCr(η6-C6H6)
Structure of ferrocene[Fe(η5-C5H5)2]:-
bis(η5-cyclopentadienyl) iron(II)
Ferrocene is known as a “sandwich compound “.
It occurs as highly stable orange crystals with a melting point of 174° C.
In solid state at low temperature, the rings are staggered.
The rotational barrier is very small, with free rotation of the rings.
At extreamly low temperature that is Below 110 K, the cyclopentadienyl (Cp) rings are ordered and eclipsed.
Terms of bonding, the iron center in ferrocene is usually assigned to the +2 oxidation state.
Each cyclopentadienyl (Cp) ring is then allocated a single negative charge, bringing the number of π electrons on each ring to six, and thus making them aromatic.
These twelve electrons (six from each ring) are then shared with the metal via covalent bonding. When combined with the six d-electrons on Fe(2+), the complex attains an 18-electron configuration.
