Difference between complex salts and double salts Method of preperation of some compounds.

The difference between complex salts and double salts.

Some important organometallic compound of T.M.

Ferrocene (C₅H₅)₂Fe

Ferrocene is the first metallocene known in which Fe-atoms remains bonded between to cyclopentadienyl ions.(C₅H₅)^-.

Methods of preparation.

1. By treating sodium cyclopentadienide with FeCl₂:

In this methods, cyclopentadiene (C₅H₆) is first treated with sodium to get cyclopentadienide.Then this compound is treated with FeCl₂ to get ferrocene.

$$2C_5H_6+2Na\xrightarrow{THF\,Solvent}2C_5H_5^-Na^++H_2↑$$

$$2C_5H_5^{-}Na^+=Sodium\,cyclo\,pentadienide$$

$$2C_5H_6=Cyclo\,penta\,diene$$

$$2C_5H_5^-Na^++FeCl_2→[C_5H_5]_2Fe+2NaCl$$

$$[C_5H_5]_2Fe=Ferrocene$$

2. By treating with C₅H₅Tl with FeCl₂

$$C_5H_6+TlOH→C_5H_5Tl+H_2O$$

$$C_5H_5Tl+FeCl_2→(C_5H_5)_2Fe+2TlCl$$

3. By treating Grignard reagent (C₅H₅MgCl) with FeCl₂.

$$2C_5H_5MgCl+FeCl_2→(C_5H_5)_2Fe+2MgCl_2$$

Structure of Ferrocene.

In ferrocene,the iron atom is sandwiched between two cyclopentadienyl rings.The planes of rings are parallel so that all the carbon atoms lie at the same distance from Fe atoms.

X-Ray study shows that ferrocene has two types of arrangement of cyclopentadienyl rings. In the gas phase, it exists in as eclipsed while in solid phase it exists as staggered.

Ligands.

Ligands are any species (molecule or ion) that contain one pair of electron and remain coordinated with the central metal atom in a complex compound are ligands.Hence, all lewis base act as ligands.

Or The neutral molecule or ion (generally anions but rarely cations containing at least one lone pair of an electron which are attached to the central metal atom or ion by coordinate covalent bonds are called as ligands.During a coordination or complex compound formation , the atoms or molecule used as ligands are capable of functioning as the electron pair donor called donor partner I,e donates one or more electron pairs to the central metal atoms or ions. Hence, the ligands are considered as lewis base (lone pair donor) whereas the central metal atom-atom as lewis acid (lewis asci acceptor).

Classification of ligands.

So many organic and inorganic molecule can act as the ligands in different geometry so the ligands are classified in many respects as follows.

1. On the basis of charge.
2. On the basis of donor atom sites.

(i) On the basis of charge.

Depending on the nature of charge present in any ligands that are classified as 3 types They are

1. Cationic ligands.
2. Anionic ligands.
3. Neutral ligands.

1. Cationic ligands.

Those ligands carrying the positive charge and are less common example NO⁺, (Nitrosyl), NH₂NH₃⁺, (hydrazinium) etc.

2. Anionic ligands.

Those ligands carrying the negative charge and are very common example include CN^-, Cl^-, CH₃COO^-, SCN^-, F^-, O^--, SO₄^--, NH₂^-, C₂O₄^--etc.

3. Neutral ligands.

Those ligands with no charge and are common example include H₂O, CO, CSC (Thiocarbonyl), HS (Thio nitrosyl) etc.

(ii) On the basis of donor atoms/ sites.

1. Monodentate ligands.
2. Polydentate /Multidentate ligands.
3. Ambidentate ligands.

1. Monodentate ligands.

The ligands which have only one donor atom and hence can coordinate to the central metal atom at one site only are called as monodentate or unidentate ligands.Such ligands may either neutral like NH₃, H₂O, CO, etc or anions like CN^-, Cl^-, OH^-, CH₃COO^-, etc or may be either cationic like NO⁺, N₂O₅⁺.

2. Polydentate / Multidentate ligands.

Those ligands which have two or more donor atoms and are coordinated to central metal ion through two or more sites are called as polydentate\Multidentate ligands.These are further divided into following types.

a.Bidentate ligands.

b.Tridentate ligands.

c. Tetradentate ligands.

d.Pentadentate ligands

e.Hexadentate ligands.

a. Bidentate ligands.

The ligands which have two same or different donor atoms and linked through two bonding sites with the central metal atom or ion for example as Oxalate ion, Ethylenediamine, Glycinate ion, Acetylacetonate

b. Tridentate ligands,

Those ligands which have three donor atoms which allow them to bind to a central metal atom or ion at three points. For example-Diethylene triamine, Triamino propane.

c. Tetradentate ligands.

These have four donor atoms which allow them to bind to a central metal atom or ion at four points. Example include Triethylenetetramine, Nitrilotriacetate

d. Pentadentate ligands.

These have five donor atom which allows them to bind to a central metal atom or ion at 5 points. Example include Ethylenediamine triacetate

e. Hexadentate ligands.

Those have 6 donor atom which allows them to bind to the central metal atom or ion at 6 sides for example ethylene diamine tetraacetic acid (EDTA).

(iii) Ambient ligands.

Some ligands may contain two or more donor atoms but only donates one lone pair and acts as unidentate ligands are called as am bidentate ligands. For example include

In NO2^-

M-NO2^-=nitro.

M-O-N=o=nitrito.

In CN^-,

M-CN=Cyano.

M-NC=Isocyanate.

In SCN^-

M-SCN=Thiocynate.

M-NCS=Isothiocynate

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.