Co-Ordinate chemistry, complex ion, coordinate number, and chelate,

Coordinate chemistry.

The branch of chemistry which deals with the study of a complex compound and their bonding behaviour is called as coordination chemistry.It is very important and is the more researched field in chemistry.Some terminologies used in coordination chemistry are discussed as below.

Molecular or addition compounds.

When the solution containing two or more salts are mixed together and allowed to evaporate crystal of new compound with different composition are formed which are called as addition or a molecular compound. These are of two types depending on their behaviour in aqueous solution.

1. Double salt.
2. Coordination or complex compounds.

1. Double salts.

Double salts are simply the combination of two different salts.When the double salts dissolved in the water these dissociated into their constitution ion and give the test of all their ions. It means the individual component of the double salts does not loose their identity in solution.

For example as below.

When the solution of ferrous sulphate and ammonium sulphate are mixed in equimolar ratio and crystallise, a double salt is called Mohr’s salt is obtaining .

$$FeSO_4\,(aq)+(NH_4)_2SO_4(aq)→FeSO_4(NH_4)_2SO_4.6H_2O$$

Similarly another example of double salts are

Chrome alum. K₂SO_4.Cr₂(SO₄)_3.24.H₂O

Ammonium alum.K₂SO₄(NH₄)2SO₄.24H₂O

Co-ordination or complex compounds.

Complex compound is those molecular compound in which same or different ligands (neutral or charged) are bonded to the central metal atom within a coordination sphere. These complex compounds retain their identities even when dissolved in water or any other solvent and their properties are completely different from those of the constituent ions. Complex compounds are mostly formed when the ligand donate the lone pair of electrons to the central metal atom and forms coordinate covalent bonds.For example

1. Simple cation and complex anion.K₄[Fe(CN)₆].
2. Complex cation and simple anion.[Ag(NH₃)₂]Cl.
3. Complex cation and complex anion [Co(NH₃)₆][Cr(C₂O₄)]₃]

Complex ion.

A complexion is either positively or negatively charged species that contain central metal atom coordinated with ligands within coordinated sphere.

Or the complexion is the electrically charged radicals which are formed by the union of a metal cation with one or more neutral molecule or anions . For example.

$$[Ag(NH_3)_2]Cl→[Ag(NH_3)_2^++Cl^-$$

Similarly [CO(NH₃)₆[⁺³, [Fe(C₂O₄)₃]^-3,[Cu(NH₃)₄]⁺², etc.

These complexion does not undergo further dissociation into its constituent ions and are written within the coordinate sphere and are of cationic (carrying a positive charge) or cationic ( with a negative charge) or neutral ( with no charge).

Complex molecule.

A complex molecule in which metal atom or ion is coordinated into two or more ligands is called the coordinated compound. Complex compounds retain their identity in the solid as well as a solution.For example, when K₄[FeCN)₆] is dissolved in water, it gives four K+ and one [Fe(CN)₆]^4- ion. It does not give any test for Fe²⁺ or CN^-.

Coordinate number.

The total number of the atoms of the ligands that can coordinate to the central metal ion is called as co-ordinated number. Generally, the coordinated number represents the total number of coordinated covalent bond formed between the central metal ion and donor atom of the ligands. So, coordination number depends either on the number of ligands or the nature of ligands with two or more donor atoms.

For example.

Coordination number of Ag+ in [Ag(CN)₂]^- is two . Similaril, coordination number of Ni⁺⁺ in [Ni(NH₃)₆]²⁺ is six.C.N of Fe⁺³ in [Fe(C₂O₄)₃]-³ is six.

The most common coordination number exhibit by metal atom is 4 or 6.

Coordinate sphere.

$$[CO(NH_3)_6]Cl_3→[CO(NH_3)_6]^{3+}+3Cl^-$$

$$[CO(NH_3)_6]^{3+}=Complex\,ion$$

$$3Cl^-=Simple\,ion$$

While writing the formula of complex compound complexion are written outside a square bracket which is called the coordinate sphere. It is actually a hypothetical boundary which distinguishes a complexion from simple ion.

Chelate.

When multidentate ligands are coordinated with a central metal atom then a complex containing one or more ring in the structure is obtained which is called chelate or chelates complex. The multidentate ligands which form chelates are called chelating agent and the formation of chelates is called chelation or cyclisation.

Chelate effect.

The complex formed by monodentate ligands contains no rings in its structure and are called non-chelating complex.Chelate complexes more stable than non-chelating complex.The enhancement of stability of a complex due to the formation of the chelate is called chelate effect.

The chelated complex is more stable than that of the non-chelating complex. This is due to the following reason.

1. Enthalpy change and entropy change for the chelate complex are always greater than non-chelating complex. The chelating complex is thermodynamically more stable .
2. When multidentate ligands are coordinated with the central metal atom, the number of bytes ( the number of linking sites ) on the central metal increased which enhance the stability of chelate.
3. The presence of one or more rings in the structure is also responsible for the greater stability of chelate.

Effective atomic number (EAN) rule.

According to the Side wick, the total number of electron present on the central metal atom included that donated by ligands is called EAN of the central metal atom of the complex. In most cases, this number becomes equal to an atomic number of their nearest noble gas. If so the complex is said to be following EAN rule and will be more stable. The complex which follows EAN are relatively less stable or may be unstable.

EAN of the central metal atom can be determined by using the following relation.

EAN = an atomic number of central metal atom + electron donated by ligands- oxidation state of central metal.

For example.

EAN of CO(III) in [CO(NH₃)₆]⁺³

Here, a total number of electron in Co-atom=27 electrons.

Number of electrons lost in CO⁺³ ion= 3 electrons.

Number of electrons gained from the ligands=6NH₃=6x2=12e^-

Therefore, EAN=27-3+12=36

Hence, EAN of CO(III) in [CO(NH₃)₆]⁺³ is equal to 36, which is evidently equal to the atomic number of Krypton (Kr) and the complex is considered to be a more stable complex.

Some common example of complex are as below.

1. [Fe(CN)6]^4- : EAN=36=(Kr).
2. [Pt(NH3)₆]⁴⁺: EAN=86=(Rn).
3. [Pt(NH₃)3Cl₃]⁺ :EAN=86= (Rn).
4. [Mn₂(CO)₁₀]Or, [Mn(CO)5]=EAN=36(Kr).

Some complexes which do not follow EAN rule which are less stable

a. [Cr(NH₃)₆]⁺³ :EAN=33.

b. [Pt(NH₃)₄]⁺² : EAN=84.

c. [Ag(NH₃)₂]⁺ : EAN=50.

d. [Ni(NH₃)₆]²⁺ :EAN=38.

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.