Chelates and chelate effect

1. Chelates and chelate
effect

2. Course Outlines
Chemistry of d-Block Elements and Coordination Complexes
• Back ground of coordination chemistry → Done
• General chemical and physical properties of transition elements →
Done
• Comparison of the elements of first transition series (3d) with
those of second (4d) and third (5d) series → Done
• Nomenclature and Structure of coordination complexes with
coordination number 2-6 → Done
• Chelates and chelate effect → Continue
• Theories of coordination complexes
• Werner's Theory
• Valence Bond Theory (VBT)
• Crystal Field Theory (CFT)
• Molecular Orbital Theory (MOT)
• Sidgwick’s electronic interpretation
of coordination
• Effective atomic number (EAN)
• Jahn-Teller theorem
• Magnetic properties
• Spectral properties
• Isomerism
• Stereochemistry
• Stability constants of
coordination
complexes

3. Ligands
• The neutral molecules or ions (usually anions) which are attached
with the central metal ion are called ligands. OR
In coordination chemistry, a ligand is
an ion or molecule (functional group) that binds to a
central metal atom to form a coordination complex.
The bonding between metal and ligand generally involves formal
donation of one or more of the ligand's electron pairs.
The nature of metal-ligand bonding can range from covalent to
ionic.
Furthermore, the metal-ligand bond order can range from one to
three.
Types of Ligands
– Neutral ligands (NH3, CO)
– Positive ligands (NH2-NH3
+)
– Negative ligands (CN-, CNS-, OH-)

4. • For example, in the complex ion, [Fe(CN)6]3- the six (CN-) ions
are the ligands.
• In most of complexes a ligand acts as a donor partner, i.e., donates
one (or more) electron pair to the central metal.
• Note that in metallic carbonyls the ligand viz CO molecule acts
both as donor and acceptor (M CO).
Substrate
• A substrate may be defined as the reactant in which some bonds
are broken and some new ones are formed as a result of the attack
of a reagent.
Substrate Product
Attacking reagent Attacking reagent

5. Chelates
• A compound containing a ligand (typically organic)
bonded to a central metal atom at two or more points.
• A chelating ligand is a bidendate or polydentate ligand
which is attached to the same central metal atom by two
or more of its donor atoms resulting in the formation of a
complex having a strain-free ring structure.
• The complex having the ring structure is called chelate or
chelated complex.
• The chelate is also called by various other names like
cyclic complex, ring-type complex etc.
• The formation of a chelate is called chelation or
cyclisation.

6. • The term chelate was first applied in 1920 by GT
Morgan and HDK Drew.
• The adjective (‫)صفت‬ chelate, derived from the great claw
(‫)پنجہ‬ or chela ( ‫کيکڑے‬‫۔‬‫بچهو‬ ) (from the Greek) of the
lobster (‫)چهينگا‬ or other crustaceans ( ‫خول‬‫دار‬ ), is
suggested for the caliper ( ‫خمدار‬‫يا‬‫دهنکی‬‫پرکار‬ ) like groups
which function as two associating units and fasten to the
central atom so as to produce heterocyclic rings.

13. Chelate effect (Stability of chelates)
• Chelates are more stable than similar non-chelated complexes of the same
metal ion.
• Greater stability of chelated complex means that there will be smaller
dissociation of the chelated complex into its components in a particular
solvent.
• The enhanced stability conferred on a chelate is called chelate effect.
• The greater stability of a chelated complex as compared to that of a non-
chelated complex is due to the increase in entropy.
• Thus when we consider the formation of [Co(NH3)6]2+ (non-chelated
complex) and [Co(en)3]2+ (chelated complex) from [Co(H2O)6]2+ as
represented by reactions (a) and (b) given below:

14. • We find that number of particles on the right hand side of reaction
(b) which represents the formation of chelated complex is greater
than that on the left hand side while the number of particles on both
sides of reaction (a) representing the formation of non-chelated
complex is the same.
• Consequently the chelated complex, [Co(en)3]2+ is more stable than
the non-chelated complex, [Co(NH3)6]2+.
• The stability of chelated complexes increases with the increase of
the number of donor atoms present in the chelating ligand.
• The stability of chelated complexes also depends on the size of the
ring formed in the complex.
• The chelated complexes containing 4-membered rings are not stable
while those containing 5- and 6-membered ring are quite stable.
• A chelated complex given by a polydentate ligand containing
double bonds is more stable than that given by a polydentate ligand
containing only single bonds.

15. • For example an octahedral complex of acetylacetonato ligand,
is more stable than the octahedral complex of ethylenediamine with
the same metal ion, [M2+(NH2—CH2—CH2—NH2)3]2+.
• Note that both the complexes are chelated complexes.
• With the increase of the number of the rings present in the chelated
complex, the complex becomes more stable.

24. The End