Complexation: Classification of complexes and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam

Complexes are compounds that result from donor–acceptor mechanisms between two or more chemical species.

Complexes can be divided broadly into three classes depending on the type of the acceptor substance:

1. Metal ion complexes
2. Organic molecular complexes
3. Inclusion complexes

Complexes are compounds that result from donor–acceptor mechanisms between two or more chemical species.

Complexes can be divided broadly into three classes depending on the type of the acceptor substance:

1. Metal ion complexes
2. Organic molecular complexes
3. Inclusion complexes                                                                                                                                                                                                        Intermolecular forces involved in the formation of complexes:

1. Van der Waals forces.
2. Hydrogen bonds (important in molecular complexes).
3. Coordinate covalence (important in metal complexes).
4. Charge transfer
5. Hydrophobic interaction.

Metal complexes: Metal ion complex (coordination complex) consists of a transition-metal ion (e.g. cobalt, iron, copper, nickel and zinc) linked or coordinated with one or more counter ions or molecules to form a complex. The ions or molecules (e.g. Cl– , NH3 , H2O, Br– , I– , CN– , etc.) directly bound with the metal are called ligands.

The number of ligands bound to the metal ion is defined as coordination number. Coordination number usually determines the geometry of the complex.

1. Inorganic type- Werner postulates:
2. There are 2 types of valences primary (ionic), secondary (coordinate).
3. Same type of anion/ radical/ molecule may be held by anyone / both type of valence.
4. Every central atom has fixed number of non-ionic valences (co- ordination number)
5. The co-ordination atoms occupy the first sphere/coordination sphere, other atoms occupy second/ ionization sphere.
6. Neutral molecules/ions may satisfy non-ionic valences.
7. The non-ionic valences are directed to specific positions in space. Ex: [Co Cl (NH3)5] Cl2 Substrate Coordination sphere Ionization sphere.
8. Chelates- These are group of metal ion complexes in which a substrate/ ligand provides 2/more donor groups to combine with a metal ion. Ligands- didentate, tridentate, polydentate. Hexadentate – ethylenediaminetetraacetic acid (EDTA)- Has a total of six points (4:0 and 2: N) for attachment of metal ions. Sequestering: This is a process in which the property of metal is suppressed without removing it from the solution. Sequestering Agent: This is a ligand which forms a stable water soluble metal chelate Ex: chlorophyll, hemoglobin.žThese are group of metal ion complexes in which a substrate/ ligand provides 2/more donor groups to combine with a metal ion. Ligands- didentate, tridentate, polydentate.
9. Olefin type- These involves Lewis acid-base reactions b. These type of complexes can be used as catalysts in the manufacturing of bulk drugs, intermediates and in drug analysis.

Organic molecular complexes: Organic molecular complexes are formed as a result of noncovalent interactions between a ligand and a substrate. The interactions can occur through van der waals forces, charge transfer, hydrogen bonding or hydrophobic effects.

1. Drug-caffeine complex- Complexes Mechanism:
2. dipole-dipole forces/ hydrogen bonding between acid (H) atom and caffeine carboxyl group.
3. Interaction of non-polar parts Ex: Caffeine + Benzocaine.èAcidic drugs (benzocaine, procaine) + Caffeine
4. Polymer type- Polymers with nucleophilic oxygen (PEG/CMC) +Drugs(tannic acid/salicylic acid/phenols)

Complexes. Disadvantages: 1. Incompatibilities in suspension, emulsion, ointments.

2.Complexes + ContainerèPolymers with nucleophilic oxygen (PEG/CMC) +Drugs(tannic acid/salicylic acid/phenols)

3. Picric acid type- Picric acid (strong acid) + weak baseèPicric acid (strong acid) + strong base  Complexes. Ex: BUTESIN PICRATE Picric acid (antiseptic) + Butesin (anesthetic) 1% ointment used for burns and abrasions.

DISADVANTAGES: Picric acid + Carcinogenic Agents èSalt. Picric acid (strong acid) + weak base èPicric acid (strong acid) + strong base.

4. Quinhydrone type- Alcoholic solutions of equimolar quantities of Hydroquinone and Benzoquinone form Quinhydrone complexes (green crystals) Mechanism: 1. Overlapping of π electrons of molecules 2. (H) bonding for stabilizing complex. Applications: Used as electrode in pH determination. Hydroquinone Benzoquinone

Inclusion complexes: An inclusion compound is a complex in which one chemical compound (the ‘host’) forms a cavity in which molecules of a second compound (‘guest’) are entrapped. These complexes generally do not have any adhesive forces working between their molecules and are therefore also known as no-bond complexes.

1. Channel Lattice Type- Host (tubular channel)- Deoxycholic acid, urea, thiourea, amylose Guest (long unbranched straight chain compounds)- paraffin, esters, acids, ethanol. Ex: Starch-iodine solution (starch-host) Urea-methyl α-lipolate (urea-host) Applications: • Seperation of isomers: Dextro, levo-terpineol are separated using Digitoxin. • In analysis of dermatological creams, long chain compounds interfere and removed by complexation with urea.
2. Layer type- Layer type complex (or intercalation compound) is a type of inclusion compound in which the guest molecule is diffused between the layers of carbon atom, to form alternate layers of guest and host molecules. E.g. Montmorillonite, the principal constituent of bentonite, can trap hydrocarbons, alcohols, and glycols between the layers of their lattices. Graphite can also intercalate compounds between its layers.
3. Clathrates- The clathrates are compounds that crystallize in the form of a cage-like lattice in which the coordinating compound is entrapped. One official drug, warfarin sodium, is in the form of crystalline clathrate containing water and isopropyl alcohol. Clathrates can be used to separate optical isomers. E.g. Hydroquinone crystallizes in a cage-like hydrogen-bonded structure, in which small molecules such as methyl alcohol, CO2 , and HCl may be trapped in these cages.
4. Mono molecular type- Monomolecular inclusion complex involves the entrapment of guest molecules into the cage-like structure formed from a single host molecule. E.g. Cyclodextrins are a family of compounds made up of sugar molecules bound together in a ring (cyclic oligosaccharides) They consist of 6, 7, and 8 units of glucose referred to as a, þ, and ç cyclodextrins, respectively.

Multiple choice questions (MCQs)

1.The number of dative bonds to the central metal ion is its

a)Oxidation number

b)Compound number

c)Coordination number

d)Dative number

2.Ions which are produced from ligands are

a)Cation

b)Anion

c)Complexation

d)None of the above

3.Different ions will split up by different compounds to give of 

a)Same colored complexes

b)Different colored complexes

c)Same density complexes

d)Same temperature complexes

4.Ligands which can form two coordinate bonds from each ion or molecule to the transition metal ion are known as

a)Ligand ions

b)Dentate ligands

c)Monodentate ligands

d)Bidentate ligands

5.Due to ligands action of splitting the color of transition metal compound, the change occurs at

a)S-orbital

b)D-orbital

c)P-orbital

d)F-orbital

6.The ______ sphere is enclosed in brackets in formulas for complex species, and it includes the central metal ion plus the coordinated groups.

a) ligand

b) donor

c) oxidation

d) coordination

7.In coordination chemistry, the donor atom of a ligand is

a) a Lewis acid.

b) the counter ion

c) the central metal atom.

d) the atom in the ligand that shares an electron pair with the metal.

8.Consider the coordination compound, Na₂[Pt(CN)₄]. The Lewis acid is

a) [Pt(CN)₄]^2–

b) Na⁺

c) Pt

d) Pt²⁺

9.Consider the coordination compound, K₂[Cu(CN)₄]. A coordinate covalent bond exists between

a) K⁺and CN^–

b) Cu²⁺ and CN^–

c) K⁺ and [Cu(CN)₄]^2–

d) C and N in CN^–

10.The central atom/ion of a coordination complex is also referred to as ________

a) Lewis acid
b) Lewis base
c) Bronsted-Lowry acid
d) Bronsted-Lowry base

11.Identify the Lewis acid in K₃[Al(C₂O₄)₃]

a) K⁺
b) Al
c) Al³⁺
d) [Al(C₂O₄)₃]^3-

12.Which of the following is the central atom/ion in [CoCl(NH₃)₅]²⁺?

a) Co
b) Co²⁺
c) Co³⁺
d) Cl^–

13.Coordination number is a characteristic of which of the following?

a)Central atom

b)Ligand

c)Coordination entity

d)Coordination compound

14.Identify the coordination sphere in the compound K₄[Fe(CN)₆]

a)K⁺
b)Fe²⁺
c)[Fe(CN)₆]^4-
d) CN^–

15.Example of chelate

a)Cisplatin

b)Iodine

c)Ferrocene

d)Haemoglobin

Solutions:

1. c) coordination number
2. c) complexation
3. b) Different colored complexes
4. d) bidentate ligands
5. b) d-orbital
6. d) coordination
7. d) the atom in the ligand that shares an electron pair with the metal
8. d) Pt²⁺
9. b) Cu²⁺and CN^–
10. a) lewis acid
11. c) Al³⁺
12. c) Co³⁺
13. a) central atom
14. c)[Fe(CN)₆]^4-
15. d) Haemoglobin

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 162-177.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 359-379.

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