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Coordination Compounds

The first synthetic dye The first synthetic dye is a result of wrong experiment. The pigment used for the above paint is ‘prussian blue’, a coordination complex named ferric hexacyanoferrate(III) ([Fe4[Fe(CN)6]3]). It is the first synthetic dye which was accidentally discovered by a German paint maker, Diesbach. He was attempting to create cochineal red pigment in his laboratory. "Cochineal" was originally obtained by crushing the bodies of cochineal insects.
Diesbach obtained potash contaminated animal oil and mixed it with the iron sulphate. He expected a red pigment but he got one that was very pale, then he concentrated the solution with more iron sulphate, but instead of a darker red he was expecting, he first got a purple, then a deep blue. He had accidentally created the first synthetic blue pigment, Prussian blue.

Learning Objectives

After completing the topic, the student will be able to:

  • Differentiate double salts and coordinate compounds.
  • Describe the constituents in a coordination compound and name the given complex compound.
  • Classify the ligands based on charge, mode of bonding and density.
  • Appreciate various bonding theories developed to describe nature of bonding in complex compounds.
  • Predict the geometry of coordination compounds.
  • Examine the color and magnetic properties of complexes.
  • Explain the different types of isomerism shown by complexes.
  • Calculate number of isomers possible for a given compound.
  • Give the synthetic methods of coordination complexes and recognize their importance and uses.
a) Hydroxyanthraquinone b) Alizarin dye a) Hydroxy anthra-quinone b) Alizarin dye
Coordination compounds

Coordination refers to the "coordinate covalent bonds". The bond is located between the ligands (small compounds) and metal atom. Metal is located at the center and is surrounded by ions or ligands.

The earliest known coordination compound is alizarin dye. It is used by the ancient Persians, Egyptians as dying agent for clothes. It has chemical name: Calcium aluminate of hydroxy anthra-quinone. Hydroxy anthra-quinone (a) is an organic compound. The hydroxy functional groups(OH) are attracted by aluminium to form a negatively charged ligand. Another metal calcium is positively charged and forms a complex structure with negatively charged ligand. At equilibrium, the total complex develops bright red color due to absorption of corresponding wavelength of light. Refer adjacent image for the structure of the complex: Alizarin dye (b).

Certain biological compounds (Hemoglobin, Chlorophyll) Persian blue, Indigo red and salts like Blue vitriol [CuSO4.5H2O] etc; are other examples of coordination compounds. The study of such metals in compounds or complexes helps to understand the bio-chemical functions in day to day life.

Chlorophyll Chlorophyll Chlorophyll is a coordination compound of magnesium (Mg) responsible for photosynthesis in plants.
Haemoglobin It is a coordination compound of iron (Fe) and it regulates the respiration process. Haemoglobin and hemoglobin–like molecules are also found in many invertebrates, fungi, and plants.
Metal complexes

When the metal in the coordination compound (in solution) retains its identity, the compound is called metal complex. For example, copper in hydrated tetra-amino cuprous sulphate solution.

Stability of compounds:
It is referred to the degree of association between metal and ligands at equilibrium.
Example: Formation of cuprammonium ion.

The addition of four amine groups to copper takes place in four stages. The change in concentration of solution at each stage is given by formation constant [Kx] and is expressed as log Kx where x denotes number of stage. The corresponding formation constants calculated on experimental data are: log K1 = 4, log K2 = 3.2, log K3 = 2.7 and log K4 = 2. The overall stability of metal complex is given by association constant (β).

The summation of individual formation constants gives degree of associatino, a measure of stability of complex. log β = Σ log Kx = 11.9.

The reciprocal of formation constant gives dissociation constant (measure of instability) of coordination complex.

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