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Saturday, August 24, 2019

Concept of colour in metal complexes :

(1) Origin of colours:
(2) d-d transition:
(3) Selection rule of d-d transition:
(4) Charge transfer:

Colour originated by charge transfer when electronic transition occurs from one part of the Complex to other part i.e such type are also called internal Redox reaction. Intensity of clour in such type transition is very high as they donot require to follow any selection rule.

(A) Charge transfer from ligands to metal:
Examples: MnO₄ ^-1 , MnO₄ ^-2 , CrO₄ ^-2, Cr₂O₇ ^-2 , [Fe(H₂O)₅(NO)]⁺² , Na₄[Fe(CN)₅(NOS)].

(B) Charge transfer from metal to ligands:
Examples: [Fe(CO)₅] , [Fe (pi-C₂H₅)₂], [Cr (pi-C₆H₆)₂],

(C) Charge transfer from metal to metal:

Examples:

Prussian Blue: (Fe(III))4[Fe(II)(CN)₆]₃

Prussian blue is formed when ferric(Fe^3) salt reacts with ferrocyanide. Here we see both oxidation states of iron but only +2 oxidation states of iron are observed to form coordination complexes with cyanide. And the other +3 oxidation state iron ions form the net coordination compound.

Turnbull Blue: [Fe(II)]3[Fe(III)(CN)₆]2

Ferrous (Fe^+2) salt reacts with potassium ferricyanide to form a blue precipitate which is called Turnbull's blue. The reaction is as follows:

$3 F e^{2 +} + 2 K_{3} [F e (C N)_{6}] \to F e_{3} [F e (C N)_{6}]_{2} ↓$

3 F e^{2 +} + 2 K_{3} [F e (C N)_{6}] \to F e_{3} [F e (C N)_{6}]_{2} ↓

3 F e^{2 +} + 2 K_{3} [F e (C N)_{6}] \to F e_{3} [F e (C N)_{6}]_{2} ↓

3 F e^{2 +} + 2 K_{3} [F e (C N)_{6}] \to F e_{3} [F e (C N)_{6}]_{2} ↓

3 F e^{2 +} + 2 K_{3} [F e (C N)_{6}] \to F e_{3} [F e (C N)_{6}]_{2} ↓

Postulates of Werner’s theory:

In coordination complex central atom metals exert two types of valencies; The important postulates of Werner’s theory are as follows:

(1) The primary or ionizable valencies which are satisfied by negative ions and equal the oxidation state of the central metal atom.

(2) The Secondary or non ionizable valencies which can be satisfied by neutral or negative ions/groups (or sometimes by cationic species -discovered later).

(3) The secondary valencies equal the coordination number of central metal atom/ion. This number is fixed for a metal.

(4) The ions/groups bound by the secondary linkages have characteristic spatial arrangements corresponding to different co-ordination numbers. In the modern terminology, such spatial arrangements are called coordination polyhedra. The various possibilities are

Coordination Number = 2 Linear

Coordination Number = 3 Triangular

Coordination Number = 4 Tetrahedral or sq. planar

Coordination Number = 5 square pyramidal or TBP

Coordination Number = 6 Octahedral

(5) The secondary valencies are generally represented by solid lines while the primary valencies are represented by dashed lines and the ions which satisfy both primary and secondary valencies will be drawn with both solid and dashed lines.

For example the complex [CoCl(H₂O)₅] Cl₂ is represented as