(a) Explain the co-ordination isomerism and ionisation isomerism with example.
(b) Explain Radio-Carbon dating.

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Solution

(a) Co-ordination isomerism:
In a bimetallic complex, both complex cation and complex anion may be present. In such a case the distribution of ligands between the two coordination spheres can vary, giving rise to isomers called the coordination isomers.
Example:
$[{C o}^{I I I} {({N H}_{3})}_{6}] [C r {(C N)}_{6}]$
Hexammine Cobalt(III) hexacyano chromate(III) and
$[{C o}^{I I I} {({N H}_{3})}_{6}] [{{C o}^{I I I} (C N)}_{6}]$
Hexammine chromium (III) hexacyano cobaltate (III)
Ionisation isomerism:
Coordination compounds having the same molecular formula but forming different ions in solution are called ionisation isomers.
Example of this type of isomerism is furnished by the red violet.
$[C o {({N H}_{3})}_{5} B r] {S O}_{4}$ - Penta ammine bromocobalt (III) sulphate and
$[C o {({N H}_{3})}_{5} {S O}_{4}] B r$ - Penta ammine sulphato cobalt (III) bromide.
(b)
This method was developed by Willard and Libby to determine the age of wood or animal fossils. This method is based on the fact that ${_{6} C}^{14}$ , radioactive isotope of carbon is formed in the upper atmosphere by reaction with neutrons (from cosmic rays)
${_{7} N}^{14} + {_{0} n}^{1} ⟶ {_{6} C}^{14} + {_{1} H}^{1}$
The $C^{14}$ atoms thus produced are rapidly oxidised to ${^{14} C O}_{2}$ which in turn is incorporated in plants as result of photosynthesis. Animals too consume $C^{14}$ by eating plants. On death, organisms cease to take in fresh carbonations. Carbon-14 begis to decay.
${_{6} C}^{14} ⟶ {_{7} N}^{14} + {_{- 1} e}^{0}$
5700 years a fossil (plant or animal) will lose half the amount of carbon-14 present in its living state. Therefore by knowing either the amount of $C^{14}$ or the number of $β$ -particles emitted per minute per gram of carbon at the initial and final stages, the age of carbon material can be determined by the following equation
$t = \frac{2.303 \times t^{1 / 2}}{0.693} log \frac{A m o u n t o f C^{14} i n f r e s h w o o d}{A m o u n t o f C^{14} i n d e a d w o o d}$

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The two compounds sulphato penta-ammine cobalt(III) bromide and sulphato penta-ammine cobalt (III) chloride represent

1)linkage isomerism

2) ionisation isomerism

3)co ordination isomerism

4)no isomerism

[C o (H_{2} O)_{3} C l_{3}] 3 H_{2} O

[C o (H_{2} O)_{6}] C l_{3}

[C o (N H_{3})_{5} B r] S O_{4}

[C o (N H_{3})_{5} S O_{4}] B r

M C l_{2} B r_{2}

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