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Introduction to CFT

When cobaltII...

Question

When cobalt $(I I)$ chloride is dissolved in water, a pink solution A is formed. When ammonia is passed through the solution, orange red solution is obtained. Then the sum of the coordination number and the oxidation state of central metal in the yellow coloured solution is :

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Solution

$C o C l_{2} H_{2} O - - \to [C o (H_{2} O)_{6}]^{2 +} Pink N H_{3} (Excess) - ------- \to [C o (N H_{3})_{6}]^{2 +}$
Here $[C o (N H_{3})_{6}]^{2 +}$ is unstable and oxidised to $[C o (N H_{3})_{6}]^{3 +}$ , which is yellow in colour.
$[C o (N H_{3})_{6}]^{2 +} \to [C o (N H_{3})_{6}]^{3 +} Yellow$
$∴$ Coordination number $= 6$
Oxidation state $= + 3$
$∴$ Sum is $6 + 3 = 9$

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