Question

why [cu(cn)4]2- is colourless

Answer 1

(_______A compound which have unpaired electron can impart colour due to d-d transition. ... Hence, [Cu(NH3)4]^2+ ion is coloured whereas [Cu(CN)4]^3–does not have unpaired electron. it is a colourless compound________)

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Answer 2

Because it does not contain unpaired electron

Explanation:

In coordination complex $[Cu(CN)_4]^{3-}$, the oxidation state of copper is +2 as cyanide is with -1 oxidation state.

x+4(-1)=-3

x=+1

Atomic number of copper is 29.

The electronic configuration of copper will be,  

$1s^22s^23s^23p^64s^13d^{10}$

The electronic configuration of $Cu^{+}$ will be,

$1s^22s^23s^23p^63d^{10}$

As unpaired electrons present in the ion show color due to the transition of the electrons from ground state to the excited state. In case of $[Cu(CN)_4]^{3-}$ there is no unpaired electron in copper that means the shells are fully filled. So, it will be colorless.

Learn more about colored and colorless salts

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