Question

Which of the following compounds is not coloured?

• A. $Na _{2}\left[ CuCl _{4}\right]$
• B. $Na _{2}\left[ CdCl _{4}\right]$
• C. $K _{4}\left[ Fe ( CN )_{6}\right]$
• D. $K _{3}\left[ Fe ( CN )_{6}\right]$

Answer 1

Answer: (B)

Oxidation state of

$Cu$ in $Na _{2}\left[ CuCl _{4}\right]=+2$ i.e., $Cu ^{2+} \rightarrow 3 d^{9}$

$Cd$ in $Na _{2}\left[ CdCl _{4}\right]=+2$ i.e., $Cd ^{2+} \rightarrow 4 d^{10}$

$Fe$ in $K _{4}\left[ Fe ( CN )_{6}\right]=+2$ i.e., $Fe ^{2+} \rightarrow 3 d^{6}$

$Fe$ in $K _{3}\left[ Fe ( CN )_{6}\right]=+3$ i.e., $Fe ^{3+} \rightarrow 3 d^{5}$

Since $Cd$ in $Na _{2}\left[ CdCl _{4}\right]$ possesses completely filled $d$ -subshell, it is a colourless compound.