The colour of $KMnO_4$ is due to

When concentrated $HCl$ is added to an aqueous solution of $CoCl_2$,its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction?

In $[Fe(CO)_5]$,the $Fe-C$ bond has

Which of the following complexes is the most stable?

Which complex cannot ionise in solution?

The $IR$ stretching frequencies of free $CO$ and $CO$ in $[V(CO)_6]^-$,$[Cr(CO)_6]$ and $[Mn(CO)_6]^+$ are $2143 \, cm^{-1}$,$1860 \, cm^{-1}$,$2000 \, cm^{-1}$ and $2090 \, cm^{-1}$,respectively. Then the correct statement about metal carbonyls is: