The strongest $CO$ bond is present in

When aqueous sodium hydroxide is added to an aqueous solution of chromium $(III)$ ions,a green-blue precipitate is first formed which re-dissolves to give a green solution. This green colour is due to

Which of the following is the most stable complex?

Given below are two statements: one is labelled as "Assertion $A$" and the other is labelled as "Reason $R$".
Assertion $A$: In the complexes $Ni(CO)_4$ and $Fe(CO)_5$,the metals have zero oxidation state.
Reason $R$: Low oxidation states are found when a complex has ligands capable of $\pi$-donor character in addition to the $\sigma$-bonding.
In the light of the above statements,choose the most appropriate answer from the options given below.

Which of the following orders is correct in the spectrochemical series of ligands?

Which one of the following complexes is violet in colour?