Which of the following is not true for a ligand-metal complex?

The reaction of nitroprusside anion with sulphide ion gives purple colouration due to the formation of

Explain the chelating effect or chelation.

Which of the following represents the correct increasing order of the spectrochemical series (field strength) for some common ligands?

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.

The tendency to form complexes increases with ..........