When ammonia is added to a cupric salt solution,the deep blue colour is observed; it is due to the formation of

The brown ring test for $NO_2^-$ and $NO_3^-$ is due to the formation of a complex ion with the formula:

Assertion : The $[Ni(en)_3]Cl_2$ ($en =$ ethylenediamine) has lower stability than $[Ni(NH_3)_6]Cl_2$.
Reason : In $[Ni(en)_3]Cl_2$,the geometry of $Ni$ is trigonal bipyramidal.

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.

On treatment of $100 \ mL$ of $0.1 \ M$ solution of $CoCl_3 \cdot 6H_2O$ with excess $AgNO_3$,$1.2 \times 10^{22}$ ions are precipitated. The complex is:

In the following reaction sequence in aqueous solution,the species $X, Y$ and $Z$,respectively,are