Which among the following cations will form the lowest stability complex if the ligand remains the same?

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.

Which one of the following complexes is violet in colour?

$A$ solution containing $2.675 \ g$ of $CoCl_3 \cdot 6 NH_3$ (molar mass $= 267.5 \ g \ mol^{-1}$) is passed through a cation exchanger. The chloride ions obtained in solution were treated with excess of $AgNO_3$ to give $4.78 \ g$ of $AgCl$ (molar mass $= 143.5 \ g \ mol^{-1}$). The formula of the complex is
(At. mass of $Ag = 108 \ u$)

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

Silver chloride dissolves in excess of $NH_4OH$. The cation present in this solution is