$3$ moles of metal complex with formula $\mathrm{Co}(\mathrm{en}{)}_2{\mathrm{Cl}}_3$ gives $3$ moles of silver chloride on treatment with excess of silver nitrate. The secondary valency of $\mathrm{Co}$ in the complex is (Round off to the nearest integer)

The number of stereo isomers possible for ${\left[\mathrm{Co}{\left(\mathrm{ox}\right)}_2\left(\mathrm{Br}\right)\left({\mathrm{NH}}_3\right)\right]}^{2-}$ is __________.

[$\mathrm{ox}=$ oxalate]

Arrange the following Cobalt complexes in the order of increasing Crystal Field Stabilization Energy (CFSE) value.

Complexes: $\begin{array}{cccc}\underset{\mathrm{A}}{{\left[{\mathrm{CoF}}_6\right]}^{3-}},& \underset{\mathrm{B}}{{\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}},& \underset{\mathrm{C}}{{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}}\text{ and }\underset{\mathrm{D}}{{\left[\mathrm{Co}(\mathrm{en}{)}_3\right]}^{3+}}& \end{array}$

Choose the correct option :