Arrange the following in increasing order of their crystal field splitting energy

I. $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$

II. $\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$

III. $\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}$

IV. $\left[\mathrm{CoF}_{6}\right]^{3-}$

$\mathrm{Pt}+3: 1$ mixture of $\left(\right.$ Conc. $\mathrm{HCl}+$ conc. $\left.\mathrm{HNO}_3\right) \rightarrow[\mathrm{X}]^{2-}$

What is the oxidation state of Pt in $[\mathrm{X}]^{2-}$ complex ion ?