$\mathrm{Pt}+3: 1$ 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 ?

Which of the following will have a spin only magnetic moment of 2.86 BM ?

The molecular formula of a coordinate complex is $\mathrm{CoH}_{12} \mathrm{O}_6 \mathrm{Cl}_3$. When one mole of this aqueous solution of complex is reacted with excess of aqueous $\mathrm{AgNO}_3$ solution, three moles of AgCl was formed. What is the correct formula of the complex ?

The sum of oxidation state and co-ordination number of central metal atom is maximum with respect to which of the following complex?