Amongst the following,the most stable complex is:
$(i)$ $[Fe(H_2O)_6]^{3+}$
$(ii)$ $[Fe(NH_3)_6]^{3+}$
$(iii)$ $[Fe(C_2O_4)_3]^{3-}$
$(iv)$ $[FeCl_6]^{3-}$

$A$ 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 $Ag = 108 \ u$)

Which among the following is a correct decreasing order of thermodynamic stability of the complexes?

Which complex cannot ionise in solution?

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

$CrCl_3 \cdot xNH_3$ can exist as a complex. $A$ $0.1 \ m$ aqueous solution of this complex shows a depression in freezing point of $0.558^{\circ} C$. Assuming $100 \%$ ionisation of this complex and that the coordination number of $Cr$ is $6$,the complex will be: (Given $K_f = 1.86 \ K \ kg \ mol^{-1}$)