What is 'X' in the given reaction?

The solubility of Ca(OH)₂ in water is :

[Given : The solubility product of Ca(OH)₂ in water = 5.5 $$\times$$ 10^$$-$$6]

Which among the following species has unequal bond lengths?

In which of the following order the given complex ions are arranged correctly with respect to their decreasing spin only magnetic moment?

(i) [FeF₆]^3$$-$$

(ii) [Co(NH₃)₆]³⁺

(iii) [NiCl₄]^2$$-$$

(iv) [Cu(NH₃)₄]²⁺