A deep blue complex is formed when conc. ammonium hydroxide is added to an aqueous solution of CuSO4 . Which of the following formulae represents the complex ? a) [Cu(NH3)4]^2+ b) [Cu(NH3)4]+ c)…

When concentrated ammonium hydroxide ($\mathrm{NH}{}_4\mathrm{OH}$) is added to an aqueous solution of $\mathrm{CuSO}{}_4$, it forms a deep blue complex due to the coordination of ammonia ($\mathrm{NH}{}_3$) molecules with the $\mathrm{Cu}{}^{2+}$ ion.

The reaction can be represented as: $$\mathrm{Cu}{}^{2+}+4\mathrm{NH}{}_3\to [\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$$ Here, $\mathrm{Cu}{}^{2+}$ acts as the central metal ion, and $\mathrm{NH}{}_3$ acts as a ligand. The coordination number of $\mathrm{Cu}{}^{2+}$ in this complex is 4, forming a square planar or tetrahedral geometry, which gives the characteristic deep blue color.

Among the given options, the correct formula of the complex is: a) $[\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$

This matches the observed coordination number and charge balance. The other options either have incorrect coordination numbers or incorrect charges.