The bond angle is maximum in:

1. $N{H}_3$

2. $N{H}_4^{+}$

3. $PC{l}_3$

4. $SC{l}_2$

Which of the following pairs contains species that are not isostructural?

1. ${\mathrm{IO}}_3^{-}\text{and\hspace{0.17em}}{\mathrm{XeO}}_3$

2. ${\mathrm{BH}}_4^{-}\text{and\hspace{0.17em}}{\mathrm{NH}}_4^{+}$

3. ${\mathrm{PF}}_6^{-}\text{and\hspace{0.17em}}{\mathrm{SF}}_6$

4. ${\mathrm{SiF}}_4\text{\hspace{0.17em}and\hspace{0.17em}}{\mathrm{SF}}_4$

In a regular octahedral molecule, MX₆, the number of X–M–X bonds at 180º is: