Question

In which of the following the repulsion is least?

• A.
• B.
• C.
• D. All of these have same repulsion

Answer 1

The correct option is A

Lets look at all the options here.

This is a trigonal bipyramidal geometry having 3 bond pairs and 2 lone pairs.

Now, first we will take repulsions on the smallest angle (which is ${90}^{\circ }$)

Lets take 1st structure,

If we take upper pyramid, the repulsions are:

1bp - bp, 2lp - bp, 0lp - lp

In the lower pyramide,

1bp - bp, 2lp - bp, 0lp - bp

Total = 2bp - bp, 4lp - bp and 0lp - bp

In II structure,

Upper pyramid,

3lp - bp, 0bp - bp, 0lp - lp

Lower pyramid,

3lp - bp, 0bp - bp, 0lp - lp

Total = 0bp - bp, 6lp - bp, 0lp - lp

In III structure, Upper pyramid,

2lp - bp, 0bp - bp, 1lp - lp

Lower pyramid,

2bp - bp, 1lp - bp, 0lp - lp

Total $\to$ 2bp - bp, 3lp - bp, 1lp - lp

Now, we know that

$lp-lp>lp-bp>bp-bp$

Now, as III structure contains 1lp - lp and other two does not. Therefore structure III will be least stable.

From I and II, we can see lp - bp repulsions are more I structure II.

Therefore structure I will be most stable.