Finding the molecule with the smallest bond angle in the following lists of elements

Question

For each of the following lists of elements, find (and give a justification for your choice) the molecule with the smallest bond angle:
a) $\ce{OSF_2, OSCl_2, OSBr_2}$
b) $\ce{SbCl_3, SbBr_3, SbI_3}$
c) $\ce{PI_3, AsI_3, SbI_3}$

Here is what I think:

a) $\ce{OSF_2}$ will have the smallest bond angle because $\ce{F}$ is more electronegative and it will attract more pairs of electrons to it.
b) $\ce{SbCl_3}$ with the same reasoning.
c) $\ce{PI_3}$ has the smallest bond angle because it is more electronegative.

Is my reasoning correct?

Answer 1

All three sets of compounds can be explained using the Bent's Rule which states

Atomic s character concentrates in orbitals directed toward electropositive substituents.

Now, let's proceed case by case here.

Case 1: Substituent atom changes, Central atom remains same

In this case, as the electronegativity of the substituent increases, the p-character of the bond increases, which reduces the bond angle.

A case similar to your first case has already been discussed on the site. I would urge you to look at this question and its accepted answer. The only difference is that the thionyl group has been substituted for a carbonyl group.

Case 2: Central atom changes, the substituents remain the same

In this case, (your case 3), we are asked to compare between three molecules where the central atom becomes more electropositive as we go through the compounds.

Due to this increase in atomic s-character in the central atom, the p-character in the bond increases, which leads to a decrease in bond angle. Therefore $\ce{PI3}$ would have the largest bond angle in your third set of questions.

Also see: Why does a larger bond angle correspond to greater s-character?