Question

The number of different bond angles for nonpolar PBr₂F₃ is?

	A.	5
	B.	2
	C.	3
	D.	4

Answer 1

B. 2

First, we consider the equatorial trigonal plane. Since the Br-P-Br bond angle is larger than 120 degrees, the Br-P-F bond angles should each be 1) less than 120 degrees and 2) still identical. If you think about it, we started with an equatorial triangle and we simply changed the length of one of the sides (the side with the two bromine atoms). This in turn changes only one of the angles of the triangle and should leave two identical bond angles in its wake. So that's one set of identical bond angles. Now we consider more steric effects and interactions between the axial and equatorial atoms. We expect the bromine to push down on the fluorines because bromine is huge compared to fluorine. This distorts the perfect 180 degree bond angle in a perfect trigonal bipyramidal molecule and similarly the 90 degree bond angles in an ideal trigonal bipyramidal molecule. This in turn creates two sets of identical bond angles for a total of three. We have: 1) F-axial, P, and Br-equatorial times 2 (there are two bromine atoms and two axial fluorines). This is somewhat greater than 90 degrees. 2) The somewhat smaller than 90 degrees F-axial, P, and F-equatorial bond. Diagram attached.