B

B. E. Douglas, D. H. McDaniel, J. J. Alexander, Concepts and Models of Inorganic Chemistry, 3^rd edition, John Wiley & Sons, New York, 1994, page 103.

Problem 2.9

Which of the following in each pair has the largest bond angle? Why?

(a) CH₄ and NH₃; (b) OF₂ and OCl₂; (c) NH₃ and NF₃; (d) PH₃ and NH₃.

Answer:

(a) CH₄ and NH₃

The lone pair repulsion in ammonia will reduce the H-N-H bond angle to below 109^o; this repulsion does not exist for methane, so CH₄ has the largest bond angle.

(b) OF₂ and OCl₂

The larger electronegativity of F will increase the repulsion between F atoms and the O lone pairs, decreasing the F-O-F angle, in comparison to Cl atoms in OX₂. The size difference between F and Cl is probably not enough to significantly affect the bond angle. Thus, OCl₂ has the larger bond angle.

The larger electronegativity of F will increase the repulsion between F atoms and the N lone pair, decreasing the F-N-F angle, in comparison to H atoms in NX₃. The size difference between F and H is probably not enough to significantly affect the bond angle. Thus, NH₃ has the larger bond angle.

(d) PH₃ and NH₃

P is a larger atom than N so the repulsion between the P lone pair and the P-H bond will be less effective than the repulsion between the N lone pair and the N-H bond. However, the P has a smaller tendency to hybridize (which is another way of saying that bond pair-bond pair repulsion becomes less important for P) so the H-P-H angle will reduce to a value closer to 90^o. The latter argument is more important so NH₃ has the larger bond angle.