Consider the molecule \(\mathrm{PF}_{4}\) Cl. (a) Draw a Lewis structure for the molecule, and predict its electron-domain geometry. (b) Which would you expect to take up more space, a \(\mathrm{P}-\mathrm{F}\) bond or a \(\mathrm{P}-\mathrm{Cl}\) bond? Explain. (c) Predict the molecular geometry of \(\mathrm{PF}_{4} \mathrm{Cl}\). How did your answer for part (b) influence your answer here in part (c)? (d) Would you expect the molecule to distort from its ideal electron-domain geometry? If so, how would it distort?

First, find the total number of valence electrons in the molecule which is composed of: 5 valence electrons from \(\mathrm{P}\), 7 valence electrons from \(\mathrm{Cl}\), and 4 × 7 (7 from four \(\mathrm{F}\) atoms) = 28 valence electrons from the \(\mathrm{F}\) atoms. The total number of valence electrons in the molecule is 5 + 7 + 28 = 40. To draw the Lewis structure, arrange the atoms in such a way that \(\mathrm{P}\) is the central atom surrounded by \(\mathrm{F}\) atoms and \(\mathrm{Cl}\). Next, place one pair of electrons between each two bonded atoms (one \(\mathrm{P}-\mathrm{F}\) and one \(\mathrm{P}-\mathrm{Cl}\)), and then fill the octets of the surrounding atoms (both \(\mathrm{F}\) and \(\mathrm{Cl}\)). Since \(\mathrm{P}\) has 5 valence electrons and it is bonded with 5 other atoms (4 \(\mathrm{F}\) atoms and 1 \(\mathrm{Cl}\)), its electron-domain geometry is described by five electron domains: 5 bonds around the \(\mathrm{P}\) atom (no lone pairs). Therefore, the electron-domain geometry of \(\mathrm{PF}_{4}\mathrm{Cl}\) is trigonal bipyramidal.

Although both fluorine and chlorine belong to Group 17, chlorine is larger in size than fluorine. The larger the atom, the longer the bond will be. Therefore, a \(\mathrm{P}-\mathrm{Cl}\) bond is expected to take up more space than a \(\mathrm{P}-\mathrm{F}\) bond.

In a trigonal bipyramidal electron-domain geometry, there are two types of positions: axial (two positions) and equatorial (three positions). The less bulky groups (or those that take up less space) occupy axial positions, whereas the bulkier groups/events occupy equatorial positions to minimize the electronic repulsion. Based on our conclusion in part (b), the \(\mathrm{P}-\mathrm{Cl}\) bond takes up more space, so it will occupy one of the three equatorial positions to minimize electronic repulsion. The remaining positions will be occupied by \(\mathrm{F}\) atoms. Therefore, the molecular geometry of \(\mathrm{PF}_{4}\mathrm{Cl}\) can be described as a trigonal bipyramid, with three \(\mathrm{F}\) atoms and one \(\mathrm{Cl}\) atom in the equatorial plane and two more \(\mathrm{F}\) atoms at axial positions.

Due to the size difference between the \(\mathrm{P}-\mathrm{F}\) and \(\mathrm{P}-\mathrm{Cl}\) bonds, there might be some distortion in the molecular geometry of \(\mathrm{PF}_{4}\mathrm{Cl}\) from the ideal trigonal bipyramidal structure. The equatorial \(\mathrm{P}-\mathrm{Cl}\) bond may slightly push the equatorial \(\mathrm{P}-\mathrm{F}\) bonds away from the ideal 120° angle, leading to a decrease in the equatorial bond angles and a slight distortion of the molecule. Axial bond angles may also differ from 180°. However, this distortion should not be significant enough to cause drastic changes in the overall molecular geometry.