[Kfjeldheim]

Our Teacher posed us a question in class the other day.  How would you draw the structural formula of PF2Cl3 if it was non polar.  I know that it molecular gemoetry is trigonal planar, and i beleive that the Cl atoms get place equtorial at 120 degrees, and the F axial at 90 degrees.  But wouldn't that make it polar because F is more electronegative?  Should F be placed at the 120 degree angles so they are further from each other?  Where would you position the F and Cl to get a nonpolar molecule?

[movies]

There two F atoms which would be placed at 180 degree angles to one another, right?  So the overall geometry is really trigonal bipyramidal.  The two P-F bonds are opposite one another so the dipoles cancel each other out.  The dipoles also cancel out in the trigonal plane with the chlorines.

[Kfjeldheim]

Yes, the F atoms are 180 degrees from each other but ony 90 degress from the Cl atoms.  Does that make a difference in the polarity?

[Donaldson Tan]

In such an arrangement, the dipole moment from a C-F bond would be cancelled out by another C-F bond because the 2 F atoms are 180 degree from each other. their dipole moment are equal in magnitude but opposite in direction.

You would not have to worry about the effect of Cl being 90degree away, because each F atom is 90degree away from 3 Cl atom, so they both experience the same degree of electronic repulsion from the neighbouring Cl atoms.

[movies]

Right, as geodome said, since the P-F bonds are orthogonal to the P-Cl bonds the dipoles don't affect each other.