PF3Cl2 does not give PF3 + Cl2.Why?

Answer 1

PF3Cl2 is in sp3d state, when it is heated and undergoes decomposition, the orbitals realign and the F atoms approach the Cl atoms closely, this prevents the "empirical" separation of PF3 and Cl2.

Answer 2

PF3Cl2
The principal axis is the C3 axis passing through the Cl-P-Cl bonds. Perpendicular to this are three C2 axes, each one through a P-F bond. There is a horizontal plane of symmetry, sh, an axis of improper rotation S3, and three sv mirror planes.
here the central atom is Phosphorous, and a glimpse at the periodic table shows you it is in column V, and therefore has five valence electrons. Trigonal Bi Pyramidial structure is simply a fancy way of saying the P atom has two bonds running up and down along the axis of the P atom, and three bonds lying flat in a plane around the equator of the P atom. Since there are only two choices for where the F and Cl atoms are, equatorial or axial, and since there are only two axial and three equatorial bonds, deduction, rather than true understanding of chemical bonding, tells us that the two Cl atoms are axial and the three F atoms are equatorial. Now I would suggest thinking about the radii of the F and Cl atoms, and doing some geometry to see how close the F and Cl atoms, can get to the P atom, and I will venture a guess that since F is highly electronegative (slightly moreso than Cl), that the F atoms can fit closer to the P atom if they are in the equatorial positions, than the Cl could if they were in the equatorial positions. By keeping the distances between the F and P small, we are reducing the potential energy of the molecule, which is thermodynamically favorable.
The bond ceasure in this molecule is not as PF3+Cl2 since the two chlorine atoms are directly linked with the P atom through co-valent bonds and hence not susptible to ceasure.