Why is diboron tetrachloride planar as a solid but staggered as a gas?

B₂Cl₄ has a B–B single bond, as expected based on the Lewis diagram for the molecule. The linked figure below shows the structure in the solid state; the B–B distance, 1.752 Å, is just what one expects for a B–B single bond. The B–Cl distances average 1.726 Å. The solid state structure shows that there are some long intermolecular B–Cl contacts roughly above and below the B₂Cl₄ plane at an average distance of 3.26 Å. (These are shown as light dotted lines.) Since the boron atoms have empty p orbitals directed into that region and the Cl atoms on neighboring molecules have lone pairs of electrons, these interactions are reasonably characterized as Lewis acid-base interactions that are more easily achieved if chloride lone-pair–lone-pair repulsions are avoided and that favors a planar B₂Cl₄ structure. In the gas phase, there are no such intermolecular contacts and Cl–Cl steric repulsions in the planar structure (3.46 Å) are eliminated of the molecule adopts a staggered structure.