Consider the structures of the following two molecules:

$X:{\mathrm{F}}_2\mathrm{C}=\mathrm{C}={\mathrm{CF}}_2$

$Y:{\mathrm{F}}_2\mathrm{B}-\mathrm{C}=\mathrm{C}-{\mathrm{BF}}_2$

In which of these two it is impossible for all the four $\mathrm{F}$ atoms to lie in the same plane?

$X:{\mathrm{F}}_2\mathrm{C}=\mathrm{C}={\mathrm{CF}}_2$,

X being an allene compound, Due to steric restrictions caused by the double-bonded carbon atoms' overlapping empty p-orbitals. Hydrogens cannot be parallel along the other side of the carbon chain since this would increase steric strain.

Two sigma bonds and two pi bonds are formed by the core carbon atom of allenes. All four of the atoms cannot lie in the same plane due to this form of bonding.