In the complex, ${\mathrm{BH}}_3{\mathrm{PF}}_3$, what is the type of back bond from B to P ?

In ${\mathrm{BH}}_3{\mathrm{PF}}_3$, the type of  back bond from B to P is $\sigma \to \pi$.

When one atom in a compound has a single electron and the other has an open orbital adjacent to it, back bonding, a type of bonding, happens between the two atoms. A molecule with back bonding possesses pi-bonding properties because pi-bonding happens after the sigma bond forms