Boron is an element of 13 group and contains three electrons in its valence shell. When its compound $\mathrm{BH}_3$ dimerises, each boron atom carry only $6 e^{-}$, that is their octet is incomplete. Hence, $\left(\mathrm{BH}_3\right)_2$ is an electron deficient compound.
In all other given molecules octet of central atom is complete.